BULLETIN

JLLINOIS QtATE [^ABORATORY

NATURAL HISTORY.

Urbana, Illinois.

VOL L'ME V

ARTICLE v.—PLANK /ON STUDIES. II. ON I'LEODORINA
ILLLNOLSENSIS, A NEfV SPECIES FROM THE PLANK-
TON OF THE ILLINOIS RIVER.

Bv C. A. KOFOID, Ph. D.

Illinois State Laboratory of Natural History,

IHHANA, II.I.INOIS.

September, 1898.



? State Laboratory of Natural History.

LABORATORY STAl K

Professor Stephen Alfred Fokbes, Ph. D.,

Director of State Laboratoi-y awl Slate Entoimlogist.

Charles Arthur Hart,

Systematic Entomologist and Curator of Collections.

Frank Smith, A. M.,

Assistant Zoologist.

Charles Atwood Kofoid, Ph. D.,

Zoologist, and Superintendent of Biological Station.

Ernest Browning Forbe.s, B. S..

Entomological Assistant.

Wallace Craic, B. S.,

Zoological Assistant.

Mary Jane Snvuer.

Secretary.

Henry Clinton Forbe.-;,

Btisiness Agent and Librarian.

Tadia Moore Hart,

Artist.



ERRATA.

Page 136, line 2, and jiage 182, line 17 from bottom, f(jr 'g^a read 'gj.

Page 226, line 2, page 263, line 17 from bottom, and page 267, lines

2 and 15, for 'g8, read 'g6.

Page 233, line 15 from bottom, for '82 read '82a.

Page 355, line 2 from bottom, for C. F. Hudson read C. T. Hudson.

Page 389, foot-note, for Vol. V. read Vol. IV.

Page 457, line 5, for Genera read Genus.

{



Article V.

—

Plankton Studies. II. On Pko<U)rhui iUinois-

ensis, a Neiv Species from the Plankton of tJie lUinois

Hirer. By C. A. Kofoid.

The genus I'lcotlorina was discovered in 1898 l)y Shaw
('94) at Palo Alto, California, and in May of the followino-

year the species Pleodorina califomtca, upon which the genus

was founded, was detected hy Mottier ('94) in water from a

shallow stagnant pool near Bloomington, Indiana. During

the same summer the form also occurred in the Illinois River

and its adjacent waters (Clinton, '94), and it has been found in

the plankton of these situations in succeeding years from

Tune to September. The distribution of the species in this

continent is thus quite extended, and it is not at all improlj-

able that continued investigation of fresh-water plankton will

demonstrate that this genus has a cosmopolitan distribution

similar to that of some other genera of the family Volvocinccc

to which it belongs.

On June 16, 1898, a form which may be referred to the

genus Pleodorina was found in the Illinois River in water

entering the stream in large part from Cook's Slough and

Quiver Lake. Owing to high water (ten feet above low-water

mark) prevailing at the time, a considerable portion of the

habitat of the form in question consisted of submerged terri-

tory, with shallow warm water abounding in growing aquatic

and semi-aquatic vegetation.

This Pleodorina could not be found in Quiver Lake collec-

tions made on the 7th of Tune, but on the 16th it was present

in the river in small numbers, increasing until the '20th, when

a maximum was reached. From this time the numbers

decreased until the 27th, when, following a rise in the river,

the species seemingly disappeared entirely from the plankton.

It was also found sparingly in Thompson's Lake during this

period, a large area of slightly su])merged territory being at

this time tributary to the lake.

Associated with this species in great abundance was Eiulo-

rina elet/anH, in all stages of asexual reproduction, and Pando-

rlna vwrunt was also present in smaller numbers and in like

:i73



274 Illi)wi>i St((ti' Lahonitorii of Natiind Histori/.

condition. Volvox, Euglena, PJuints, Lepoc'uuils, Trachclo-

monas, Dinobryoii, Synura, Mallomonas, Uroglena, Melosira,

and Frafi'dlar'ta occurred in varying fre(inency,l)ut only a single

specimen of Pleodorina caUforn'icd was found in collections

containing the species described in this paper. The animal

plankton was represented in the main hy rotifers, Polyarthni

being most abundant, while Synclueta, Enchlams, Pterodina,

Brachionaa, and Anurcea were also present. Dijflugia,

CodoneUa, Bosmina, Cy(dops and nauplii complete the list

of the more common associates of this Plcodoruia in the

plankton.

Pleodorina ilUnoiseii-'iis n. sp.

The species here described consists of an ellipsoidal cteno-

bium or colony of 32, rarely 16 and still more rarely (54,

bifiagellate cells. The shape is quite constant, occurring in

the youngest colony and continuing throughout the asexual

cycle until the daughter colonies abandon the gelatinous

matrix of the maternal organism. Among the large number

examined only a few specimens were seen which approached

a spherical form. Measurements of twelve seemingly full-

grown colonies from material freshly killed in 2f\^ formalin

showed a range of 101 to 137 /< in long diameter, and an

average of 113 /^. The transverse diameter ranged from 84

to 102 jj, and averaged 94 /<. Individuals in which the

gonidia have begun to divide show a considerable swelling of

the hyaline gelatinous envelope. One specimen containing

2- and 4-cell stages measured 178x155 //, and when

the young colonies are ready to escape, the parent may meas-

ure as much as 200x175 /<. At the time of escape the

young colonies measure 46x38 yw. The measurements of

the colonies approach very closely those given *by Biitschli

('80-89, p. 840) for Eudorina; viz., 100-150 /<; and the

colonies of this genus found in association with the form here

described exhibit dimensions almost, if not quite, identical

with those above recorded for the Pleodorhui.

The colony (PL XXXVI., Fig. 1) contains, as a rule, 32

cells arranged, as Henfrey ('56) first noted for Eudorina, in



On Pleodor'nid lllino'isensis. 275

live circles, two of which are pohir and contain four cells

each, while eight cells are found in each of the remaining

three circles, one of which is equatorial and the other two lie

l)etween the latter and the polar circles. The cells resemble

those of Kudorind in that they are situated in the periphery

of the hyaline gelatinous matrix and are not closely crowded

together, the degree of separation depending upon the age of

the colony and varying considerably in different cases.

Their inner ends do not approach the center of the colony as

is the case in Ptindor'tna. No trace of any protoplasmic

connection between the cells of a colony could be detected in

the living organisms, nor in material killed in formalin or in

chromo-acetic acid and afterwards stained in fuchsin, hiema-

toxylin, or Bismark l)rown. Specimens treated l)y Zograf's

method (1% osmic acid followed .by 4% crude pyroligneous

acid) or by 1% osmic acid followed by picrocarmine, showed

no connection between the cells.

The colony is surrounded by a common gelatinous sheath

(•sA.) increasing in thickness (3.5 to 12 /<) with the age of the

organism. This membrane or sheath is of equal thickness in

all regions and consists of two parts : an outer, thin, denser,

more highly refractive layer {o.l.) ; and an inner homogeneous

one (/./.), which shows no traces of the concentric structure

found in Pandor'inn. It is within this latter layer that

the increase in thickness takes place in the older colo-

nies. It is limited centrally by a thinner and less highly

refractive layer {vi.iii.) which encloses the common matrix

(in.) in which the cells of the colony lie. Frequently among
the older organisms there occur upon the posterior end of the

colonies blunt, pseudopodia-like protuberances (PL XXXVL,
Fig. 4) of the sheath, of irregular form and of no constant

number. Their position and the fact that they are often,

though not always, found in old colonies from which some of

the daughter colonies have already apparently escaped, sug-

gest that they may mark the place of exit of the young indi-

viduals from the parent. Similar protuberances were ob-

served upon EudoriiKi and Pandoriiia, under similar con-

ditions, in the collections in which the Pleodorlna under



276 Illinois State lAilioiuitorii of N<itnv<d History.

discussion was I'oiind. Wills ('80) found that the daughter

colonies of Volvox globator escaped through a rift in the pos-

terior hemisphere of the parent, and Klein ('89) observed the

same phenomenon in Volvox aureKS. The escape of the

daughter colonies in Pleodorina has not been observed by me.

The sheath stains deeply in an aqueous solution of methylen

blue, more deeply, in fact, than the enclosed matrix, the outer

layer taking the deeper stain. It also shrinks to about one

fourth its former thickness. This shrinkage, together with

that of the central matrix, causes the sheath to wrinkle along

lines which bound hexagonal areas from whose centers the

cells now project, thus giving the appearance of a division of

the surface of the colony into regular polygons. The sheath

shows no trace of the layer of radial rod-like structures

found by Klebs ('86) in Pandor'tna, but iodine or methylen

blue demonstrates a finely granular condition like that

described for Eudorina. The sheath is traversed by the pairs

of flagella which arise from the outer ends of each of the cells.

The matrix (»?.) is a gelatinous substance of some consist-

ency, filling the colony inside of the inner membrane. In

the living colonies, in those which were killed in the

various reagents mentioned above and afterwards stained,

and in disintegrating material, no traces of any divisions can

be detected in this substance that are not due to wrinkling

caused by shrinkage. Methylen blue or iodine causes the

matrix to show a faintly reticulated or vacuolated appearance

due to different densities of staining. That the substance of

the matrix has considerable consistency even in the swollen

condition found in the maternal colonies, is shown by the

fact that the flagella of the young forms, before rotation

begins, can be seen to penetrate the matrix of the parent very

slowly. Their ends are often blunted or even knob-like and

their lateral motion is very limited. The movement of the

young colonies through the matrix is a very slow and gradual

one, showing the gelatinous consistency of the substance in

which they are imprisoned.

The striking feature in the structure of this species, as in

the case of P. cidifornica (Shaw, '94), is the presence of two



On Pleodorlna ilUnoisensis. 277

distinct types of cells in the colony (PI. XXXYL, Fig. 1),

the vegetative {v. c.) and the gonidial (g. c.) cells. The
presence of these tAvo types of cells at once places this new
species in the genus Pleodorhia rather than in KinJor'nxi—
which it otherwise closely resembles.

The vegetative cells (r. c.) are four in number and consti-

tute the anterior polar circle, being always directed forward

in locomotion, as in the other species of the genus. Their

number remains the same in the smaller colonies of sixteen

cells and in the larger ones of sixty- four. The diameter of

these cells ranges from 9.5 to 16.8 /^, twelve cells averaging

12.25 //. The size of these cells varies even in the matured
colonies, measurements at this stage ranging from 9.G to

15.G /.'. At birth the cells of the young colonies vary in

diameter from 3.5 to 5 /< in different parents. In the

daughter colonies while still in the maternal matrix, no dis-

tinction in size between the vegetative and gonidial cells can
be detected, nor can this distinction l)e made in the younger
free-swimming colonies, it being thus impossible at this

stage to distinguish the young V. ilUnoisensis from the simi-

lar stages of Eudorina elcgans with which they were asso-

ciated. When the young colonies have attained dimensions

of 46x38 /<, the vegetative cells measure 4 /^ and fhe gonidia

4.8 //. A like similarity between the two kinds of cells in the

3'oung colonies exists, according to Shaw ('94), in 1'. call-

fortiica.

In structure the vegetative cells (PI. XXXVI. , Fig. 2) are

in most particulars similar to the gonidia, described below.

They sometimes appear to be a trifle lighter green in color

—

a difference which may be due to their smaller size. The
principal differences lie in the smaller number of pyrenoids

and the larger size, both absolute and relative, of the stigma

or eye-spot.

As to the fate of the vegetative cells, the evidence at hand
is insufficient and conflicting. In three colonies in which

the daughters were moving about in the maternal matrix,

some having already escaped, the vegetative cells showed
very evident signs of degeneration, the contents being



278 Illinois State Lahoratorn of Natural History.

shrunken and irregular. In the larger number of instances

of this stage under observation the cells appeared normal,

showing no trace of degeneration or division. In one instance

only have I found a specimen in which the vegetative cells

had divided beyond question. This was a colony in which

the gonidia had completed their division but had not escaped.

Three of the vegetative cells were in the two-cell stage and one

was undivided. In two instances matured colonies have been

found in which four smaller daughter colonies (of eight and

sixteen cells respectively) were present at one pole.

The gonidial cells (PI. XXXVI., Fig. 3) constitute the

remainder of the colony. They usually number 28, rarely

12 or 60, and occupy the parts behind the anterior polar

circle of vegetative cells. These cells in most instances can

be easily distinguished by their larger size. In form they are

spherical, though some specimens in preserved material are

slightly flattened on their inner ends. In diameter they range

in seemingly full-grown colonies from 15 to 25 /<, averaging

in twelve specimens 1 9.2 yu. Their dimensions just before their

division, that is in colonies in which division has begun, also

show the extreme range quoted above, the smaller diameter

having been found in a sixteen-cell colony. As a rule the

gonidia are all of the same size, but occasionally specimens

have been found in which one or more dwarf cells occur

among them. These are irregular in their distribution and

can be distinguished at once from the vegetative cells by

their position. Similar dwarf cells were found in both Pan-

clorina and Eudorina. In matured colonies gonidial cells are

frequently found which fail to divide. The gonidia are of a

light green color, a trifle darker than the vegetative cells.

Their color in general is similar to that in Eudorina, and is

somewhat lighter than that in Volvox and Pandorina, with

which they are associated.

A distinct cell membrane (c. m.) is found about each of the

cells. In the living condition and in the material preserved

in formalin it forms a highly refractive hyaline layer, about

1 jj in thickness, outside of the green contents of the cell. It

stains very faintly in hsematoxylin and assumes a deep brown



On Pleodorina illinoisensis. 279

tint with long-continued action of iodine and sulplmric acid.

In the case of diseased colonies hereafter mentioned the cell

membranes persist, often retaining their original form and

shape, after the entire disappearance of the contents.

The greater part of the cell contents consists of what seems to

be one large chromatophore (PI. XXXYI., Fig. 2, chr.), which

occupies all of the cell except the centrally placed nucleus

with its enveloping protoplasm, and a slender column (jk c)

passing from this region to the anterior end of the cell. In

m^ny cells a faintly marked notch or furrow ifi(., Fig. 1) is to

be detected on one side of the chromatophore at the anterior

end of the cell. This seems to mark the line of contact of

the sides of the chromatophore which has surrounded the

nucleus. In the 2- and 4-cell stages of the gonidial cells

the nucleus and the protoplasmic mass are plainly seen

to occupy one side of the cell (PI. XXXYIL, Fig. 7, 8),

but in the cells of the young colony it again occupies a central

position. The chromatophore is uniformly of a bright chloro-

phyll-gi'een, and shows a finely granular structure under high

magnification. In the youngest colonies each cell contains

but a single spherical pyrenoid (pr.), which occupies a lateral

position in the chromatophore, in the inner hemisphere of the

cell. In the older colonies the number of pyrenoids increases,

as many as twelve having been found. They are scattered

irregularly through the chromatophore, and may occur in

any part of it. A similar increase of pyrenoids is reported

by Shaw ('94) for P. californica. In the vegetative cells the

number of pyrenoids is often but 2-4, and is, as a rule, less

than that of the gonidial cells. In a very few instances as

many as eight have been found, and in one old colony the

vegetative cells seemed to be packed full of pyrenoids. In the

young colonies the pyrenoids have a diameter of about 1 //,

and in the older colonies of 2.5 /'.

The nucleus (».) lies in about the center of the cell in the

midst of a mass of protoplasm enclosed by the chromatophore.

In mature gonidial cells before division it has a diameter of

7-8 }.i, and contains a sub-central nucleolus (ncl.) whose

diameter is 3 yw. The nucleolus stains deeply with picro-



280 Illinois State Ijahoratoni of Ndtiiral Ilistorif.

carmine, and is by this means easily distinguislied from the

pyrenoids, which it resembles in appearance and size. The

nuclear membrane is detected with difficulty. It encloses a

faintly stained nuclear reticulum (r.). In the younger cells

the nucleus is much smaller (4-5 /<), the nucleolus is rela-

tively larger, and the reticulum is not evident. In the living

cell the nucleolus alone can be seen in the midst of the

grayish protoplasmic mass at the center of the cell. The

protoplasm is continued from this central region peripherally,

in the axis of the cell as a slender column (j;. c), to the

anterior end, where it includes the stigma and bases of the

two flagella. A protoplasmic mantle enclosing the chromat-

opho^'e was not demonstrated.

The stigma or eye-spot (s.) lies at the anterior end of the

cell, near its axis, and is often so placed that an equilateral

triangle may be drawn with it and the bases of the two liagella

as apices. It is of a bright reddish brown color, though in

some of the posterior cells the color is often very faint, giving

the stigma the appearance of a slightly tinged oil-globule.

It is of an elongated hemispherical shape when seen from the

side, and has a circular outline when seen from above. Its

upper end often projects slightly so as to elevate the cell

membrane. The application of killing agents and alcohol

soon removes its color, and even in formalin this fades out in

the course of a few days, leaving merely a colorless, highly

refractive structure. The larger stigmata have a diameter of

2.5 /< and a depth of 2.8 //, and are to be found in the cells

in the anterior part of the colony, especially in the four vege-

tative cells of the anterior polar circle. Posteriorly the stig-

mata are less prominent, and are often not to be found at all

as brightly colored spots but merely as pale globules whose

position alone affords a clue to their real character.

This specializa,tion of the stigmata in the anterior end of

the colony occurs also in Endorina, Pandorina, and Volvox,

and Shaw ('94) states that in P. californica the stigmata,

which are present in the posterior part of the young colonies

(in gonidial cells), become less conspicuous and disappear as

the colony enlarges and the differentiation of the cells pro-



On Pleodorina illino'isensis. 281

ceeds. This prominence of the stigmata in the anterior end,

together with the facts that this end is always directed fore-

most in locomotion and that the species showing this ditter-

entiation are positively phototactic in the vegetative condition

when the dilt'erentiation is prominent, all point toward the

participation of the stigmata in the function of light percep-

tion. An interesting phenomenon occurs at the time of the

division of the gouidia, for the stigma of the mother cell

persists and is passed on through the five successive cell

divisions to the outer end of one of the cells of the daughter

colony, situated in the margin of the cup which arises from

the plate of cells and closes to form the ellipsoidal daughter

colony. Inasmuch as this cup always closes from the inside

out, that is with the opening directed outward, it is evident

that the stigma must traverse the distance between the outer

end of the mother cell and its inner end, which corresponds

to the outer ends of the cells of the daughter colony. New
stigmata arise in the cells of the daughter colony, but being at

first very small are thus quickly distinguishable from the

persisting stigma. The ultimate fate of this persisting stigma

has not been traced.

No contractile vacuole was observed in the living cells, and
careful search with a Zeiss ^^^-inch oil-immersion lens for

this structure in preserved and stained material has led to no

positive identification of a vacuole. The bleached stigma

and what seem to be the enlarged bases of the Hagella are the

only areas discernible in the anterior end of the cell which at

all resemble a contractile vacuole. Shaw ('94) finds in picro-

nigrosin material a single vacuole in the anterior end of the

young cells of P. californica.

The flagella (,/'.) are two in number for each cell, and unite

with the cell at the anterior end adjacent to the stigma. The

two flagella have the same proportions, and in adult colonies

they measure 40 « in length. In the young colonies they

are relatively longer. They are visible on the young colonies

shortly after the cup closes, and persist upon the maternal

colony during the early divisions of the gonidia.

The locomotion of the colonies of Pleodor'nta iUiiioisensis



282 Illhwis State Laboratory of Nattij'al History.

is of the type prevailing among other spherical or ellip-

soidal genera of the Volvocinefe ; viz., rotation about the

principal or long axis of the colony, either from right

over to left or the reverse, frequently with one direction

predominating, progression being usually along the line

of the axis, the same end of the colony always leading.

In P. illinoisensis the vegetative pole always leads in loco-

motion in horizontal, oblique, and vertical movements, and

is therefore the anterior pole of the colony. Under normal

conditions,when under observation, this species is rarely quiet

during the period of growth. While still within the matrix of

the mother colony the ceasless rotation with its frequent rever-

sals begins. Colonies in the life cell, while favorable condi-

tions prevail, can be seen in active movement, jostling one

another and their neighbors in their seemingly aimless wan-

derings. When an object is met which does not yield to their

persistent rotation, their movements may slacken for a time

to be resumed shortly in some line of less resistance. The

rotation of this species is prevailingly from right over to left

as the following tables show, which indicate the number and

direction of the reversals of rotation in ten individuals in one

minute.

Direction



On Pleodorhtu illlnolsensis. 283

rotations. This is indicated in a general way in the second

table by the plus and minus signs, which show the direction

in -which the rotation was of longer and shorter duration.

In conclusion it may be said that both directions of rotation

occur, though that from right over to left is more frequently

met with, or, in other words, is of longer duration.

"With regard to locomotion in P. cnliformca, Shaw ('04)

says that " the movement of the plant in the water was fol-

lowed in the case of a few individuals bearing well-developed

gonidia. In swimming through the water the vegetative pole

is directed forward and the plant revolves to the right (in

observed cases) on the axis connecting the vegetative and
reproductive poles. The path is parallel to this axis in up-

ward vertical as well as in horizontal movement." The
polarity of this genus thus expressed physiologically in the

movements of the oolony is accompanied by a corresponding

structural differentiation of the cells composing the organism.

In Goniuni, according to Fresenius ('56), the motion of the

colony resembles that of a wheel, progression taking place in

the line of the axis of rotation. According to Biitschli

('83-'87, p. 858), locomotion is accomplished by the rotation

of the plate-like colony around its shorter axis, the direc-

tion of rotation being to the right in some individuals and to

the left in others. Pfeffer ('84), on the other hand, describes

the rotation during the forward movement as alternately

from the right and the left. Migula ('90) calls attention to

the wavering, often backward, and irregular movements of

this genus, and also notes its rotation about an axis through

the middle of the colony. This rotation is either to the right

or to the left, no predominance being mentioned. Polarity

is thus marked in the activity of the Goninm colony, though

not expressly marked in its structure except as it appertains

to the individual cells.

In Stephanospluera the polarity in structure is but slightly

marked in the colony, being indicated in some colonies by

the asymmetrical position of the cells, but there is a physio-

logical differentiation in that one pole of the colony leads in

locomotion. In this genus also, according to Cohn ('52), the



284 Illinois State La}>or(itori/ of Natural Historji.

rotation is in either of the two directions and is subject to

frequent change. No predominant direction was noted by

him.

In I'diidorina the only structural expression of polarity is

found in the greater development of the stigmata in the cells

in the anterior end of the colony. In other particulars the

poles are not differentiated. Braun ('51) maintains that in

this genus the rotation is constantly around the long axis of

the colony in the direction of the hands of a clock, when the

motion is toward the observer. Nageli {Jide Biitschli, '83-'87,

p. 858), on the other hand, observed rotation in both direc-

tions. My own observations upon Pandorlna mornm show

beyond question that the direction of rotation is not constant,

as the following table demonstrates.

Direction



On Pleodorina illinoisensis

.

285

precise statement as to locomotion in this genus. As ol)-

served by me, it closely resembles that described above for

Pleodorina illinoisensis ; viz., rotation around the long axis of

the colony, the same pole constantly leading in progression.

The direction of rotation is frequently reversed, though it was
predominantly from right over to left in the cases observed.

A functional polarity thus exists in this genus.

In T'o/ro.r, according to Klein ('90), there is a polar differ-

entiation as regards the stigmata that is even more marked
than it is in the genera previously mentioned. He finds that

the cells of the pole directed forward in locomotion each

possess a stigma which is especially large and intensely

colored ; that the color fades out and the stigmata become
smaller and paler as the equator is approached ; and that

beyond this they are usually represented merely by a color-

less oil-drop, which in some cases may even disappear. The

posterior hemisphere is also marked by the development

there of the gonidia, as was first shown by Colm ('56), and

occasionally ellipsoidal colonies are found whose long axis

connects the anterior and posterior poles. Locomotion in

Volvox is accomplished, as elsewhere in the family, by the

rotation of the colony about its principal axis. Wills ('80)

observed the predominance of the rotation to the right and

its occasional brief reversal. Klein ('89) states that this

preference is found in T\' (ilohator, but that it is not shown by

J', aureus] In this latter species the changes are frequent and

are often separated l)y a brief pause. Backward motion is

rarely seen and lasts but a short time. In the case of Volvox

the axis of rotation is slightly oblique, the center of the

colony remaining in the line of progress, but the axis of

rotation being inclined from above the line at the anterior

pole to below it at the posterior one.

We thus find that Pleodorina illinoisensis, which exhibits

both a structural and physiological polarity, shares with

most, if not all, of the genera of the family to which it

belongs, the physiological differentiation which is expressed

in locomotion, and also, in observed cases, exemplifies the

extreme form of a predominance of rotation in one direction.



286 Illinois State Laboratory of Natural History.

We also find that the structural diflferentiation shown in the

decadence of its posterior stigmata obtains in varying degrees

in the other spherical and ellipsoidal genera of the family

—

least in Pandoriua, most in Volvox. The genus Pleodorina

agrees with Volvox in having a structural polarity based upon

the division of the colony into vegetative and gonidial regions,

but the differentiation is simpler. Of the two species of Pleo-

dorina, the one here described exhibits the simplest possible

differentiation of the colony consistent with the symmetry

of the organism; viz., the differentiation of the anterior

polar circle of four cells as vegetative members of the colony.

Of the two species of the genus it thus stands nearer Eudo-

rina, while its sister species P. californica approaches more

closely to Volvox both in the number of cells and in the

extent of the differentiation.

The discovery of this additional species of the genus Pleo-

dorina thus supports the opinion expressed by Shaw ('94),

who founded the genus, that it was intermediate between

Eudorina and Volvox but nearer the former. Judging merely

from the asexual stage, P. illinoisensis affords additional evi-

dence of the close relationship of Pleodorina and Eudorina.

Throughout the preparation of this paper the writer has

had constantly in mind the possibility that the form here

described is merely a stage in the life cycle of Eudorina. A
number of facts lend support to this hypothesis : (1) the

occurrence of Pleodorina illinoisensis with Eudorina elegans

;

(2) their marked similarity, aside from the four vegetative

cells, in structure and measurements; (3) the impossibility

of separating the youngest free-swimming colonies of the

two forms; (4) a considerable variation in the size of

the vegetative cells in Pleodorina, grading toward the

condition in Eudorina
; (5) some evidence that in certain

cases at least the vegetative cells may divide, one case of a

2-cell stage having been seen in the hundreds, if not thou-

sands, of specimens examined, and one instance noted in

which a maternal colony containing thirty-two daughter

colonies had at one pole four colonies which were slightly

smaller than the remaining twenty-eight ; and (6) the occur-



On Pleodorina illinoiscnsis. 287

rence of pleomorphism in the family VoIvoc'duuc, Klein ('89

and '90) citing no less than twenty-four " comhinations " in

the case of Volvox aureus. It may then be that the form here

described as Pleodorina iUino'iaeusis is only a " Pleodorina

stage" of Eudorina.

The abrupt disappearance of this supposed new species

from the plankton prevented the carrying out of breeding

experiments designed to test its validity, and it seems that

the matter must remain undecided for the present. In the

absence of satisfactory proof that the form here described is

but a phase of the life cycle of Eudorbia it has seemed best

to the writer to make the above suggestion and to take the only

course open in publication, namely, the description of the form
as a new^ species, inviting the criticism of subsequent investiga-

tion. The dilemma here presented is byno means an isolated

one in plankton work, nor is it new to the family Volvoclnea'

:

witness the long confusion which existed over the two species

of Volvox, aureus and glohator, which has been at last cleared

up by the excellent work of Klein ('89, '89a, '90) and Overton

('89). Another instance is often presented when Paudorina

and Eudorina both occur in the same collections and the

plankton statistician must decide to which genus each speci-

men observed must be referred. Typical specimens of each

can be found, but all individuals do not conform to the type,

or they may present conditions in which the conformation is

obscured by some phase of the life cycle.

The asexual reproduction of Pleodorina iUinoisensi.s (PI.

XXXVII.) resembles that of other species of the genus in that

it is accomplished by the repeated division of the gonidial

cells, resulting in the formation of daughter colonies in the

maternal matrix. These escape later from the parent organ-

ism, and by growth attain the adult condition with the differ-

entiation of the four vegetative cells. Five successive cell di-

visions, pervading all the cells of the parent organism except the

vegetative cells, are necessary for the completion of the process,

and result in the 2-, 4-, 8-, 16-, and 32-eell stages of the form-

ing colonies. The first two of these divisions result in the forma-

tion of a quadrangular plate of cells—a form which is retained



288 Illinois State Lahordtori/ <>/ Ndtnnil Tlistorj/.

through the two succeeding divisions, which produce the 8-

and IG-cell stages. The cupping of this phite, which results

in the formation of an ellipsoidal colony, is apparent as early

as tlie 4-cell stage (PL XXXYIL, Fig. 9) and continues

through the later stages (Fig. 11, 13), so that hy the time

the 16 -cell plate is formed it has almost the curvature of a

saucer. With the formation of thirty-two cells the closure

of the cup proceeds and is soon completed. The orifice of

the cup is directed outward in all cases, and thus the ends

of the cells of the daughter colony which are formed from the

oKtcr end of the maternal gonidial cell come to lie in the

inner side of the cup, and are the inner ends of the cells of

the daughter colony. In the matured colonics the young

usually lie with their long axes parallel to the surface of the

parent. I have not, however, been able to identify the point

of closure of this cup with this region or positively with any

other.

The sequence and position of cleavage planes which produce

the quadrangular plate of the 16-cell stage is, in the main,

similar to that described by Goroschankin ('75) and Braun

('75) for Eiidorina and Volvox. Beyond this stage there is

some doubt as to the agreement. A full discussion of the

subject is beyond the scope of the present paper, for which the

following brief description must suffice. The first cleavage

plane (I) divides the gonidial cell into two hemispheres along

the axis of the cell, and the daughter nuclei, with the sur-

rounding protoplasm, are placed close together in the center

of the opposing faces of the new cells (PL XXXVII., Fig. 7),

The second plane (II) is at right angles (Fig. 8-9) to the first

and also passes through the regions representing the axis

of the ancestral cell. In this instance also the nuclei are

gathered near the center of the young colony, which exhibits

to an appreciable extent the curving indicative of the later

formation of the cup. The 8-cell stage results from the

divisions of each of the quadrants of the 4-cell stage by a

plane (III) which is parallel to one of the previous planes

and perpendicular to the other, meeting the latter at a point

about midway between the center and the circumference. By



On Plcodur'uia illinoinensis. 289

a sul)seqnent adjustment of the cells the four more centrally

placed ones come to form a sort of a (Ireek cross whose anoles

are filled by the other four (PI. XXXVII. , Fig. 10, 11). The
lC)-cell stage is formed by four additional planes, each of

which divides one of the cross-cells and its corner neighlxn-.

The location of these planes may be described in the same
terms as the last excepting that they meet the radial planes, I

and II, at about one fourth the distance from the circum-

ference toward the center. The cupping of the plate soon

advances to such an extent that it consists of a square of four

centrally placed cells, upon each of the four sides of which
there overhangs a row of three cells, of three grades of eleva-

tion (PL XXXVII., Fig. 12, 13). The succeeding division and
the completion of the cup (PI. XXXVII. , Fig. 14) result in the

young colony's assuming the ancestral form. Throughout
these divisions the number of pyrenoids in the daughter cells

grows steadily less. But one can be found in each cell in the

32-cell stage, while in the 16-cell stage two are readily recog-

nizable in each cell. In the earlier stages and before division

the number often varies, and the pyrenoids are frequently so

crowded that enumeration is difficult if not impossible. It

seems not improbable that these structures also must undergo

some division during the process of cell multiplication. Dur-
ing the processes of division the nuclei continue to occupy

a position near the inner ends of the cells (in the new colony),

and it is only after the divisions have been completed that

they come to occupy their usual positions at the center of the

cells—perhaps as a result of the growth of the chromatophore.

No stage of sexual reproduction has been positively identi-

fied for this species.

A peculiar condition of the colonies of this species, also

occurring in Pandorina and Kiidorina, deserves passing

notice. It occurred with considerable frequency in all three

genera and resulted in each case in the destruction of the

entire colony affected. The early stages of the disease, if it

be such, are indicated by the homogeneous condition of the

cells and the fading out of the color, together with a flattening

of the cell contents into a disk- or lozenge-shaped mass (PL



290 Illinois State Ijahorator/t of Ntitiinil Historij.

XXXYI., Fig. 4). In the subsequent stages this mass assumes

a yellow and then a brownish color, takes on an irregular shape

(PL XXXVI., Fig. 5), and disintegrates, leaving the empty cell

walls occupying the matrix. In spite of the suggestion in the

above description there was never any trace of the formation

of spermatozoa in the colonies presenting these phenomena,
neither was there any indication of encystment. There was
no indication of either a fungous or an algal parasite, and it

seems not improbable that the occurrence of these diseased

forms may have been due to some unfavorable local condition

in the water tributary to the habitat of the genera affected.

The following brief synopsis of the prominent characters

of this genus and its two species will serve as a convenient

diagnosis for their determination.

IHcodurina Shaw.—Colony consists of a spherical or ellip-

tical coenobium of greenish biflagellate cells of two types,

vegetative and gonidial, in the anterior and posterior parts

of the colony respectively, which lie in the periphery of a

hyaline gelatinous matrix and are surrounded by a common
hyaline envelope. Cells each with one reddish stigma, which is

more prominent in the anterior part of the colony. No con-

necting filaments between the cells. Non-sexual reproduc-

tion by gonidia, which are formed by increase in size of a

part of the cells of the colony. Daughters escape from
parent as small colonies of biflagellate cells which at this

stage are all similar. Sexual reproduction not known.
P. caZ//or;?/ica Shaw.—Number of cells in colony 64 or 128.

Maximum diameter of colony 175-340 /^. Vegetative cells

constituting approximately one half the colony. Gonidial cells

2-3 times diameter of vegetative cells. Known habitat : ponds,

ditches, and streams in California, Indiana, and Illinois.

P. illinoisensls n. sp.—Number of cells in colony usually

32, rarely 16 or 64. Dimensions of colony range from

46x38 M to 200x175 /^. Vegetative cells always four in

number. Gonidial cells approximately 1.1—2 times diam-

eter of vegetative cells. Known habitat : submerged lands

along the Illinois River. Types deposited in collections of

Illinois State Laboratory of Natural History and United

States National Museum.

Illinois Biological Station, Havana, 111., .luly 25, 1898.



On Pleodorhia illinoise^isis. 291

BIBLIOGRAPHY.

Braun, A.

'51. Betrachtungen iiber die Ersoheinuiio; iler Verjun>i^uii»:; in der

Natiir, insbesoiulere in der Lebens- uud Bildungsgeschichte der

Prtanze. XVI-3C4 pp., 3 Taf. Leipzig.

'75. Uber einige Volvocineen. Ber. d. Gesellsch. naturforsch.

Freiinde zu Berlin, 1875, pp. 9-18.

Butschli. O.

'80-'89. Protozoa. Bronn's Klassen und Ordnungen des Thier-

reichs, Bd. 1., Tli. l.-lll. 2035 pp., 79 Taf. Leipzig und Heidelberg.

Carter, H. J.

'58. On Fecundation in Eudorina elegans and Cryptoglena. Ann.

Mag. Nat. Hist., Ser. 3, Vol. IL, pp. 237-253, PI. VIIL

Clinton. G. P.

'94. Pieodoriua in Illinois. Botan. Gazette, Vol. XIX., p. 383.

Cohn, r.

'52. Uber eine neue Gattung aus der Familie der Volvocineen.

Zeitschr. f. wiss. Zool., Bd. IV., pp. 77-110, Taf. VI.

'56. Observations sur les Volvocinees. Ann. d. Sci. Kat., Botan.,

IV. Ser.,T. V., pp. 323-332; also, with some changes, in 34 Jahresber.

d, Schles. Gesellsch. f. vaterl. Cultur f. 1856, pp. 39, 40. 77-83.

Fresenius, G.

'56. Uber die Algengattungen Pandorina, Gonium und Raphidium.

Abhandl. der Senckenberg. naturforsch. Gesellsch., Bd. II. , pp.

187-200, Taf. VII 1.

Goroscliankin, J.

"75. Genesis ira Typus der palmellenartigen Algen. Versucb einer

vergleichenden Morphologie der Familie der Volvocinea (German
title of Russian article). Mittheil. der kais. Gesellsch. natur-

forsi'h. Freunde in Moskau, Bd. XVI. 40 pp., 2 Taf.

Henfrey, A.

'56. Notes on some Fresh-water Confervoid Algw New to Britain.

Trans. Micr. 8oc. London, N. S., Vol. IV., pp. 49-54, Pi. IV.

Klebs, G.

'86. Uber die Organisation der Gallerte bei einigen Algen und
Flagellaten. LTntersuch. a. d. botan. Inst, zu Tiibingen, Bd II., pp.

333-418, Taf. III., IV.

Klein, L.

'89. Morphologische und biologische Studien iiber die Gattung

Volvox, Priugsheim's Jahrb. f. wiss. Botan., Bd. XX., pp. 133-211,

Taf. X.—XII.



292 Illinois State Ldhordtorii of NcttKnil Historij.

'89a. Xeue Beihiige znv Keiintniss der Gattuno^ Volvox. Ber. d.

cleutsoh. botan. Gesellsch., Jahrg. 1889, Btl. VI 1., pp. 42-53, Taf.

HI.

'90. Ver.ijleidieiule Vntersuchuiio^en iiber Morpholo^ie luul Biologie

(ler FoitpHanziing bei der Gattiinj;- Volvox. Ber. d. iiatiirf. Ge-

sellsch. zu Freiburg- i. B,, Bd. V., pp. 29-120, Taf. II.—YI.

Migula, W.
'90. Beitriio^e zur Kenntniss des Goniiim pectorale. Botan. Centralbl.,^

Bd. XLIV., pp. 72-76, 103-107, 143-14G, Taf. II.

Mottier, D. M.
'94. Pleodorina in Indiana. Botan. Gazette, Vol. XIX., p. 383.

Overton, E.

'89. Beitrao: zur Kenntniss der Galtiin^- Volvox. Botan. Centralbl,,

Bd. XXXIX., pp. 05-72, 113-118, 145-150, 177-182, 209-214, 241-246,

273-279, Taf. I.-II I.

Pfeflfer, W.
'84. Locomotorisehe Richtungsbewe<?ungen durch cbemische Reize.

I'ntersuch. a. d. botan. Inst, zu Ttibingen, Bd. I., pp. 363-482.

Shaw, W. R.

'94. Pleodorina. a new genus of the Volvocinea'. Botan. Gazette,Vo].

XIX., pp. 279-283, PI. XXVII.

Wills, A. W.
'80. On the Structure and Life History of Volvox globator. Mid-

land Xaturallst, Vol. III., pp. 209-214, 233-237, PI. VII., VMI.



0)1 Ph'odorina iUlitoisensis. 2i)H

EXPLANATION OF PLATES.*

AHimKVIATIONS.

A.,



Plate XXXVI.



ll.ATE XXXVII.