COLONIES OR COENOBIA IN THALLOPHYTA 27 



granulatum, a common freshwater alga, in which most of the cells are shown 

 emptied of contents. It will be noted that the cells around the margin of the 

 table-like colony differ in form from the inner cells ; each of them possesses 

 two horn-like processes, or, to express it shortly, is two-armed. The inner cells 

 have not these. But when the cells are aggregating to form the colony, as is 

 shown in the young stage, Fig. 3, C, all the cells have the capacity to produce 

 two arms; the inner ones are however prevented from assuming this form by 

 the closeness with which they are packed together. Were the regular arrangement 

 of the cell-colony hindered at its birth, all the cells would certainly be two-armed. 

 Here then a reciprocal influence, not of a very far-reaching character, affects the 

 cells. The single cells of the colony of Pediastrum are, according to Chodat, 

 polyergic at an early period, and in the formation of a new colony the energids 

 which proceed out of one cell separate from one another. What force brings 

 about the expansion of the colony in one plane we do not know ; perhaps the 

 direction of rays of light has something to do with it. 



b. A special interest attaches to the relationships of configuration of the colonies 

 of Volvocineae. Amongst them we find isolated monergic forms, like Chlamydo- 

 monas, with freely motile cells, and also more highly developed ones, like 

 Volvox, consisting of colonies of many cells exhibiting a polarity and division 

 of labour. 



Chlamydomonas is a nearly ellipsoid or spherical energid possessing a 

 membrane and provided with two cilia, and it multiplies by division. In Chlamydo- 

 monas Brownii, for example, four daughter-cells, seldomer two, are produced 

 in asexual propagation, according to Goroshankin. A cell which is about to 

 divide becomes motionless, and then a longitudinal wall is formed within it, 

 followed by a second at right angles to the first. The four daughter-cells then 

 separate from one another. They lie in one plane and would make a four- 

 celled colony such as occurs in Gonium J were they to remain united. The colony 

 in this genus is flat and table-like, built up Out of four to sixteen quite similar 

 cells which are surrounded by a common gelatinous envelope, but are not, as was 

 formerly supposed, in connexion with one another by protoplasmic continuations. 

 How slightly the cells are united with one another is shown by the fact that in 

 Gonium pectorale, as Biitschli states, single cells often leave the colony and swarm 

 around it, each having all the appearance of a Chlamydomonas; frequently also 

 the colony breaks up entirely into isolated cells. Each of these cells may become 

 the foundation of a new colony, but once the colony is formed a vegetative increase 

 of the cells no longer takes place. If we imagine one of the flat colonies of Gonium 

 invaginated to a hollow sphere we should obtain the colonies of Eudorina, and 

 the colonies of Pandorina arise in a manner only slightly different ; in these genera 

 also all the cells of the colony are equal in value, and are not in continuation 

 one with the other. Volvox 2 itself differs from these. In the first place the 



1 See Mignla, Beitrage zur Kenntniss des Gonium pectorale, in Botan. Centralblatt, xliv (1890). 



2 See specially Klein, Morphol. und biolog. Studien Uber die Gattung Volvox, in Pringsh. Jahrb. 

 xx ; Id., Vergl. Unters. Uber Morphologic und Biologic der Fortpflanzung bei der Gattung Volvox, 

 in Ber. der Naturf.-Gesellsch. zu Freiburg i. B., Bd. v (1891), where the literature is cited. 



