COLONIES OR COENOBIA IX TIIALLOPHYTA 29 



within the vegetative cells, inasmuch as the anterior ones are somewhat differently 

 organized from the posterior ; and secondly between generative and vegetative cells. 

 The vegetative cells work for the generative cells and then disintegrate, and thereby 

 the colony is able to multiply itself rapidly under favourable conditions, and to 

 send strong daughter-colonies into the world. As a matter of fact there is under 

 such conditions an uncommonly productive multiplication. In the colonies of 

 other Volvocineae the cells exercise a reciprocal influence though it is less profound, 

 and it is shown in their common swimming movement which of course presupposes 

 a regulating of the ciliary capacity of the single cells. It is because the relationship 

 of Volvox to the colonies of other Volvocineae is so clear that we reckon it as 

 a colony ; considered by itself one might regard Volvox as a true pluricellular 

 plant and term it a cell-dominion. There is no experimental evidence as yet 

 available which would enable us to say whether in the event of the destruction 

 of its generative cells any other cell could take up their work; but it is probable 

 if the destruction look place without any great injury to the colony, and at 

 a sufficiently early period, that this substitution would occur. I do not here intend 

 to describe the organs of propagation in the Volvocineae, I will only recall the fact 

 that we find in the group a very instructive series of gradations from isogamy 

 to oogamy. The spermatozoids of Volvox present a parallel formation with the 

 spermatozoids of Archegoniatae ; their elongated form is, in my view, developed 

 here, as there, to enable them to pierce into the gelatinous coating of the ovum. 



B. FIXED COLONIES. 



These are attached to the substratum either through the excretion of fixing 

 substance, or through the construction of special anchoring organs which frequently 

 appear in consequence of external stimuli, the so-called contact-stimuli, and 

 perhaps owe their origin primarily to such a sensitiveness. The existence of an 

 anchored base is a further stage of differentiation of the colony, which may then 

 readily pass into the condition of a cell-dominion if the end of the colony opposite 

 to the base acquire the form of a vegetative point. This step has indeed frequently 

 been taken, but I must first of all depict some examples in which it has not taken 

 place. 



The colonies of Apiocystis, which are invested by a common gelatinous 

 envelope, excrete at their base a fixing substance in the form of a kind of disk. The 

 peculiar 'pseudo-cilia' of this alga which stretch out from the gelatinous envelope 

 I consider to be organs for the interchange of material, especially of gases, 

 because the thick gelatinous envelope makes this process difficult. 



Amongst the Diatomaceae occur colonies, both free-floating and fixed, of the 

 most different forms, in which the single cells are held together usually by a gela- 

 tinous excretion, and this excretion serves as the anchoring substance in the fixed 

 colonies. A division of labour among the cells of the colony is not known unless 

 we see this in the circumstance that in many of them, as for example in the 

 genus Achnanthes, the thread-like colonies are fixed at one end of the thread 

 and their basal cell alone excretes the mucilage for fixing the organism. This 



