190 VOLVOX, PLEUROCOCCUS 



H amatococcus than to Chlamydomonas, since they have thick 

 mucilaginous walls (g.) traversed by protoplasmic processes, 

 which correspond in adjacent individuals. Movement is effected, 

 as in Eiidorina, by the combined lashing of the pairs of cilia 

 borne by all except a few large cells (Fig. i : o, A, d.) located in 

 that part of the colony which is always directed backward 

 during movement. It is these large cells alone that divide to 

 form daughter-colonies, the latter being at first liberated into 

 the hollow interior of the parent (Fig. loo. A), where they grow 

 until set free by its rupture. 



Volvox thus affords a simple example of the setting apart 

 of certain cells for special purposes ; most are purely vegetative 

 and perish with the death of the parent, whilst a few are destined 

 to undergo division and persist as daughter-colonies. This is 

 in marked contrast with Eudorina, where every cell is capable 

 of performing all the hfe-functions (nutrition, growth, repro- 

 duction, etc.). The specialisation exhibited by Volvox brings 

 with it the death of the greater part of the colony, a feature which 

 is generally associated with division of labour. Eudorina and 

 Volvox also exhibit a rather advanced type of sexual reproduction 

 which is, however, so rarely observed that its description may be 

 dispensed with. 



A large proportion of the simple unicellular and colonial green 

 plants are, however, non-motile throughout the vegetative phase 

 of their life-history, and only exhibit movement, if at all, in 

 connection with reproduction. They thus recall the Palmella- 

 stages of Chlamydomonas, to which some, indeed, show consider- 

 able resemblance, e.g. the bright green gelatinous masses of 

 Tetraspora, commonly found in spring in small ponds, and con- 

 taining numerous cells arranged in groups of four (Fig. loi, D). 



As an example of a motionless unicellular plant, the very 

 widely distributed Pleurococciis, which commonly forms the green 

 powdery covering on tree-trunks, palings, etc., may be studied. 

 Under the microscope the green powder is seen to consist of 

 small groups of 2, 4, or rarely more, cells, intermingled with 

 which are more or less numerous rounded individuals (Fig. 102, A), 

 all representing different stages of Pleurococcus. Each cell 

 has a moderately thick wall, and contains a single nucleus and 

 a lobed chloroplast (Fig. 102, B). The isolated cells constitute 



