GREEN ALGAE 



391 



A simple example of the Volvocales, a series of free-swimming organisms 

 which retain their cilia in the vegetative stage, is seen in Chlamydomonas 

 (Fig. 329). Each cell has a nucleus, a chromatophore with pyrenoid, an eye- 

 spot, and two cilia by which it is actively motile. Externally is a swollen 

 wall of cellulose. Multiplication is by zoospores formed by division of one 

 cell into 2, 4, or 8 (Fig. 329, fc). They are enclosed at first in the cell-wall, 

 but finally escape as separate individuals. There is also sexual reproduction 

 by pairing of similar biciliate gametes, or in some species of dissimilar 



Fie. 329. 



Chlamytiumonas. a, unicellular plant motile by two cilia. 6, result of division 

 into four. (Dr. J. M. Thompson.) 



gametes, to form a zygote. The sexual differentiation thus initiated in 

 Chlamydomonas is carried to the point of distinct oogamy in Volvox, a larger 

 spherical, free-swimming organism, in which, in addition to vegetative pro- 

 pagation of daughter-spheres within the parent, there are well-defined 

 antheridia and oogonia. These organisms, which are sometimes described 

 as animals, stand near to the Flagellatae, and illustrate progression both in 

 vegetative structure and in sexual method. 



In the Protococcales the vegetative cells are non-motile, though 

 in some of them motile zoospores and gametes may be produced. 

 A common and very simple example is seen in Protococcus viridis, 

 Ag. ( = Pleurococcus vulgaris), found as a green incrustation on the 

 windward side of tree-trunks, etc. The plant retains its vitality 

 after drying, as from its habitat it would need to do. Each cell 

 is limited by a cell-wall, and contains a nucleus and a definite chloro- 

 plast, which being lobed gives the appearance of several, though it 

 is stated that only one is really present (Fig. 328). Multiplication 

 is by cell-division, the products of which remain loosely related to 

 one another in groups ; finally they separate by rounding off of the 

 cells. No motile stage has been proved to occur. It is as though 

 that plant represented an unlimited repetition of the encysted stage 

 of a Flagellate, such as Euglena. 



