GREEN ALGAE 



391 



A simple example of the N'olvocales, a series oi Iree-swimming organisms 

 which retain their ciha in tlie vegetative stage, is seen in Chlantydomonas 

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

 spot, and two ciha by whicli 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, h). They are enclosed at first in the cell-walJ, 

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

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



v-i 



Fig. 329. 



Chhmydunivi:as. u, uuicelJular plant motile by two cilia, b, rcbult of duiiioii 



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



ya metes, — to form a zygote. The sexual differentiation thus initiated m 

 Clilamydomonas is carried to the point of distinct oogamy in Volvo.v, 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 aniiuals, stand near to the Flagellatae, and illustrate progression both in 

 vegetative structure and in sexual method. 



Ill the Protococcales the vegetative cehs are non-motile, though 

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

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

 Ag. { = Pleurococciis 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). JMultiplication 

 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 Eugleiia. 



