318 BOTANY 



plant, while in each end of the cell there is a small vacuole containing minute 

 crystals of gypsum in constant motion. Many Desmids are characterised by 

 heliotactic movements ; they protrude fine mucilaginous threads through the cell 

 walls, by means of which they can push themselves along, and take up a position 

 in a line with the direction of the incident rays of light. 



Multiplication is effected by cell division. This is accomplished by the forma- 

 tion of a partition wall across the middle of the cell after the nuclear division is 

 completed. Each daughter cell eventually attains the size and form of the mother 

 cell, by the outgrowth of a new half on the side towards the new division wall (Fig. 

 237, A). After the completion of their growth, the two cells separate from each 

 other. 



The conjugation of the protoplasts takes place, in the case of the Desmidiaceae, 

 outside their cell walls. Two cells approach each other, and surround them- 

 selves with a mucilaginous envelope. Their cell walls rupture at the constriction, 

 and parting in half allow the protoplasts to escape, which then unite to form a 

 zygospore. The zygospores of the Desmidiaceae frequently present a very character- 

 istic appearance, as their walls are often beset with spines (Fig. 237, G). The four 

 empty cell halves may be seen close to the spore. 



Class VI 

 Chlorophyceae (Green Algae) 



In the Chlorophyceae are included the majority of the Algae pro- 

 vided with green chromatophores. They group themselves naturally 

 into three orders, according to the structure of the thallus : the Proto- 

 coceoideae, which include all the unicellular forms, whether living as 

 isolated cells or as cell colonies ; the Confervoideae, comprising forms 

 consisting of simple or branched cell filaments or cell surfaces ; the 

 Siphoneae, with a thallus variously developed, but usually consisting of 

 a single, multinuclear, tabular cell. 



Sexual reproduction has not been demonstrated for all species of 

 the Chlorophyceae. In the simplest cases it is effected by the conjuga- 

 tion of naked gametes, of similar form and equal size. The gametes, 

 as distinct from those of the Conjugatae, are motile ciliated protoplasts, 

 and are known as Planogametes. In other genera there is a differ- 

 entiation of the sexual cells into a female non-motile egg-cell or 

 OOSPHERE and a motile ciliated male cell or spermatozoid. Examples 

 of this advance from ISOGAMY to OOGAMY are afforded by each of the 

 above three orders. 



In addition to asexual reproduction, the Chlorophyceae almost 

 always exhibit an asexual mode of reproduction by the formation of 

 motile ciliated swarm-spores (zoospores) which resemble the piano- 

 gametes. 



The cells in which the swarm-spores are formed are termed 

 SPORANGIA; similarly those producing gametes are designated game- 



