40 



PLANT MORPHOLOGY 



are thiek-walled and divide in any plane. They may remain in groups or 

 become separate. With the return of favorable conditions, they produce 

 a new filamentous body. 



Protococcus. One of the commonest and most widely distributed 

 of the green algae is Protococcus,^ a terrestrial form growing on the shaded 

 side of damp tree trunks, moist rocks, walls, etc. It is a unicellular alga, 



Fig. 28. Drapamaldia, portion of plant, a branching filament with a marked differentiation 

 in size of vegetative cells, X 200. Each cell has a central nucleus obscured by the peripheral 

 band-like chloroplast with many pyrenoids. 



consisting of a spherical protoplast enclosed by a cell wall (Fig. 29). 

 It has a small nucleus and a large, peripheral, irregularly lobed chloroplast 

 usually without pyrenoids. Reproduction occurs entirely by cell 

 division, spores and gametes being unknown. Permanent colonies are 

 not formed but, instead of separating immediately, the cells usually hang 

 together temporarily in small groups. In the presence of excessive mois- 

 ture, the number of cells in a group is greatly increased and sometimes 

 some of them grow into short filaments. 



In most unicellular algae the division of a cell involves the formation of 

 a new cell wall completely around each daughter protoplast and the 

 disintegration of the wall of the parent cell. In Protococcus, however, a 



' Often called Pleurococcus. 



