34 



PLANT MORPHOLOGY 



In young colonies the cells are uninucleate but later become multi- 

 nucleate (coenocytic), as many as eight nuclei being present. Young 

 cells have a single peripheral chloroplast with one pyrenoid, while older 

 cells have several pyrenoids, the chloroplast becoming diffuse. 



In asexual reproduction a protoplast divides generally into as many 

 daughter protoplasts as there are cells in the colony, but often into twice 

 as many. These become biciliate zoospores that escape as a group 

 enclosed in a common membrane (Fig. 24A). The zoospores then come 

 together and form a new colony within the membrane (Fig. 24i?, C). 



Fig. 22. Four-celled colony 

 of Scenedesmus, X 750. Each 

 cell contains a small nucleus 

 and a large peripheral chloro- 

 plast with a pyrenoid. 



Fig. 23. Young colony of Pediastrum 

 horyanum, its cells forming a plate, X 750. 

 Some of the cells have become binucleate. 

 Each has a peripheral chloroplast and a 

 pyrenoid. 



Sexual reproduction also takes place, Pediastrum being isogamous. 

 Division of a vegetative protoplast results in the formation of many 

 biciliate gametes. These escape separately and fuse in pairs to form 

 zygotes. After increasing in size, the zygote gives rise to a group of 

 zoospores. These escape into the water, swim freely and, after coming 

 to rest, develop into thick- walled polyhedral cells (Fig. 24Z)). The 

 polyhedrons enlarge and divide internally to form a group of zoospores 

 that escape in a common membrane, within which they construct a new 

 colony by coming together without further division (Fig. 2^E-G). 

 Hydrodictyon. This remarkable alga, common in fresh water, is 

 a free-floating colony having the form of a large hollow net, the polyg- 

 onal meshes of which are made up of elongated cylindrical cells arranged 

 end to end (Fig. 2b A). Each mesh consists usually, but by no means 

 always, of six cells. A single colony may reach a length of 20 to 30 cm. 

 At first each cell contains a single nucleus and a chloroplast with one 

 pyrenoid, but later there are many nuclei and a large number of pyre- 



