270 BOTANY: PRINCIPLES AND PROBLEMS 



produce new plants whenever a suitable environment again 

 appears. 



In another group of Protococcales the body cells are non- 

 motile, independent movement being limited here to the zoos- 

 pores and the gametes. The individuals are grouped in colonies 

 which are simple, few-celled plates in Pediastriim (Fig. 146) but 

 form a complex network in the water-net, Hydrodidyon (Fig. 147). 

 The zoospores do not escape from the mother plant and swim 

 about, as is usually the case, but the group of zoospores formed 

 within a single mother-cell displays the remarkable habit of 

 uniting, while still within this cell, to form a minute colony of 

 non-motile cells which is finally liberated and grows into a 

 mature colony. 



The Protococcales are of especial interest because of the light 

 which they throw upon the development of the multicellular 

 individual and the differentiation of the sexes. 



2. Confervales or Conferva-like Algae. — These include most 

 of the common thread-like and membranous green algae of salt 

 and fresh water. They all reproduce asexually by zoospores and 

 also exhibit various types of sexual reproduction, simple in the 

 lower forms but relatively complex in the higher ones. The order 

 is large and varied and contains many species which are but 

 distantly related to one another. Three typical genera will give 

 us an idea of the group. 



Ulothrix (Fig. 148) is a common thread-like or filamentous alga, 

 its short cells each containing a single nucleus and one large 

 chloroplast. The contents of any cell may become divided 

 into a group of zoospores which escape and may each form a new 

 plant ; and smaller binucleate motile cells, produced in the same 

 manner as the zoospores and often indistinguishable from them, 

 act as gametes and conjugate in pairs to produce zygospores. 

 Ulothrix is often cited as another good example of the origin of 

 sexual reproduction. 



Oedogonium (Fig. 149) is also a common genus and its filament 

 is anchored by a modified basal cell, the holdfast. In certain cells 

 the contents may round up and produce a single large zoospore 

 with a circle of cilia near one end, and this soon settles down, 

 develops a holdfast, and grows into a new filament. Other cells 

 also become much enlarged, each producing a single rounded, 

 non-ciliated cell, well supplied with chloroplastids and food 

 material. This is the female gamete or egg, and the cell which 



