52 



PLANTS AND MAN 



straining wall and the protoplasm, by cell division, develops into 

 the new plant. 



In other thallus plants (pond sciims and seaweeds) spores are 

 frequently produced which are capable of swimming about 

 independently; such motile spores are known as zoospores. In the 

 pond scum Ulothrix, which is somewhat similar to Spirogyra, 



each cell may participate in the 

 reproductive process. The cell 

 contents undergo division to form 

 several pear-shaped zoospores (fig. 

 26), each with four hair-like 

 flagella which by their vibration 

 bring about locomotion of the cell 

 through the water. Since the zoo- 

 spore can thus swim away from 

 the parent filament, dispersal is 

 brought about coincident with 

 reproduction. 



Further specialization in 

 asexual reproduction occurs in 

 algae such as the kelps {Laminaria) 

 where the zoospores are not pro- 

 duced by every cell of the thallus 

 but in certain cells which have 

 assumed this particular function; these special cells form spore 

 sacs, and are known as sporangia. In all plants other than the 

 thallus species, spores are always produced in multicellular 

 sporangia. 



Reproductive variation among the algae, in itself of little 

 significance to man, is of theoretical importance because it shows 

 how sexual reproduction may have evolved from primitive 

 asexual types. Returning to our Ulothrix plant, we discover that 

 zoospore formation often alternates with another type of repro- 

 ductive activity. The contents of a cell undergoes division to 

 form a number of motile cells which are smaller than the zoo- 

 spores. Upon escaping from the cell they swim about just as 

 zoospores do; but before they exhaust themselves they unite with 

 one another in pairs to form fused cells (fig. 27). Each fused cell 



Fig. 27. — Ulothrix cells can also 

 form motile bodies known as 

 gametes which must fuse in pairs 

 before germination. 



