186 



STEUCTURAL BOTANY 



sp 



on the same individual. Both kinds of sporangia arise 

 as lateral branches, either sessile or stalked. 



The unilocular sporangia are simply globular or pear- 

 shaped cells, borne at the sides 

 of the branches (Fig. 79). 

 They become densely filled 

 with protoplasm, and their 

 contents divide up into a great 

 number of small zoospores. 

 Each zoospore contains a 

 nucleus and a brown plastid, 

 and bears two cilia, which are 

 inserted at the side, not at one 

 end of the cell. The sporan- 

 gium dehisces at its apex, and 

 the zoospores escape and swim 

 off into the water. During 

 their movements one cilium 

 points forward in the direction 

 of movement, and the other 

 trails behind. In all Phse- 

 ophycese the motile cells have 

 two cilia each, and they are 

 always inserted laterally, a 

 point which distinguishes 

 them from the corresponding 

 bodies in the green Algae. 

 These zoospores come to 



FIG. 78. General view of part of , ' . , 



thethallusof^ocr^s,show. rest after a time, and grow 



ing several branches, sp, sp, up i n fc o new plants. Hence 

 lurilocular sporangia, borne f ., . c 



the unilocular sporangia are 

 organs of asexual reproduc- 

 tion. 



pl 



laterally on the branches ; 

 g, g, intercalary growing-points, 

 where cell-division is going on. 



Magnified 56. (R. S.) 



