SIPHONALES 



95 



compound structure must be regarded as representing a group of 

 biflagellate zoospores which have failed to separate. The zoospore 

 is motile for about 15 min., after which it comes to rest and germi- 

 nates, the first thread often being more or less colourless. ' ' Zoospore ' ' 



Fig. 65. Vaucheria. A, V. sessilts, germinating zoospore. B, V. piloboloides, 

 developing aplanospore. C, V. piloboloides, escape of aplanospore. D, V. ge- 

 minata, thread with cysts. E, escape of amoeboid protoblast from cyst. 

 F-I, V. repens, development and escape of compound zoospore. J, regeneration 

 and formation of septa in injured thalli. K, sex organs of V. sessilis ( x 100). 

 L, sex organs of V. terrestris (x 100). M, sex organs of V. geminata (x 100). 

 N, V. geminata, germinating aplanospore. O, germinating zygote. P, zygote 

 with four haploid nuclei. Q, portion of compound zoospore, much magnified. 

 (A, D, E, N, O, after Oltmanns; B, C, F-J, Q, after Fritsch; K-M, after Hop- 

 paugh; P, after Hanatschek.) 



formation can often be induced by transferring the plants from light 

 to darkness, or from a nutrient solution to distilled water. 



Under dry conditions aplanospores may be formed at the 

 ends of short laterals or terminal branches, whilst if exposed to 

 greater desiccation the threads of the terrestrial forms become 

 septate and rows of cysts are formed, thus giving the '' Gongrosira'^ 

 stage. When conditions become more favourable these cysts 

 germinate either into new filaments or else into small amoeboid 



