326 



BOTANY 



chromatophores (Fig. 247, A). The cell walls of the vesicle and rhizoids of each 

 individual enclose but one protoplast. Multiplication may take place vegetatively, 

 by budding, resulting in the outgrowth of a new vesicle from the aerial portion of 

 the thallus. After enlarging considerably in size and sending down rhizoids into 

 the substratum, the young plantlet isolates itself from the mother vesicle by a 

 new cell wall. Asexual reproduction is provided for by the formation of swarm- 

 spores. In this process the whole plant becomes converted into a single sporangium 

 by the division of its protoplasmic contents into numerous swarm-spores, which make 

 their escape through an opening at the apex. Each swarm-spore has two to four 

 chromatophores, but only a single cilium, which is situated at its anterior, colourless 



Fig. 247. — Botrydium granvhitum. A, The 

 whole plant ; B, swarm-spore ; C, piano- 

 gametes ; a, a single gamete ; b-e, two 

 gametes in process of fusion ; f t zygote. 

 (A X 28 ; B, C X 540.) 



Fig. 248. — Vauclieria sessilis. A, B, A sporangium in 

 process of formation ; C, B, E, formation of a swarm- 

 spore (X 95) ; F, swarm-spore (X 25); G, portion of 

 the colourless peripheral protoplasm in the anterior 

 end of the swarm-spore ( X 950). 



end (Fig. 247, B). The formation of swarm-spores occurs only when the thallus is 

 covered with water. After coming to rest the heliotactic swarmers (p. 243) invest 

 themselves with a cell wall and give rise to new plantlets. Sexual reproduction 

 may also occur. For this purpose, in summer or in times of drought, the proto- 

 plasm of the vesicles becomes broken up into a number of rounded or angular non- 

 motile spores or aplanogametes. These spores may remain at rest, perhaps for a 

 period of a year or more, until supplied with water, when numerous small sexual 

 planogametes (C, a) are formed from their contents. These planogametes are each 

 provided with two cilia and a red eye-spot, and, by conjugating in pairs, give rise 

 to zygotes (b-f). The zygotes round themselves off and germinate, either directly 

 or after a period of rest. The planogametes are also heliotactic. Through the for- 

 mation of the gametes within the resistant resting-spores the latter acquire the 

 character of gametangia. 



