CHAETOPHORALES 



67 



the germlings in which the erect filament arises first and the 

 prostrate portion subsequently or vice versa. By increasing the 

 osmotic pressure or by adding toxic salts to the environment the 

 thallus passes into a palmelloid state, whilst under other conditions 

 akinetes can be formed. The plants are confined to well-aerated 

 fresh water though they have also been found growing on fish living 

 in stagnant water, but in these cases the movements of the fish 

 presumably provide adequate aeration. 



Trentepohliaceae : Gongrosira {gongro, excrescence; sir a, chain). 



Fig. 47- 



A genus which lives on stones and the shells of gastropods that 



are to be found in fresh and salt water, although there is one 



species that is terrestrial. The cushions or plates are frequently 



B A 



Fig. 47. Gongrosira. A, portion of G. circinnata showing formation of zoospores. 

 B, dehisced sporangium of G. stagnalis. (After Fritsch.) 



lime encrusted and form a tough green stratum with a base that is 

 composed of one or more layers of cells which give rise to dense, 

 erect, branched filaments. The sporangia are usually borne 

 terminally on these erect threads, and in some species they can 

 even be distinguished morphologically by their greater size, 

 although generally they do not differ from the vegetative cells. The 

 sporangia produce biflagellate zoospores, and any of these which are 

 not able to escape become converted into aplanospores. Biflagellate 

 isogametes develop from the lower cells of the thallus, whilst the 

 prostrate portion can also give rise to akinetes. 



5-2 



