449 



The chromatophore is a parietal plate with a large lateral 

 pyrenoid, protruding far into the lumen of the cell. 



The sporangia occur at the base of the branches; in the spe- 

 cimens found one, two, or, more rarely, three upon each branch, 

 The sporangia are pedicellate or more rarely the uppermost 

 sessile. The sporangia are oval in shape; they have a thick wall 

 especially at their upper end. They are about 21 // long and 13 n 

 broad. 



From this plant Acr. flexuosum Vickers chiefly differs in 

 its thinner filaments and by the presence of ramuli, upon which 

 the sporangia are placed. 



Our plant ought also to be compared with Acr. Daviesii 

 (Dillw.) Nagl. showing in its short, thick-walled cells and whole 

 ramification great likeness to this species. But it differs in an 

 essential way especially by the lack of the repeatedly ramified 

 branchlets. 



The plant was gathered in shallow water at the end of Fe- 

 bruary. 



St. Croix: Salt River. 



17. Acrochsetium unipes B0rgs. 



18. opetigenum B0rgs. 



19. Acrochsetium robustum Bergs. 



When I described this plant I had not come across young 

 specimens. In fig. 418 the basal parts of two young plants are figured ; 

 in these the basal discs are not yet 

 developed. From these figures it 

 seems quite clear, that the germinat- 

 ing spore during its growth produces 

 downwards the process which pene- 

 trates into the tissue of the host. 

 The process has an acute base and 

 thick walls. It is not separated from 

 the original germinating spore by 

 any wall. The process is the only 

 endophytic part of the plant; the ori- 

 ginal spore and the small disc gradual- 

 ly developed round it are epiphytic. 



Fig. 418. Acrochsetium robustum 



Borgs. Bases of two young plants. 



(About 250:1). 



29 



