94 SEAWEEDS 



vegetative mode of propagation. The thallus origin- 

 ates in a creeping filament which sends up at frequent 

 intervals perpendicular branches. These grow at first 

 by division of most of their cells, but eventually a 

 growing-point or meristematic region is localised, 

 above which the filaments assume the character of 

 the assimilative filaments, either remaining as simple 

 rows of chromatophore-containing cells, or, by cell- 

 divisions parallel to the axis of each filament, be- 

 coming polysiphonous (or consisting of tiers of cells), 

 or even attaining a parenchymatous character. The 

 portions of the original filaments below the meri- 

 stematic region give rise to the basal tissue, the cell- 

 rows of which branch, and the whole ultimately in 

 some cases attains a compact character. This tissue, 

 which remains almost colourless, bears the assimi- 

 lative filaments, and among them the reproductive 

 organs and paraphyses. The assimilative filaments 

 are frequently shed, exposing the reproductive organs, 

 and in one case at least this shedding of the filaments 

 precedes the development of the plurilocular spor- 

 angia. Kjellman thinks it possible that there may be 

 perennial forms which periodically shed the assimi- 

 lative filaments and renew them from persistent 

 growing-points. In the case of the more simple free 

 filamentous forms there is naturally no such sharp 

 distinction of tissue. 



The Reproductive Organs. The unilocular sporangia 

 are ovate or pyriform, and mostly arise laterally from 

 the base of the assimilative filaments. They are 

 generally sessile and relatively large. The pluri- 

 locular sporangia of Elachista are the most simple 



