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 Hlachista are the most simple 
