256 SEAWEEDS 
several trichomes, or bundles of trichomes, diverge at 
the extremity of their common sheath, in which 
they remain partially embedded, while the free parts 
become clothed each with a separate sheath. Where 
two trichomes thus emerge, it sometimes occurs that 
a fairly regular dichotomous false branching is pro- 
duced. In the Lyngbyec false branching (in spite of 
the absence of heterocysts), arises from a breaking 
of the trichome, sometimes by its mere length, some- 
times by a curvature of the filament, and the new 
ends breaking through the sheath grow out, or 
sometimes only one does this. The tribes Vagin- 
ariee and Lyngbyew have thus the first a terminal, 
the second a lateral branching. 
The trichomes grow at all points, but generally 
towards the apex there are indications of apical 
growth in a greater number of short cells. At the 
apex itself the terminal cell, more or less conical 
or like a cupola in shape, has a thick protective 
outer membrane, and its presence or absence and 
its precise form afford a systematic character in 
classification. 
The structure of the sheath has been very care- 
fully studied in this group by M. Gomont, and his 
observations are probably generally applicable to all 
Nostocacee. The sheath and cell-wall proper exhibit 
different chemical reactions, and while the latter 
appears to resemble cutine, the sheath consists of 
a substance nearly allied to cellulose. At the 
same time the sheath appears to become cutinised, 
when it becomes coloured under the influence of 
light. 
