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 Lynglyece 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- 

 ariece and Lyngbyece 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 

 Nostocaeca?; 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. 



