254 SEAWEEDS 



the ordinary vegetative cells, from which they may 

 be farther distinguished by their greater refraction. 

 They adhere to the sheath, and at the point of their 

 attachment to the neighbouring cells there is a little 

 button-like projection of cell- wall. Besides the true 

 branching in the tribe of Sirosiphoniece there are 

 false branchings in other forms, occasioned by modes 

 of growth, by the sticking of the heterocysts to the 

 sheath, and by the development of hormogonia 

 which have not escaped from the sheath, or have 

 become fixed on the filaments. In the last case, of 

 course, there is no sort of regularity. 



Reproduction is most frequently is ordinarily 

 effected -by the production of hormogonia. These 

 are mere segments of the trichome, to be distinguished 

 from it with difficulty in some genera (e.g. Nostoc. 

 Anabaina), but in others more specialised. In some 

 cases the production of hormogonia terminates the 

 existence of the thallus, and in fact involves its de- 

 struction ; in others it begins at an early stage of 

 development, while the filaments are still small, and 

 proceeds more or less actively during the life of the 

 plant. The hormogonia escape by sliding towards the 

 opening at the end of the sheath. Some fix them- 

 selves on the parent plant, but most descend to the 

 substratum. On coming to rest they either develop 

 at once into a new filament, or rest for a longer or 

 shorter time while their cells increase in size and 

 their sheath grows in thickness, sometimes becoming 

 larger than the ordinary filaments. In other cases 

 the hormogonia divide and subdivide, while they 

 elongate and multiply for a time before assuming 



