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 Sirosiphonicw 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 (eg. 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 
