THE ALG^E 203 



the whole plant consisting of a repeatedly branched 

 filament, the main axis of which is comparatively thick, 

 the successive branches becoming more and more slender, 

 while the ultimate ramifications terminate at the tips in 

 delicate hairs (see Figs. 88 and 89). The filament is 

 at first only one cell in thickness throughout. In the 

 lower part of the thallus, however, a peculiar kind of 

 secondary cortex is formed, as the plant grows older ; 

 the basal cell of a lateral branch gives rise to delicate 

 filaments, which grow in a downward direction, attach 

 themselves closely to the membrane of the main axis, 

 and eventually form a complete coating over it. This 

 mode of forming a cortex by means of adherent branches 

 is by no means uncommon among filamentous Algse, both 

 of the red and brown divisions. 



Each cell of the thallus contains, in addition to the 

 colourless protoplasm, a number of plastids (the bearers of 

 the combined red and green pigments) and, at least when 

 young, a single nucleus. The cells communicate with 

 each other by pits in their transverse walls ; the pit- 

 membrane is covered on either side with a pad of callus 

 like that in sieve- tubes (see Part I. p. 60). Fine strands 

 of protoplasm extend through the callus and pit-membrane, 

 thus connecting the contents of the adjacent cells. We 

 see, then, that the protoplasm is continuous in these 

 Algae, as well as in higher plants. 



The growth of the thallus goes on entirely at the apex 

 of the various branches ; each branch terminates in an 

 apical cell, which divides by transverse walls to form the 

 successive segments composing the filament. When 

 a fresh branch is to be formed, an oblique wall is 

 produced in a segment which has just been cut off from 

 the apical cell. By the oblique wall two unequal cells 



