THE ALGAE 



169 



Ceramium rubrum 



This plant is among the commonest of all the British marine Algae, 

 occurring all round our coasts, chiefly in rock pools of the midshore zone, 

 attached to rocks, stones or to other Algae. The plants can be recognized 





^i Ai^S^^ 



¥.^ 







Fig. 160. — Ceramhim diaphaymm. Portion of a thallus, 

 natural size, showing beaded appearance due to 

 incomplete cortication of the central siphon. 



under a hand lens by the incurved (connivent) tips of the apices of the 

 filaments (Fig. 160). The plants usually grow in tufts, varying from 2 to 

 as much as 30 cm. in length, and rather irregularly dichotomously branched. 

 The fronds are clear red in colour when living in deep sea, or reddish brown 

 or yellowish green when living in shallow tidal water. 



The plants are apparently dioecious, the antheridia being formed in 

 sessile patches on the upper branches of the male plants, while the cysto- 

 carps are formed at the nodes of the females. The asexual plants resemble 

 the sexual ones, the tetrasporangia being developed in cells of the cortex. 



Structure of the Th.\llus 



The apex of each filament is formed of a single apical cell which cuts 

 off segments basally. Branching begins by the formation of two obliquely 

 intersecting walls in the apical cell, each segment thus cut off becoming the 

 apex of a new branch. This oblique division of the apical cell usually occurs 

 after about a dozen axis segments have been formed, and the further elonga- 

 tion of the axis is due to the expansion of these cells. Before elongation the 

 branches have a characteristic incurved form, and each elongating branch is 



