i6o A TEXTBOOK OF THEORETICAL BOTANY 



Chondrus crispus is, or used to be, employed in the preparation of jellies 

 as a substitute for gelatine, especially in invalid cookery. In recent years 

 it has also proved to be valuable as a substitute for agar. 



Ceramiales 



The Ceramiales are Rhodophyceae in which the auxiliary cell is only cut 

 off from the basal cell of the procarp branch after fertilization. In many 

 species two auxiliary cells and two procarp branches are cut off, one from 

 each side of the basal cell. The carpospores arise from very short goni- 

 molobes derived from the auxiliary cell and consequently the carposporophyte 

 is small. The carpospores are formed in a cluster and are developed on short 

 side branches of the main filaments. They are sometimes enclosed in a 

 cystocarp. 



There is a diplobiontic alternation of generations (see p. i8i) in which 

 the tetrasporic plants are morphologically similar to the gametophytic ones. 



The order includes a number of common genera of which Polysiphonia is 

 probably the best known in British seas, but other genera are also common, 

 and we may mention Ceramium and CalUthamiiion as other members of the 

 order. We shall consider as our types the abundant species Polysiphonia 

 nigrescens and Ceramium rubrum. 



Polysiphonia nigrescens 



The thallus of Polysiphonia (Fig. 148) looks superficially like a simple 

 branched filament (Fig. 149), but it is actually composed of a system of 



f t '^ ■ ] 



Fig. 148. — Polysiphonia fibrillosa. Habit of growth. 



parallel filaments, termed siphons. There is a central siphon composed of 

 relatively large, elongated cells surrounded by a variable number of peripheral 

 or pericentral siphons. The number of these varies between four and 

 twenty, but it is fairly constant in each species (Fig. 150). 



At the extreme apex the thallus consists only of the cells of the central 



