SEA WEEDS 363 



C. gracillimum, of the lower rock pools, with very slender gelatinous 



fronds, swollen nodes and small fan-shaped branchlets ; C. tenuissi- 



mum, closely resembling C. strictum in 



general appearance, but distinguished by 



having its tetraspores only on the outer 



side of the nodes ; and the Transparent 



Ceramium (C. diaphanum), which may 



be found throughout the year on rocks 



and weeds in the rock pools. The last 



species is the largest and most beautiful 



of the genus, and may be readily recog- _,, 



* _ J FIG. 250. Ceramium 



msed by its light-coloured, transparent diaphanum 



stem with swollen purple nodes, and its 



conspicuous spore-clusters near the tips of the filaments. 



Our last example of the genus is the Common Red Ceramium 

 (C. rubrum), which may be found in the rock pools at all levels. 

 It is very variable in form, but may be known by its contracted 

 nodes, in which the red tetraspores are lodged, and its spore- clusters 

 surrotinded by three or four short branchlets. It differs from 

 most of the other species in having both nodes and internodes 

 covered with cortex-cells, and hence the latter are not transparent. 



The order Spyridiacece has a single British representative 

 which may be found in various localities on the south coast. It is 

 Spyridia filamentosa, a dull-red weed with thread-like, tubular, 

 jointed fronds, from four inches to a foot in length. The main stem 

 is forked, and densely clothed with short and slender branchlets. 

 The frond is covered with a cortex of small cells. The spore- 

 clusters are grouped together, several being enclosed in a mem- 

 branous cell in conceptacles, or external sacs, at the ends of the 

 branchlets; and the tetraspores are arranged singly along the 

 jointed branchlets. 



The next family (Cryptonemiacece) is an extensive one, con- 

 taining nearly twenty British genera of red or purple weeds, 

 with unjointed, cartilaginous, gelatinous, and sometimes mem- 

 branous fronds. The spores are irregularly distributed, and are 

 contained either in sunken cells or in conceptacles. The tetra- 

 spores are either in cells at the edges of the frond or collected 

 together in compact groups. 



Of the genus Dumontia we have only one species (D. filiformis), 

 the frond of which is a simple or branched tube, from an inch to 

 more than a foot in length, containing a loose network of filaments 



