202 BRITISH SEA-WEEDS. 



according to the size of the specimen, sometimes of greater 

 diameter at the base than a hog's bristle, sometimes much 

 less, always becoming gradually thinner towards the upper 

 part, irregularly, forkedly branched ; the ultimate branch- 

 lets incurved or hooked at the tips ; the nodes of the stem 

 and branches contracted ; the internodes in the lower part 

 of the frond about twice as long as broad ; both nodes and 

 internodes coated all over with coloured, cortical cellules. 

 Spore-clusters on the sides of the branches, embraced by 

 three or four short branchlets ; tetraspores arranged round 

 and sunk in the nodes. 



This species is the most common of all the Red weeds 

 found on our shores. It is annual^ and more luxuriant 

 in summer and autumn than at other seasons. It grows 

 on rocks and stones, or parasitically on other weeds, and 

 extends from near the extreme limit of high tide to some 

 distance beyond low-water mark. Subject to so many 

 vicissitudes, it is not remarkable that it should be liable 

 to great variety of form ; some of these are so distinct 

 that they have been described and figured as separate 

 species ; but later research, and the comparison of ex- 

 tended series of specimens, have demonstrated that they 

 are not entitled to specific rank. The following varieties 

 are described by Professor Agardh, and I therefore insert 

 them : — 



Ceramiiun rabrum, var. decurrens. 



Frond regularly forked, or furnished with forked branch- 

 lets ; internodes with a narrow transparent stripe in the 

 centre, caused by the absence of surface cellules. 



This is one of the forms hitherto considered distinct. 

 It is figured under the name of C. decurrens in Harvey's 



