110 BRITISH SEA-WEEDS. 



Subdivision 3. — Frond forked ; tubes numerous, flattened, 

 arranged round a large, jointed, central cavity containing 

 hags of coloured endochrome. 



Polysiphonia fastigiata. Flat-topped Poly- 

 siphonia. 



Fronds rigid, from one to two inches long, growing in 

 globular tufts ; stems as thick as horse-hair, much divided, 

 forked ; branches and branchlets forked, their axils acute, 

 their tips awl-shaped ; tubes about eighteen ; articulations 

 shorter than they are broad. Spores in egg-shaped, stalk- 

 less conceptacles ; tetraspores immersed in the terminal 

 branchlets. 



This species grows parasitically on two or three kinds 

 of Fuci, but most commonly on F. nodosus, which is 

 very rarely free from it ; wherever the larger plant is 

 found, and that is almost everywhere, it is sure to be 

 fringed with dark-brown, hair-like tufts of the smaller. 

 Indeed the two are so nearly inseparable that they mu- 

 tually impart a character, the one to the other. P. fas- 

 tigiata is a perennial, and attains its greatest luxuriance 

 of growth in summer and autumn. It possesses a very pe- 

 culiar microscopic character, which may perhaps be over- 

 looked if I do not specially describe it. I allude to the 

 bags of endochrome which exist in the centre of the cavity 

 round which the tubes are arranged. No other British 

 species of Polysiphonia is furnished with these organs, 

 on which Dr. Gray, in his ' Handbook of British Water- 

 weeds/ has founded a new genus under the name of 

 Vertebralia, in allusion to their being, as it were, the 

 backbone of the plant. If a small section of the stem 

 be crushed between two glass slides, and then examined 

 under a microscope, these bags will be seen among the 



