204 INTEODUCnON TO CEYPTOGAMIC BOTANY. 



Haloplegma* and some others, are true or doubtful members 

 of Ceramiacew; while Dictyurus is an undoubted ally of 

 Polysiphonia. In this curious genus the net forms a spiral 

 web round the principal stem, exactly after the fashion of the 

 confluent leaflets of Riella, as we shall see amongst the 

 He'paticcB.^- It will be seen, from the figure, that the branchlets 

 from which the net-work grows are precisely like those of 

 Polysiphonia. These plants are simply beautiful objects, of 

 no practical utihty. 



182. Under the head of Chlorosperms, I took occasion to 

 speak of the variations to which many of the species are 

 subject. These, however, are few in comparison to what take 

 place among Rhodosperms. One species alone, Gelidium, cor- 

 neum, varies to such an extent that the forms may not only 

 be considered as distinct species, but even as belonging to 

 different genera. In point of thickness and degree of division 

 of the frond, the limits are almost boundless, but the fruit 

 always remains essentially the same, though the position some- 

 times differs, as in Odonthalia dentata, where it is axillary or 

 marginal, and this applies to either kind of fruit. On the 

 contrary, there may be great similarity of frond, with a total 

 diversity of structure ; and characters of habit are sometimes 

 extremely deceptive. The limits of species are no less vague 

 amongst the filiform than the more frondose Algas, and it is 

 not every difference of the arrangement of branchlets that 

 must be considered as of specific importance. 



183. There are many circumstances which have great 

 influence on the form of Rhodosperms as upon other Algse. 

 Comparative depth, difference of temperature, the quantity 

 of saline matter contained in the water, exposure to the action 

 of currents, and many other points, operate in effecting changes 

 of more or less gravity. Almost every species of Rhodosperm 

 varies more or less, as indeed is the case with all vegetable pro- 

 ductions, and characters which are most relied upon sometimes 

 fail unexpectedly. Nemaleon multifidiLm is often perfectly 



* Decaisne, in Ann. d. So. Nat. s6r. 2, vol. 2, p. 233. 

 t In Thalassiophyllum Glaihms, the leafy expansions are spirally 

 developed round the branching stipe. 



