206 INTRODUCTION TO CRTPTOGAMIC BOTANY. 



these plants never growing together, though both affect similar 

 situations on different shores (Harv. Phyc. Brit. tab. 125). It 

 is not, however, supposed that they are mere forms of one 

 species. 



186. Though Algse in general derive their nutriment from 

 the surrounding medium, cases occur in which the substance 

 on which they grow seems to have some influence. Delesseria 

 alata, and its form angustissima, grow together on the same 

 stem of the digitate Laminavia, but this same matrix, in 

 other cases, apparently exercises considerable influence on the 

 plants which grow upon it. Polysiphonia urceolata, for in- 

 stance, when growing upon rocks exposed at low water, is 

 more robust and much more branched than when it grows 

 on Laminaria digitata, in which case the branches are 

 shorter, their ramuli squarrose, and often turned back. Bho- 

 dymenia palmata, on rocks, is broad and but slightly divided, 

 on Fucus serratus it assumes the laciniate form of R. sobo- 

 lifera. 



187. Where there is any doubt as to specific identity, the 

 first point is to ascertain the fruit which in general does not 

 vary in the same species. In Corallina squamxxta, however, 

 as already mentioned, so great a diversity exists as to render 

 received opinions, as to the value of the fruit in that group, very 

 doubtful. All characters, on the contrary, which depend on 

 mere outline, on the degree in which the fronds are cleft, 

 comparative thickness, &c., are to be received with great cau- 

 tion, even in specimens from distant localities ; the disposition 

 of the fruit, on the contrary, its intimate structure, and that of 

 the frond, are in general points of great importance. 



188. In many cases, finally, differences which at first appear 

 specific, depend merely on age. The old emarginate leaflets of 

 Leveillea Sckimperi (Fig. 50), for instance, are very different 

 from the acuminate form of these organs when young, with their 

 flagelliform terminating threads. The older frond, also, may 

 be much less simple in structure than the younger, from the 

 addition of superficial cells. But these are points which will 

 be readily appreciated by every attentive observer. The 

 reproach which was cast upon Algologists a very few years 



