96 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



such structures as Chordaria, representing simple or branched 

 bodies hke whipcord, or Miaceous expansions, often of con- 

 siderable breadth, as the larger Ulva3. By a still further differ- 

 entiation, we have, at once, bodies consisting of condensed and 

 often elongated tissue, analogous to stems, and foliaceous ex- 

 pansions resembling leaves. We cannot, however, regard these 

 as of exactly the same nature as their analogues in Phseno- 

 gams. If we compare the different species of Nitophyllum, we 

 shall perceive that the midribs of the leaves are the same 

 organs with the stems, and that the fronds themselves, how- 

 ever they may simulate leaves, and answer the same end 

 of exposing a greater surface to the surrounding medium to 

 profit by its influences, have, by no means, the same organic 

 value. The main end is rather one of nutrition than of 

 aeration, though it is true that, in certain cases, even the 

 leaves of Phsenogams may, on occasion, answer both ends at 

 once. But there, at any rate, analogy ceases ; they are not, 

 hke the leaves of Phsenogams, essentially symmetrical organs, 

 on the due arrangement of which, after certain laws, the 

 formation of the fruit-bearing organ depends ; for there can 

 be no pretence that the fruit of either kind is a transformation 

 of any external organs, in the same sense in which it is under- 

 stood in Phsenogams. 



75. There is, however, in many species an additional resem- 

 blance to true leaves, in the fact that, in certain conditions, they 

 are deciduous, and re-appear the following year in new beauty, 

 though not from buds, in consequence of nutriment stored 

 up in the thicker and firmer part of the plant. Every one, for 

 instance, who has observed these productions in their native 

 spot, knows how vast a difference there is between the winter 

 and summer state of Delesseria sanguinea; and though, 

 perhaps, less generally observed, a series of the larger Lami- 

 naricB, in their progress from a little strap-shaped frond to the 

 enormous whips with a hundred thongs, heavy enough to load 

 a man, present a constant development of a new frond at the 

 base of the old one. 



76. It will be seen presently that these productions vary no 

 less in colour than in form and structure ; and, within certain 



