INTRODUCTION. xiH 



or with the aspect and elasticity of gristle ; others tough 

 and coriaceous, or resembling leather ; while the stems of 

 some of the larger kinds are almost woody. The leaves 

 of some are delicately membranaceous, glossy and trans- 

 parent ; of others, coarse and thick, and either wholly des- 

 titute of nerves, or furnished with more or less defined 

 ribs, or beautifully veined. Several have the power of 

 withdrawing carbonate of lime from the water in which 

 they grow, and laying it up, in an organized state, in their 

 tissues. Among fresh-water species, particularly of the 

 Rivularia, we find the first imperfect exhibitions of this 

 remarkable power, but in some of these the lime occurs in 

 such lumpy masses, that it may perhaps rather be regarded 

 as an incrustation, through which the plant continually 

 grows. In the marine Corallines, and in several of the 

 orders Siphonaceae and Batrachospermacea, the secreting 

 process is too perfect for the lime to be considered as an 

 incrustation. It is obviously necessary to the perfect de- 

 velopment of the vegetable. Some of the least perfect of 

 the Corallines, the Melobesia or Nullipores, resemble 

 masses of calcareous matter, not at all unlike the incrusta- 

 tions formed in water strongly impregnated with carbonate 

 of lime ; but when we place these apparent rocks into acid 

 for a short time, until the lime is partly dissolved, there re- 

 mains a delicately cellular structure, of the full form and 

 size of the original mass, and built in a perfectly regular 

 manner. In the cells of this body, and the interstices be- 

 tween them, the particles of lime had been arranged. 

 Among the most minute kinds, many (comprising the 

 family Diatomacea} are cased with organized silex, and 

 these cases, which resist the action of fire, are found in 

 countless myriads in a fossil state, in many countries, cover- 

 ing miles of ground, or forming mountains, and presenting 



