THE AIR CAVITIES. 11 



n 



very numerous, and the form of which is nearly constant 

 in each species ; because, although it he an accident, it is 

 one caused by the structure and growth of the species. 

 In certain species these cavities are, as we have seen, 

 large enough to be visible to the naked eye ; at other 

 times they are so small, that they can only be perceived 

 with a lens or even a microscope ; in this last case, the 

 spaces which result from these fissures in the cellular 

 tissue, or from these dilations of the intercellular 

 passages, bear a great resemblance to vessels, from which 

 they only differ in their being less regular. Some 

 naturahsts even think that all the vessels are true acci- 

 dental cavities produced by the growth of the plant ; and 

 they support this opinion either upon the analogy of the 

 large air cavities, or upon the fact that vessels are not 

 visible in very young embryos. But in this bold 

 hypothesis, there would be much difficulty in explaining 

 the extreme regularity in the form of vessels — the 

 peculiar situation of the trachefp, which, as I have 

 shown, differ much from other vessels — the constant 

 direction which the sap takes at the first moment of 

 growth, &c. Lastly, whatever opinion one adopts in 

 this respect, it must always be admitted that the vessels 

 are formed long before, and with much greater regularity 

 than the air cavities. 



These cavities contain air, but we cannot assert that 

 it performs anj direct fmiction in the act of vegetation ; 

 we must not liken them completely to certain air cavities 

 which are formed in some organs by a true dilatation of 

 the tissue, as we see in the floating bladders of some 

 species of Fucus, of Trapa nataiis, Utricularia, 8fc. 

 We shall again have occasion to revert to these organs 

 hereafter. 



VOL. I. 



