234 VEGETABLE ORGANOGRAPHY. 



The number of fibres in a leaf of a.given size is that 

 which most influences its texture : when they are very 

 numerous, the cellular tissue occupies proportionally a 

 less space, and the leaf is composed of a firmer, a more 

 fibrous tissue. When the fibres are fewer or more 

 separated, the cellular tissue is more developed, and the 

 leaf is softer or more fleshy. If the leaves of the 

 Fir and the Orange, on the one hand, be compared with 

 those of the Tobacco and the Ficoids, on the other, we 

 shall have nearly the extremes of these differences ; 

 it is also met with in the leaves of the same species ; 

 thus, of two similar plants, that which grows in a more 

 fertile soil vdll have its leaves softer, — the natural number 

 of the fibres is not changed, but the development of the 

 cellular tissue is greater ; that which grows in a barren 

 soil will have, with the same number of fibres, a less 

 developed cellular tissue. The leaves of the same indi- 

 vidual may present analogous differences, according as 

 they are more or less favoured in their growth. 



These considerations, so elementary that it would 

 perhaps appear better to omit them, give us the reason 

 of some facts of delicate anatomy ; thus, for example : 

 — 1st, we have seen, in speaking of the Stomata, that 

 they appear to be the upper orifices of the sap-vessels ; 

 consequently, the larger the number of fibres, or in 

 other terms, the more fibrous a leaf is, the proportional 

 number of stomata will be greater in a given space ; 

 thus, there are as many as fifty or sixty on the leaves of 

 the Orange, in the same space as in i\\e Mesembryanthe- 

 mum there are but five or six. 2d, we have seen, in 

 speaking of Hairs, that, when they exist, they always 

 arise along the nerves or their ramifications ; conse- 

 quently when a leaf is young, its nerves existing akeady 

 formed, the hairs are more numerous in a given space, 

 and in proportion as its development goes on, the eel- 



