ACTION OF LEAVES. 39 



rally grows. Now, as this structure is capable of 

 beiiig ascertained by actual inspection with a micro- 

 scope, it follows, as a necessary consequence, that the 

 natural habits of an unknown plant may be judged 

 of with considerable certainty by a microscopical ex- 

 amination of the structure of its epidermis. The rule 

 will evidently be, that plants with a thick epidermis, 

 and only a few small stomates, will be the inhabitants 

 of situations where the air is dry and the supply of 

 liquid food extremely small ; while those with a thin 

 epidermis, and a great number of large stomates, will 

 belong to a climate damp and humid ; and interme- 

 diate degrees of structure will indicate intermediate 

 degrees of atmospherical and terrestrial conditions. 

 It is, however, to be observed, that the relative size 

 of stomates is often a more important mark in inves- 

 tigations of this nature than their number ; those or- 

 gans being in many plants extremely numerous, but 

 small and apparently capable of action in a very lim- 

 ited degree ; while in others, where they are much 

 less numerous, they are large and obviously very ac- 

 tive organs. Thus the number of stomates in a 

 square inch of the epidermis of Crinum amabile is 

 estimated at 40,000, and in that of Mesembryanthe- 

 mum at 70,000, and of an Aloe at 45,000 ; the first 

 inhabiting the damp ditches of India, the last two 

 natives of the dry rocks of the Cape of Good Hope : 

 but the stomates of Crinum amabile are among the 

 largest that are known, and those of Mesembryanthe- 

 mum and Aloe are among the smallest ; so that the 

 70,000 of the former are not equal to 10,000 of the 



