68 ITS PEOPERTIES. 



of large stomates, will belong to a climate damp and hiamid; 

 and intermediate degrees of structure wUl indicate intermediate 

 atmospherical and terrestrial conditions. It is, however, to be 

 observed, that the relative size of stomates is often a more 

 important mark in investigations of this nature than their 

 number; those organs being in many plants extremely 

 numerous, but small and apparently capable of action in a very 

 limited degree ; while in others, where they are much less 

 numerous, they are large and obviously very active organs. 

 Thus the number of stomates in a scLuare inch of the epidermis 

 of Crinum amabile is estimated at 40,000, in that of Mesem- 

 bryanthemum 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 Mesembryanthemum and Aloe are among the gmaHest, 

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

 Crinum. Again the Yucca aloifolia has four times as many 

 stomates as a species of Cotyledon in my collection, but those 

 of the latter are about the y-j-g- of an inch in their longer 

 diameter, large and active, while the stomates of the Yucca are 

 not more than -a-sVo- of an inch long in the aperture, and 

 comparatively inert. The Yucca, therefore, with its numerous 

 stomates, has weaker powers of perspiration and respiration 

 than the Cotyledon. 



A leaf, then, is an appendage of the stem of a plant, con- 

 sisting of an expansion of the cellular rind, into which veins 

 are introduced, and enclosed in a skin through which respiration 

 and perspiration take place. It is in reality a natural con- 

 trivance for exposing a large surface to the influence of external 

 agents, by whose assistance the crude sap contained in the 

 stem is altered and rendered suitable to the particular wants 

 of the species, and for returning into the general circulation 

 the fluids in their matured condition. In a word, the leaf of a 

 plant is its lungs and stomach, traversed by a system of veins. 



It is well known to Gardeners that the eflloieney of leaves is much 

 promoted by their being kept perfectly clean. The great cause of the 

 Tinhealthiness of plants in towns is the amount of dirt which unavoidablv 



