40 VITAL ACTIONS. 



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 7 J, 

 of an inch in their longer diameter, large and active, 

 while the stomates of the Yucca are not more than 

 .2 jo? of an inch long in the aperture, and compara- 

 tively inert. The Yucca, therefore, with its. nume- 

 rous stomates, has weaker powers of perspiration and 

 respiration than the Cotyledon. 



62. A leaf, then, is an appendage of the stem of a 

 plant, consisting 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 contrivance for ex- 

 posing 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. 



63. As the leaf is an extension of the rind of a 

 stem, its epidermis is also an extension of the skin of 

 the same part ; and hence it is that in plants which 

 produce no true leaves, such as the Stapelia, the 

 office of the leaf is performed by the rind and epider- 

 mis of the bark. 



64. The functions of respiration, perspiration, and 

 digestion, which are the particular offices of leaves, 

 are essential to the health of a plant ; its healthiness 

 being in proportion to the degree in which these func- 



