GENERAL STRUCTURE OF LEAVES. LEAFLESS PLANTS. 147 



217. The cuticle of the leaves is furnished more abundantly 

 than those of any other parts, with stomata (. 91), by which 

 watery vapour and gases can pass out, and air can enter ; but 

 these are chiefly, and often entirely, confined to the lower surface. 

 The woody skeleton of the leaf forms what is commonly known 

 as the midrib, and the veins proceeding from it. These veins or 

 nerves (as they are commonly termed) must not be confounded 

 with similar parts in animals, since they do not in the least 

 resemble them. They are principally composed of woody fibre, 

 and of spiral vessels and ducts; and they proceed from the 

 neighbouring stem or branch, constituting the greater part of 

 the footstalk or petiole of the leaf, from which they afterwards 

 spread out. 



218. This general account of the structure of the leaves of 

 Flowering-plants, will suffice to enable us to compare them 

 with the corresponding parts in Cryptogamia. It also enables 

 us to see how beautifully they are adapted, by the immense 

 amount of surface they present, by its thinness and delicacy, - 

 by its numerous apertures, and by its expansion to the light 

 of day, for the purpose they have to perform ; the exposure 

 of the crude sap to the air and sun, under the influence of 

 which, it is elaborated or digested, so as to become a highly- 

 nutritious fluid. 



219. There are few Flowering-plants, in which the stem and 

 branches are not, at some part of the year, clothed with these 

 beautiful appendages ; and the exceptions are chiefly in those 

 forms unknown as natives of temperate climates, but common 

 in tropical regions, in which the stem itself is so altered in 

 structure, as to be able to perform the functions of leaves. Most 

 of these are included under the designation of Cactus or Prickly- 

 Pear tribe. Their stems, instead of being firm and woody, are 

 comparatively soft and fleshy ; their substance is moist, and com- 

 posed almost entirely of cellular tissue ; their surfaces are green, 

 and covered with a distinct cuticle, which is furnished with 

 stomata ; and their form is often flattened, so as to expose, like 

 leaves, a large surface to the air. It is interesting to observe, 

 hew completely the deficiency of one organ is here supplied by 



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