

VEGETABLE PHYSIOLOGY. 15 



the grasses, the canes with hollow stalks, and in the various 

 farrinaceous seeds, where it is most essential that their structure 

 should be protected, from the action of insects ; nature, to render 

 it more strong and resisting, has given it a glassy rind of net- 

 work, composed principally of a silicious earth, which Sir H. 

 Davy has ascertained in many instances, is capable of striking 

 fire upon the application of the steel. 



2. The cellular integument, which lies immediately under 

 the cuticle, and is for the most part of a green color, at least in 

 the leaves and branches. This is in general the seat of color, 

 and in that respect analogous to the rete mucosum, or pulpy 

 substance situated under the human cuticle, which is pale in the 

 European, black in the Negro, and red in the American ; but 

 we must carry the analogy no further, for these two parts 

 perform no functions in common. Du Hamel supposed this 

 pulp to form the cuticle ; but this is improbable, as his experi- 

 ments show, when that membrane is removed, that the cellular 

 integument exfoliates, at least in trees, or is thrown off in 

 consequence of the injury it has sustained, and a new cuticle, 

 covering a new layer of the same succulent matter, is formed 

 under the old one. Annual stems or branches have not the 

 same power, any more than leaves. 



Mirbel remarks that " leaves consist almost entirely of a plate 

 of this substance, covered on each side by the cuticle. The 

 stems and branches of both annual and perennial plants are 

 invested with it ; but in woody parts it is dried up and reproduced 

 continually, such parts only having that reproductive power. 

 The old layers remain, are pushed outward by the new ones, 

 and form at length the rugged, dry, dead covering of the old 

 trunks of trees." 



The petals of flowers are almost entirely composed of cellular 

 texture, the cells of which are filled with juices fitted to refract 

 and reflect the rays of light, so as to produce the brilliant and 

 delicate tints which constitute so great a portion of their beauty. 



3. The cortex or true bark of the plant, known to every one 

 by the name of bark. It consists of but one layer in plants and 

 branches only one year old, and often not distinguished from 



