12 VEGETABLE PHYSIOLOGY. 



Preservative Organs. The shells of seeds, the 

 protuberant envelope of bulbs, the pulpy mass of 

 tubers, the bark of trees and shrubs, and the gene- 

 ral cuticle of plants, are the preservative appendages. 

 The coverings of seeds are various in number, tex- 

 ture, and durability. Some of them yield quickly to 

 the decomposing action of the air; whilst others, 

 defended by the horny texture the oleaginous or 

 resinous quality of the shell are wonderfully per- 

 sistent, whether exposed on the surface of, or buried 

 deep in, the ground. 



Bark is the common covering of trees, &c. It is 

 composed of distinct layers of cellular matter annually 

 detached from the vital membrane, and consequently 

 increasing in thickness every year during the life of 

 the tree. In some cases, however, the outer layer is 

 thrown off when two or three years old, but in gene- 

 ral it is persistent. The exterior layers being first 

 deposited, they of course must gradually give way to 

 the subsequent internal accretion of the stem, and 

 consequently are parted into longitudinal rifts to give 

 room to the new layers of wood and liber. There 

 are some smooth barked trees whose annual layers of 

 liber are remarkably thin and distensible ; the first 

 layers of which, instead of being fractured perpen- 

 dicularly, are stretched horizontally, embracing the 

 axis like a series of hoops. 



The ligneous axis of a tree may be deemed a pre- 

 servative member, because, though when left by the 

 vitality, and no longer a channel for the sap, it sooner 

 or later decays, yet before this takes places, it answers 



