198 SPUING. 



after the full height has been attained, and the stem 

 is left hollow: the second part is the epidermis, by 

 which the plant is covered to the minutest part, and 

 which in the living plant is always of such a texture 

 as to resist the decomposing action of water, in which 

 some of the internal parts of many plants are in certain 

 stages of their growth soluble. Were it not for this 

 property of the epidermis, great part of many vegetables 

 would be melted by every shower of rain. A consider- 

 able quantity of silica or flint-earth is contained in the 

 epidermis, in some of the reeds, the grasses, and 

 marsh plants, such as the shave-grass (equisetum hye- 

 male), which is there the hardest of vegetable sub- 

 stances ; and being finely toothed in the equisetum, it 

 answers well for polishing either wood or metal. In 

 many trees the epidermis is destroyed after the parts 

 within become indurated; and these are then cracked 

 and champed, and easily scale off. 



The particular fuctions which the pith and the 

 epidermis perform in the living plant, are not very well 

 known, and in all probability they are only temporary, 

 the pith to give stiffness while the rest of the sub- 

 stance is a pulp, and the epidermis to form a core and 

 preserve the shape. In long lived plants they cease to 

 be useful, and often to exist. No pith can be found 

 in the root out of an old box tree, for instance, and 

 the epidermis of some trees, as the birch and some 

 sycamores, peels off every season. Even in young 

 subjects the pith can be removed without much effect 

 on the vegetable action. 



The third part of the vegetable stem varies with the 

 age. If it be the production of the year, and early in 



