Vegetable Physiology. 147 



the previous one, it is obvious that they are both produced at 

 the same spot, and that the newest layers of both will always 

 be in contact with each other. They seem to be produced in 

 this way. In tne spring, the bark becomes loosened, and may 

 often be readily separated from the wood. A kind of mucila- 

 ginous fluid, called the Cambium, is then formed between them. 

 This is gradually developed into cells, and from these are 

 formed the ducts and cellular portion of the woody layer, and 

 the layer of bark is at the same time added, both being inter- 

 sected by the medullary rays. Bark sometimes is nearly all 

 composed of cellular tissue, and becomes thick and spongy, as 

 is the case with cork, which may be regarded as a kind of exter- 

 nal pith. The inner bark, or liber, however, is usually thin and 

 delicate in texture, and has been applied to various useful pur- 

 poses. One of these is indicated by the meaning of the word 

 liber, which signifies both a book and the inner bark. In fact it 

 was used by the Romans for writing upon. By many of the Pa- 

 cific Islanders, the liber of several trees is used for cloth, mats 

 and sails. It is in the vessels and woody tubes of the albur- 

 num that the fluid absorbed by the roots is carried through the 

 stem, and these vessels communicate with those of the leaves, 

 which receive it from them. In the liber, on the contrary, the 

 fluid, after being elaborated by the leaves, and converted into 

 nutritious sap, descends again through the trunk, for the purpose 

 of nourishing its various parts. A part of this sap is carried 

 inwards by the medullary rays, which thus diffuse it through 

 the whole stem, as also through the substai.ce of the roots, 

 down which it is conveyed by their bark. In this descent it 

 mixes with the ascending current, particularly at its lower part, 

 and being much superior in density, adds to that of the ascend- 

 ing fluid, and thus maintains the conditions necessary for end os- 

 mose. The vessels of the bark down which the sap runs, 

 form a complete network, in which it may be seen to move in 

 various directions. 



The endogenous stem is formed very differently. The woody 

 bundles are distributed irregularly through the mass, and in- 

 stead of being united into rings, remain separate, and only 

 form any thing like hard wood, on the exterior, where they are 

 pressed together by the addition of new matter within. Each 



