14 DESCRIPTIVE BOTANY. PAHT I. 



always the same. In the several products of vegetation 

 woods, gums, resins, &c. the proportions between 

 those three elements vary considerably; and even a 

 fourth element, azote, enters as a fundamental ingre- 

 dient into some of them. It should seem that the 

 atoms which compose the organic molecules in the 

 elementary textures of vegetables, are held together by 

 some vital property, rather than by the laws of chemi- 

 cal affinity ; for although these substances may, with 

 certain precautions, be long preserve"?! in much the same 

 state as that in which they were left when the vital 

 principle was first abstracted from them, yet there ap- 

 pears to have been no very definite chemical union 

 between their atoms, which are no sooner abandoned to 

 the influence of surrounding media, than they enter into 

 new combinations distinct from that under which they 

 existed in the living plant. 



(15.) J-'/cnii'ii (art/ Tixxnrx. There are two element- 

 ary tissues, which are respectively composed of the two 

 kinds of elementary organs, the cells and tubes already 

 noticed. The one is c.ilkd the " cellular" tissue, and 

 consists entirely of cells, and constitutes the chief bulk 

 of every vegetable: the other is the "vascular" tis- 

 sue, and is made up of tubes ; but this latter tissue is 

 found only in certain families of plants. The vascular 

 penetrates the cellular tissue in thin cords, which are 

 composed either of single tubes, or more frequently of 

 bundles of tubes, running continuously throughout the 

 plant, and passing into the leaves, where the tubes 

 separate, and diverge in various directions, and form 

 the veined-like appearance which these organs generally 

 present. 



(16.) O'ffntitr Tissue. If a fragment of any plant 

 be allowed to macerate for some days in water, or if 

 it be subjected to the action of nitric acid, the several 

 elementary organs of which it is composed will sepa- 

 rate from each other, and may then be examined in an 

 isolated state. When thus detached, the cellular parts 

 are found to have been made up of minute vesicles, or 



