\xn GENERAL PRINCIPLES 



VII. HAIRS. 



193. Hairs are minute expansions of transparent cellular tissue proceeding from 

 the surface of plants. They are of two kinds, lymphatic and secreting. 



194. Lymphatic hairs are formed by vesicles of cellular tissue placed end to end, 

 and not varying much in dimensions. 



195. Secreting hairs are formed by vesicles of cellular tissue placed end to end, 

 and sensibly distended at the apex or base into receptacles of fluid. 



196. Lymphatic hairs are for the protection of the surface on which they are 

 placed, and for the control of evaporation through the stomata (44). They always 

 proceed from the veins, while the stomata occupy the interjacent parenchyma. 



197. Secreting hairs are receptacles of the fluid peculiar to certain species of 

 plants, such as the fragrant volatile oil of the sweet brier, and the acrid colourless 

 secretion of the nettle. 



VIII. FOOD AND SECRETIONS. 



198. Plants are nourished by the absorption of food from the earth, in conse- 

 quence of which they grow, and produce their peculiar secretions. 



199. The growth of plants is very rapid ; that of the leaves is such that they often 

 acquire six or seven times their original weight per hour. 



200. The food of plants consists of water, holding various substances in solu- 

 tion. The roots have the power of separating these substances, and selecting such 

 only as are congenial to the nature of the species. 



201. As soon as it is absorbed, it begins to ascend into the stem. 



202. The ascending fluid is called sap ; it consists chiefly of water, mucilage, and 

 sugar, mixed with a small quantity of such peculiar secretions of the plant as it 

 may dissolve in its course. It does not alter its nature materially until it is dis- 

 charged into the leaves. 



203. It is put in motion by the newly developing leaf-buds, which, by constantly 

 consuming the sap that is near them, attract it upwards from the roots as it is 

 required. Therefore, the movement of the sap is the effect, and not the cause, of 

 the growth of plants. It depends upon a vital irritability, and is independent of 

 mechanical causes. 



204. This irritability is indicated not only by the motion of the sap, but by seve- 

 ral other phenomena of vegetation ; such as, 



a. The elasticity with which the stamens sometimes spring up when touched, and 

 the sudden collapsion of many leaves when stimulated : 



6. The apparently spontaneous oscillation of the labellum of some Orchideous 

 plants : 



c. The expansion of flowers and leaves under the stimulus of light, and the col- 

 lapsion of them when light is withdrawn. This phenomenon in leaves is called the 

 sleep of plants : 



d. By the effects of mineral and vegetable poisons being the same upon plants as 

 upon animals. Mineral poisons kill by inflammation and corrosion ; vegetable 

 poisons by the destruction of irritability. 



205. After the sap has been distributed through the veins of the leaves, it be- 

 comes exposed to the influence of air and light, and undergoes peculiar chemical 

 changes. In this state it is called the proper juice. 



200. When the proper juice has been once formed, it flows back along the lower 

 stratum of veins, and descends towards the roots, passing off horizontally into the 

 centre of the stem. 



207. Hence the great importance of leaves to plants, and the necessity of ex- 

 posing them to the full influence of light and air, for the purpose of securing a due 

 execution of their natural functions. 



203. Hence also the impropriety of mutilating plants by the destruction of their 

 leaves. 



209. In Exogenous plants (80) the upward course of the fluids is through the al- 

 burnum, their downward passage through the bark, and their horizontal diffusion 

 takes place by the medullary r i 



210. Hence the peculiar principle of such plants are to be sought either in the 

 bark or the heart-woo,-] (10(1.) not in the alburnum (101.) 



211. As they are the result of the growth of a plant, they will be found more 

 abundantly in annual plants at the end than at the commencement of their growth. 



