34 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



taneous oscillation of the labellum of some Orchidaceous plants ; the expansion 

 of flowers and leaves under the stimulus of light, and the collapse of them 

 when light is withdrawn (this phenomenon in leaves is called the sleep of 

 plants) ; and by the effects of mineral and vegetable poisons being the same 

 upon plants as upon animals. Mineral poisons kill by inflammation and cor- 

 rosion ; vegetable poisons by the destruction of irritability. 



264. After the sap has been distributed through the veins 

 of the leaves, it becomes exposed to the influence of air and 

 light, and undergoes peculiar chemical changes. In this state 

 it is called the proper juice. 



265. When the proper juice has been once formed, it flows 

 back, and descends towards the roots, passing off horizontally 

 into the centre of the stem. 



266. Hence the great importance of leaves to plants, and 

 the necessity of exposing them to the full influence of light and 

 air, for the purpose of securing a due execution of their natu- 

 ral functions. Hence also the impropriety of mutilating 

 plants by the destruction of their leaves. 



267- In Exogenous plants (95), the upward course of the 

 fluids is through the young wood ; their downward passage 

 through the bark, towards, or into the root ; and their horizon- 

 tal diffusion takes place by the medullary rays. 



268. Hence the peculiar principles of such plants are, in 

 trees and shrubs, to be sought either in the bark or the heart- 

 wood (118), not in the alburnum (119). But in plants whose 

 stems are annually destroyed while the roots are peren- 

 nial, the latter are the chief reservoir of secretions ; and in 

 annuals, whose root and stem both perish, the secretions are 

 dispersed equally through the stem and root. 



269. 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. 



270. In Endogenous plants (95) it is probable that the up- 

 ward course of the fluids is through the bundles of vascular 

 and woody tissue, and that the downward and horizontal pas- 

 sage takes place through the cellular tissue. 



271. The precise direction of the sap in Acrogens (95) is 

 unknown. 



VIII. FLOWER-BUD. 



272. The FLOWER-BUD consists of a fixed point, surround- 

 ed by imbricated, rudimentary, or metamorphosed leaves, 



