CHAP. I. VITAL PROPERTIES AND STIMULANTS. 1?1 



external agents. Unless such be present, the vital force 

 remains dormant, even where it is not extinguished. 

 Thus for example, seed will not germinate unless it be 

 placed under peculiar circumstances with regard to 

 moisture, temperature, and the atmosphere ; but when 

 a sufficient supply of these three stimulants is provided, 

 the seed swells, bursts, and the plant is gradually de- 

 veloped. The principal stimulants to vegetation are 

 light, heat, air, and water ; and the conjoint action of 

 at least three of these four is generally requisite to se- 

 cure a healthy condition to most plants. 



(154.) Light. The action of light, as we shall show 

 more distinctly when we are describing some of the 

 functions of vegetation, is of the greatest importance. 

 We shall here notice only onq phenomenon, to which 

 we have already alluded (art. 148.), where the presence 

 of this stimulant exerts a decided influence. 



(155.) Sleep of Leaves. The phenomenon to which 

 we allude is termed the sleep of plants. This consists in 

 a periodic change in the position of an entire leaf, or of 

 the several leaflets of which a compound leaf is formed. 

 The petioles, or leaf stalks, either bend upwards or 

 downwards, so that the flattened surface or limb of the 

 leaf is elevated or depressed. There are about a dozen 

 different modifications in the manner in which the 

 leaves are inclined to the stalks on which they grow ; 

 some raise their leaflets so that their upper surfaces are 

 brought into contact, and others depress them so that 

 the under surfaces meet together. This phenomenon 

 is best exhibited by various species of the two natural 

 orders, the Leguminosae (which includes both the pea- 

 flowering plants, as clover, &c., and the acacias and 

 mimosas, &c. which have regular flowers) and the 

 Oxalidese. These phenomena depend upon a special 

 physiological law, subject in some degree to the sti- 

 mulating effects of light and heat, which elicit and 

 control them, but which are not themselves the pri- 

 mary causes of these effects. When the sensitive-plants 

 are confined in a dark room, their leaflets periodically 



