CHAP, ii.] CHEMISTRY, ETC., OF PLANT LIFE. 67 



its young shoots are thus pressed close to the wall or 

 tree up which it is growing, and kept in this position 

 until they have become plainly anchored by means of 

 the numerous root-like organs of attachment that enter 

 the crevices of the supporting body. Plants that bend 

 away from the light are said to be negatively heliotropic. 



Light also acts in a marked manner on many of the 

 lower forms of plant life, some moving towards the light, 

 others away from it. 



Water, as already stated, is indispensable to all plants 

 during active growth. Growing points require much 

 water for the purpose' of conveying the formative mate- 

 rials required for the building up of new tissues, and a 

 certain amount of water is also used up in such forma- 

 tions. A cell containing protoplasm saturated with a 

 large amount of water will give up water to a cell con- 

 taining a smaller proportion of this substance, and as 

 the young cells are almost entirely filled with dense proto- 

 plasm, there is a constant slow movement of water towards 

 growing points induced by osmotic action and by the 

 water being used up as it reaches these points. A 

 second rapid movement of water is distinct from the slow 

 movement, and supplies the water that plants exhale in 

 the form of watery vapour through the stomata of the 

 leaves into the air. The water is taken up by the root 

 and is conveyed along the walls of the youngest portion 

 of the xylem to the leaves. In darkness the stomata 

 are almost or quite closed, so that little or no transpira- 

 tion takes place. As light and heat are increased the 

 stomata open, being specially influenced by bright light, 

 and under these conditions the amount of watery vapour 

 given off increases. The rigidity of leaves and young 



