I 



The Striicfiire of Leavex. 1 1 



atmosphere, whicli cannot be obtained by the root from tlic soil.* 

 This, however, is not the only use of leaves, besides this they 

 throw off throuf^h myriads of liule " stomata," or breathing pores, 

 all superfluous fluid drawn up by the root. Again, the green leaves 

 keep the ground about the roots of the tree moist, by keeping ofl' 

 the sun ; and may not this be the reason why nature has made the 

 leaf (jreen, viz., beeause this is the best colour to resist the sun's 

 rays, and that to do this is one of the duties of leaves. Nor is this 

 all, for the coloured leaves of the flower attract insects, which by a 

 curious ])rocess fructify flowers — i.e., make them bear fruit and 

 seed. 



Next let us consider how it is that in the autumn leaves become 

 yellow, or at any rate change their colour. The explanation of 

 this is very curious, and is much disputed, but the following appears 

 to approach the truth. 



The green colouring of leaves is due to two colouring principles, 

 a yellow and a blue, derived from the sun's rays, the blue increas- 

 ing in proportion to the heat of the sun. These act upon a certain 

 juice in the plant dependent on the root, which is shown by leaves 

 losing their colour when the roots are unable to perform their func- 

 tions properly, as in hot weather or hot rooms. Now, according as 

 the amount of "yellow" or "blue" prevails, the leaf is the more 

 green or the more yellow. Thus in very young leaves there is 

 hardly any blue as vet, since the sun has but little power in early 

 spring ; therefore they are of a yellow tint, but as the sun gets 

 more powerful, the "blue" predominates, and they get "greener;" 

 again, in autumn, as the sun gets less powerful they lose their blue 

 colonring,t and so get yellow again. In connection with this it is 

 worth while mentioning that flowers appear to follow an analogous 

 rule. For in the winter we have the " snowdroj)," which is quite 

 white ; then, as spring comes on, the " primrose," renowned for 

 its pale yellow colour; next, the celandine, which is of a much 

 deeper yellow ; and after these comes the purple violet. Other 

 autumn tints that are not yellow may be accounted for by the 

 coloured contents of the tissues. 



We now come to the question, Why do leaves die ? 



The reason is made clear by merely burning a few leaves in the 

 autumn, and examining the ashes ; there will then be found quan- 

 tities of earthy substances in the ashes, which have been drawn up 

 by the root and passed on to the leaves, and thus choked them up. 

 If, then, leaves were not to die, they would soon get so choked 



• Nitrogen. 

 t As a further proof of this I have a " geranium," which, growing out of doors in. 

 a protected place, has thrown out a few new leaves in the beginning of December, 

 but these are yellow, and that of a very i^ale tint. 



