24 GENEEAL PKmCIPLES. 



they liave on their growth and productiveness. It is be- 

 lieved that the opening of the leaf bnds in spring induces 

 the formation of new roots ; this is doubtful, as new roots 

 may be seen forming at times when there are no leaves 

 on the tree and apparently no growth whatever going on 

 in the buds. But if the roots are not roused into action 

 by the leaves, it is well known they will not continue and 

 grow long if leaves do not make their appearance. We 

 observe in the case of trees the tops of which have been so 

 much injured by drying and exposure that scarcely a sound 

 bud is left to grow, in this case the roots, although in 

 perfect order, remain nearly dormant until new shoots and 

 leaves are produced, and in proportion as the leaves in- 

 crease so do the roots. The fact of the absorption and 

 exhalation by leaves of certain fluids, has, to a very con- 

 siderable extent, established the theory that the sap of trees 

 is taken up from the roots through the cells or sap vessels 

 of the wood of the trunk and branches in a crude state^ 

 and j)asses into the leaves ; that in their tissue spread out 

 under the sun's rays, it receives certain modifications. 

 Carbonic acid, which has been taken in a state of solution 

 from the soil and by the leaves from the atmosphere, is 

 decomposed, its oxygen is given off into the air, carbon 

 becomes fixed, and thus the component f)arts of the tree, 

 starch, sugar, gum, &c., are formed. After passing 

 through this purifying or concentrating process, the sap 

 acquires a more solid consistence, and is called ecmibium / 

 so prepared it returns downwards through the nerves or 

 vessels of the leaf to the base of the leaf stalk, and then 

 betv\'een the wood and bark of the stem, forming new 

 layers on its passage. Such is, at present, the most j)opu- 

 lar theory of the functions of the leaves and the ascent, 

 assimilation, and descent of the sap. Some distinguished 

 writers on the subject reject this theory, alleging that — 

 " there is no such thing as crude sap, that as soon as it 



