LEAVES. 45 



to grow 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 

 increase, 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 iip from tlie roots, throxigh the cells or sap 

 vessels of the wood of the trunk and branches, in a crude 

 state, and passes 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 oiF into the air, carbon 

 becomes fixed, and tlius the component j^arts of the tree, 

 the material of the cells, starch, sugar, gum, etc., are 

 formed. After passing through this purifying or concen- 

 trating process, the sap acquires a more solid consistence, 

 and is called cambium ; so prepared, it returns down- 

 wards through the nerves or vessels of the leaf to the base 

 of the leaf stalk, and then between the wood and bark of 

 the stem; forming new layers on its passage. Such is, at 

 present, the most popular 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 enters the roots it becomes assimi- 

 lated, and fit for the production of new cells, and that it 

 passes upwards, forming new wood or cells by a chemical 

 process."* Observation, however, has clearly established 

 that, in the leaves of healthy trees, cliemical processes, 

 depending on light and heat, and absolutely essential to 



* Schleiden's Principles of Botany. 



