136 THE STEM. 



closely pressed together, and form, as it were, coarse threads of 

 wood ; but on reaching the ground they assume the appearance 

 and functions of real roots. Every transition is found between 

 this arrangement and that in which they are united and blended 

 with one another in a continuous ligneous tissue. 



228. Nevertheless, it is carrying such conclusions much too far 

 to assert, with Thouars and Gaudichaud, that wood is the roots 

 of buds or of leaves, and to insist that each branchlet or branch 

 contributes a distinguishable or definite portion to the trunk below, 

 which is prolonged into a particular root or set of roots. In Palms, 

 indeed, according to the high authority of Martius, there are no 

 other threads of wood in the trunk than those which have proceed- 

 ed from the bases of the leaves. But in exogenous stems, of 

 which most is known, although the principal growth commences 

 and proceeds in the manner above described (224), yet it undoubt- 

 edly goes on from year to year by the continual multiplication and 

 growth of cells (32, 203-205) over the whole extent of the cam- 

 bium-layer nearly simultaneously, irrespective, at least in the trunk 

 and roots, of any direct connection with buds or leaves above. The 

 formation of wood is resumed each spring where it was interrupted 

 the previous autumn. This is shown in the case of stumps which 

 have been kept alive for several years, in consequence of the natu- 

 ral ingrafting of some of their roots with the roots of adjacent trees 

 of the same species, and which have continued to form annual lay- 

 ers, although very thin ones, while they survived, notwithstanding 

 they bore no leafy shoots, or scarcely any.* The cambium-layer, 

 however it may have originated in the first instance, blends into a 

 common stratum, which appears to possess an inherent power of 

 continuing and reproducing itself, while it is nourished by the 

 elaborated sap, which is generally supplied by the foliage above. 

 It is well known that the ascending sap is laterally diffused with 

 great readiness through the whole circumference of the sap-wood ; 

 if this be destroyed on one side of the tree, the sap that ascends on 

 the other is equably supplied to all the branches throughout. The 

 branches of each year's growth are, therefore, kept in fresh com- 



* The ascertained fact, that the fibro-vascular tissue of secondary roots orig- 

 inates independently in the parenchyma, adjacent to, but not at first in con- 

 tact with, the wood of the stem, is decisive against the Thouarsian hypothesis, 

 as strictly carried out. 



