130 THE STEM. 



of wood, reaching from the highest leaves to the remotest rootlets.* 

 As the exogenous tree, therefore, annually renews its buds and 



* The layers of wood and bark, by which the exogenous stem annually in- 

 creases in diameter, are formed by the multiplic-ation of the cells of the cambium- 

 layer throughout its whole extent. That the organic material to supply this 

 growtii in ordinary vegetation descends in the bark, for the most part, and that 

 the order of growth in the formation of M'ood is from above downwards, and 

 also the general dependence of this growth upon the action of the foliage, may 

 be inferred from a variety of facts and considerations. The connection of the 

 wood M'ith the leaves is shown : — (1.) By tracing the threads of soft woody 

 Endogens, such as Yucca, directly from the base of the leaf into the stem, and 

 thence downward to their termination, towards which they become attenuated, 

 lose their vessels, and are finally reduced to slender shreds of woody tissue. 

 (2.) The amount of wood formed in a stem or branch, other things being equal, 

 is in a relation to the number and size of the leaves it bears ; its amount in any 

 portion of the branch is in direct proportion to the number of leaves above that 

 portion. Thus, when the leaves are distributed along a branch, it tapers to the 

 summit, as in a common Reed or a stalk of Indian Corn ; when they grow in a 

 cluster at the apex, it remains cylindrical, as in a Palm (Fig. 184). Consequently 

 the increase of the trunk in diameter directly corresponds with the number and 

 vigor of the branches. The greater the development of vigorous branches on a 

 particular side of a tree, the more wood is formed, and the greater the thickness 

 of the annual layers on that side of the trunk. (3.) In a seedling, the wood 

 appears in propoi-tion as the leaves are developed. (4.) If a young branch be 

 cut off just below a node (156), so as to leave an internode without leaves or 

 bud, little or no increase in diameter will ordinarily take place down to the first 

 leaf below. But if a bud be inserted into this naked internode, as the bud de- 

 velops, increase in diameter, with the formation of new wood, recommences. 

 That the formation proceeds from above downwards, or that the elaborated sap 

 which furnishes the material for the growth is diffused from above downwards, 

 appears from the effect of a ligature around exogenous stems, or of removing a 

 ring of bark. It is a familiar fact, that, when a ligature is closely bound around 

 a growing exogenous stem, the part above the ligature swells, and that below 

 does not. Every one may have obsciwcd the distortions that twining stems thus 

 accidentally produce upon woody exogenous trunks, causing an enlargement on 

 the upper side of the obstruction. When the stem is girdled, by removing a 

 ring of bark so as completely to expose the surface of the wood, the part above 

 the ring enlarges in the same manner ; that below does not, until the incision is 

 healed. The wood of the roots is manifestly formed in a descending direction. 

 But this is continuous with that of the stem ; and its first layer, the extension of 

 the wood of the radicle into the primary root, agrees in composition with the 

 wood of the succeeding layers in the stem, having no spiral vessels, but only 

 ducts. Still, whatever analogy there may be between the growth of the wood 

 in the stem and of roots, there is no real basis for the ingenious concc])tion of 

 Thouars and of Gaudichaud, that wood is the roots of buds or leaves, or that 

 it is absolutely dependent upon them for its formation. 



