ORIGIN OF THE WOOD. 133 



225. That the wood thus originates in connection with the leaves 

 is shown, (1.) By tracing the threads of soft woody Endogens, 

 such as Yucca, and some Palms, 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.) Because the 

 amount of wood formed in a stem or branch is in proportion to 

 the number and size of the leaves it bears ; its amount in any por- 

 tion 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. 166). Consequently the aggregate 

 diameter of the branches is (cateris paribus) equal to that of the 

 trunk from which they arise ; as is beautifully illustrated by the 

 excurrent stem of Pines and Firs, (carried directly upwards by the 

 continued growth of the leading shoot, 155,) the diameter of which 

 regularly diminishes as the lateral branches are given off. Conse- 

 quently the increase of the trunk in diameter directly corresponds 

 with the number and vigor of the branches. The greater the de- 

 velopment 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 ap- 

 pears just in proportion as the leaves are developed. (4.) If a 

 young branch be cut off just below a node (141), so as to leave an 

 internode without leaves or bud, no increase in diameter will take 

 place down to the first leaf below. But if a bud be inserted into 

 or ingrafted upon this naked internode, as the bud developes, 

 increase in diameter, with the formation of new wood, recom- 

 mences. 



226. These facts conspire to show, not only the general depend- 

 ence of the wood on the leaves for its formation, but also that the 

 wood is produced from above downwards.* The following are 

 some of the considerations that may be adduced in confirmation of 

 this view : (1.) When a ligature is closely bound around a grow- 



* There is an article by James Warren, in the first volume of the Memoirs 

 of the American Academy of Arts and Sciences, published in 1785, ingen- 

 iously maintaining the downward growth of the wood, apparently from orig- 

 inal observations altogether. 

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