312 ANNUAL OP SCIENTIFIC DISCOVERY. 



This discussion of the conical growth of trees leads us necessarily to 

 the investigation of the source from whence they derive their elabo- 

 rated formative material. This is undoubtedly the leaf. Now, this 

 law is plainly apparent in the single shoot, the figure of which depends 

 on the manner in which the leaves are disposed about its surface ; for 

 as the wood is formed by the leaves, when these are placed in regular 

 order over every part of the circumference of the shoot, as in the beech 

 and lime, the shoot is always necessarily cylindrical, for the woody 

 matter proceeding from the leaves is then distributed equally on all its 

 sides. On the contrary, when the leaves on the single shoot are oppo- 

 site, or in pairs, placed at right angles to each other, as in the spindle- 

 tree and mapfce, the descent of nourishing matter from them is neces- 

 sarily limited to that portion of the stem immediately below them, and 

 consequently the young shoots and branches of these trees are square. 



But not only the form of the single shoot, but also the extent to 

 which it is conical, depends on the leaves. If the vital activity of the 

 leaves is too enfeebled to form wood, if they remain crowded together 

 into clusters at the top of the shoot without separating, the shoot may 

 increase in length, but there is no increase in breadth. Two shoots of 

 the horse-chestnut are now lying before me, placed side by side for 

 comparison, and the contrast between their figure is not only very 

 perceptible, but also highly instructive. The shoot in the one case is 

 conical ; in the other, cylindrical. The conical shoot is the growth of 

 a single year ; the cylindrical shoot is the growth of ten years ; yet 

 both are nearly the same size. As the elaborated woody matter form- 

 ing the substance of these shoots was derived from the leaves with 

 which they were clothed, and as, in the case of the ten years' shoot, 

 very little was supplied, that shoot is cylindrical, not conical, like the 

 one year's shoot. 



It follows, too, that the breadth of the wood-rings Termed annually, 



t" J 7 



and which are visible on the transverse section of the stem, must also 

 correspond w r ith the amount of active leaf-surface which is put forth 

 into the atmosphere during the vegetative season. In order to verily 

 this truth, it is only necessary to select branches, the leaves of whose 

 side-shoots are annually put forth as leaf-clusters, and which therefore 

 take a minimum of development, and consequently exercise the small- 

 est possible amount of physiological influence on the branch, and where 

 powerful growths are suddenly succeeded by growths greatly retarded. 

 One such branch now lies before me, seven years old, whose main 

 stem is eighteen inches long, and whose side-shoots are abortive in 

 their growth. It grew the first three years five inches annually, or 

 altogether fifteen inches ; but in the last four years the growth stag- 

 nated, or averaged only nine lines (a line is the twelfth part of an 

 inch) annually ; and the cross section of the branch actually shows the 

 three inner rings or woody layers, formed by the leaves of the first 

 three years, to be much broader than the four outer rings, the leaf-de- 

 posits of the last four years. 



These investigations and others lead irresistibly to the conclusion, 

 that the breadth of the wood-rings is determined not only by the activ- 

 ity of the leaves of the terminal shoot of the main stem, but that the 

 leaves of the side-shoots or of the whole system of shoots cooperate ; 

 and therefore that the leafage of each season forms a common source, 

 whence is derived not only the nutriment forming the new layer or 



