_g66 SECONDARY CHANGES. 



as in Platanus and Pyrus Malus, the new ones appear on the surface laid bare 

 by its removal. In trees with an adherent^ longitudinally cracked bark, as Robinia, 

 Prunus domestica, species of Populus, and Ginkgo, the living lenticels lie at the 

 "bottom of the longitudinal furrows. The first cortical cracks pass through the first- 

 formed lenticels themselves, and give them the position indicated. New ones then 

 arise in the peridermal layers which are successively laid bare at the bottom of the 

 furrow, by the further extension of the crack. 



According to the few existing enumerations, and judging from appearances, 

 the number of the lenticels occurring on the same transverse portion constantly in- 

 creases with the dilatation, at least in many treeS ; and probably to a greater extent, 

 the smaller the dilatational growth of the individual lenticd. 



As follows from the facts discussed above, the distribution of the lenticels on a 

 shoot is in general determined by that of the stomata, by the structure of the older 

 cortical surface, the form of the bark, &c. An additional phenomenon, independent 

 of all these relations, fiirther appears, namely, that while the distribution is uniform 

 all round on upright shoots, in those growing horizontally the upper side has fewer 

 lenticels than the lower side. The amount of difference between the two sides varies, 

 according to the species, and according to the age of the shoots of a tree; in the 

 latter respect in such a manner that it is greatest in young stages, and becomes more 

 and more equalised as growth in thickness proceeds. Of the numbers found by 

 Haberlandt for this relation, in several species of "tree (species of Gleditschia, Tilia, 

 and Ulmus campestris), some may be given here, which also illustrate the successive 

 increase in the total number of the lenticels. The number on the upper side is 

 printed as the numerator, that on the lower side as the denominator of a fraction. 

 Pifece of branch, 20 c™ long, of — 



istyear. 



Gleditschia triacanthos: -sfs; 



Ulmus campestris : |^ ; 



