﻿Trees of New York State 51 



P. Lenticels 



An external feature of twigrs which is of minor value in identi- 

 fication is the presence of lenticels (Figs. 67 and 68) which vary 

 greatly in number, size, and color. Lenticels are organs of aera- 

 tion which nature has provided to insure a supply of oxygen 

 to tissues underlying the corky periderm. These often contain 

 chlorophyll which is active in the elaboration of plant food ; in 

 Rixy case the living cells making up these tissues require oxygen 

 for their maintenance. The initial covering of all twigs is known 

 as the epidermis or outer skin. It is usually a single layer of 

 cells, the outer walls of which are covered witli a special water- 

 proofing material known as cutin, and has, as its chief function, 

 the protection from desiccation of the deeper lying tissues. In 

 most woody plants the epidermis is comparatively sliort lived 

 and no provision is made for its expansion to accommodate the 

 enlarging twig; it ruptures and sloughs off the second or third 

 year. It is at this time that many twigs are covered with a gray, 

 evanescent skin, the old epidermis, which is no longer a func- 

 lional part of the tree but still clings to the twig. But prior to 

 the rupturing of the epidermis, a new water-and air-proof layer, 

 several cells in thickness, has arisen from the renewed division 

 of cells underlying the epidermis. This is known as periderm 

 and assumes the protective functions of the epidermis after the 

 latter is ruptured. The cells of the periderm are strongly suber- 

 ized, that is, corky, and are quite impervious to the passage of 

 air or moisture, but here and there breaks in the continuity of 

 the corky layer occur which consist of unsuberized, loosely 

 pttached cells, the so-called lenticels. 



Lenticels make their appearance on the new growth during the 

 first summer soon after the unfolding of the new leaves. In some 

 species they are relatively abundant and conspicuous structures 

 and stand out clearly against the green background of the young 

 stem, while in others they are sparse and relatively inconspicu- 

 ous. At this stage they are usually punctate and orbicular in 

 outline or somewhat elongated, either vertically or longitudinally 

 and vary in color from white through shades of orange, brown 

 and pink, to nearly black. Lenticels are generally uniformly dis- 

 tributed over the surface of the twig but in some cases there is 

 a tendency toward aggregation below nodes or on the underside 

 of twigs. As the twigs mature into branches and larger limbs, 

 lenticels finally disappear owing to the exfoliation of the bark 



