Trees of New York State 53 



coarse brandies, may be conspicuous twig features. Ailanthus 

 and Kentucky Coffee Tree both have stout twigs with conspicu- 

 ous medulla, but in the first case it is ochraceous, while that of 

 the Kentucky Coffee Tree is salmon pink. Tn different woody 

 plants the color varies from white to pale or dark brown, with 

 shades of i)ink, red or pale green as the exception, rather than 

 the rule. 



H. Bark 



The most important feature in the identification of standing 

 tr<'es in winter is the bark, though one of the most dif^cult to 

 describe. Bark characteristics can best be taught by the visual 

 method, by actual studies in the field supplemented by photo- 

 graphs and lantern slides. 



In the preceding pages mention was made of the fact that the 

 epidermis, as found in twigs, rarely functions for more than one 

 year, but is soon replaced by a corky layer or periderm which 

 takes over the protective function. The first layer of periderm 

 is usually superficial in origin and has its inception in the layer 

 of cortex immediately underlying the epidermis. It continues 

 to function for a long period, often for 20-40 years, provision 

 l)eing made for the increase in girth through the additions of 

 new layers from the dividing cells underneath and the imper- 

 ceptible sloughing off of cell particles from without. Sooner or 

 later deeper cork formation begins and is immediately evinced 

 by a roughening of the bark due to the desiccation and death 

 of deeper-lying tissues. Deep cork formation may result from 

 new jieriderms concentric with the first and give rise to ringed 

 bark, as in Cherry and Birch, but in the majority of cases the 

 new corky layers are in the form of arcs which encroach into the 

 living tissues for varying distances. The thickness of bark varies 

 greatly between species and individuals and depends largely on 

 such factors as (1) the rapidity of formation, (2) time of the 

 beginning of exfoliation, and (3) the rate of exfoliation. Ash, 

 Platanus and Osage Orange are thin barked; and in most of the 

 Oaks and Black Locust the bark is thick. It is due to the many 

 minor variations in the time and manner of deep cork formation, 

 the time of exfoliation, as well as the organic content of the tis- 

 sues concerned that trees have bark characteristic of the species. 



Some species have very rough bark with deep longitudinal fis- 

 sures, as in the Chestnut, Oak and Black Locust. In others the 

 longitudinal fissures are comparatively shallow a»d are separated 



