112 



THE STEM 



and possess thick, slightly stratified outer walls overlaid by a 

 cuticle; in most instances, inclusions and the remains of the dis- 

 integrated protoplast are present in the epidermal cells. 



(6) Immediately within the epidermis occurs the periderm 

 which is composed of three layers of tissue, viz. : (1) the phellem 

 or cork which is differentiated centrifugally from the phellogen 

 and consists of a varying number of layers of narrow, laterally 

 compressed cells which are arranged in definite radial rows and 

 are densely packed (except the two outermost layers) with dark 

 brown material which consists probably of substances classed un- 

 der the general head of tannins; (2) the phdlo<jen which is a 

 uniseriate layer of meristematic cells found next to the innermost 

 layer of phellem cells. In Tilia, as in so many woody stems, the 

 phellogen is initiated by the tangential division of the outermost 

 layer of cortical cells. (3) Internal to, and directly next to the 

 phellogen occurs a single laj^er of cells characterized in this in- 

 stance by their rectangular form and obvious protoplasts. This 

 layer of cells is known as the phcUodcrm and differentiates centri- 

 petally from the phellogen. Note that the corky layer of the 

 stem is definitely broken at certain points which appear as some- 

 what lens-shaped areas; these regions are known as Icnticels. In 

 studying the structure of the lenticels of the basswood stem, ob- 

 serve particularly the continuity between the phellogen and 

 phelloderm of the lenticel and these tissues as they occur at either 

 side of the lenticel. In this particular developmental stage of 

 the stem in Tilia the lenticels have been formed from the first or 

 primary cork cambium ; as the stem increases in diameter from 

 year to year, new lenticels are formed at various points on the 

 surface of the bark by the activity of subsequently formed 

 phellogen layers; thus even in relatively old branches, lenticels 

 develop and represent the necessary areas through which exchange 

 of gases between the living tissues and the afmosphere can take 

 place in the essential process of respiration. 



(r) Directly internal to the periderm occurs the cortex, which 

 consists of two rather definite regions, viz. : 



(?) An outer region composed of four or more laj^ers of 

 coUenchyma cells, whicli in the position of their thickened pri- 

 mar}^ walls appear intermediate in character between "angular" 



