THE ANATOMY OF THE SHOOT 69 



which the activation of pericyclic or cortical tissues, or both, 

 results in the formation of secondary bundles and interfascicular 

 parenchyma. 



Secondary thickening is accompanied by changes in the cortical 

 and epidermal tissues of the stem. When the development of 

 secondary stelar tissue is not extensive, it may be compensated for 

 by the grov^^th and division of the cells of the outer regions of the 

 axis; but w^here there is considerable secondary thickening, it 

 usually involves a loss of the epidermis and often portions of the 

 cortex. In such cases, a periderm is formed following the develop- 

 ment of a phellogen which is initiated in the cortical or pericyclic 

 parenchyma; or, infrequently, in the epidermis. The phellogen 

 produces phellem (cork cells) centrifugally and phelloderm cen- 

 tripetally. (See Chapter I.) 



Lentkels, which serve as aerating structures, are developed in 

 the periderm when that tissue becomes extensive. They are com- 

 monly initiated at points beneath stomata, and the first step in their 

 formation consists in the division of the cortical cells which form 

 a group of loosely organized cowplementary cells. Later, the true 

 phellogen arises and the growth of the peridermal layers causes an 

 outbulging of the mass of complementary cells rupturing the 

 epidermis at the stomatal opening. Continued activity of the 

 phellogen at this point, or the production of successive phellogens, 

 results in the formation of layers of complementary cells. More 

 compact closing layers are also developed which serve to hold the 

 spongy mass of complementary cells together. The closing layers 

 are progressively ruptured by growth and stretching so that the 

 margins of the lenticel appear irregular and fragmented. A typical 

 lenticel is elliptical, somewhat raised above the surface of the bark, 

 and is usually oriented with its greatest dimension in the vertical 

 plane. They may become tangentially stretched where there is a 

 persistent periderm as in Prunus and Betula. 



Vascular Transition. — In describing the ontogeny of the root 

 and stem axes, no statement has been made with respect to the 

 interconnection between the vascular systems of the two. The 

 vascular organization is different in orientation and mode of 

 development in the stem and root which together constitute the 

 axis of the plant, and it follows that there must be a region of 

 vascular transition at some point where continuity of the radial 

 exarch protostele of the root with the endarch dictyostele of the 



