THE ANATOMY OF THE SHOOT 77 



loosely arranged, with intercellular spaces that form a system of 

 air passages that are continuous with the substomatal cavities. 

 Both palisade and spongy cells are chlorenchymatous, the number 

 of plastids usually being greater in the former; and chloroplasts 

 also occur in the guard cells of the epidermis. The border paren- 

 chyma, adjacent to the vascular elements of the smaller bundles and 

 to single tracheids, may be lacking in chlorophyll. 



The veins resemble the vascular bundles of the stem with respect 

 to their primary tissues; and, in most instances, consist of collateral 

 bundles that are oriented so that the xylem lies toward the adaxial 

 surface. In plants which have bicoUateral stem bundles, such 

 vascular organization may also occur in the leaf; but it is com- 

 monly wanting in the lateral veins and in the peripheral portions of 

 the main ones. (Fig. 19.) The secondary veins are progressively 

 smaller with fewer vascular elements; and, ordinarily, phloem ele- 

 ments do not occur in the ultimate veinlets which consist of one or 

 two xylem elements, usually tracheids, accompanied by non-chloro- 

 phyllose parenchymatous cells. Such cells presumably serve in 

 translocation as a connecting link between the photosynthetic cells 

 of the mesophyll and the phloem of the larger veins. The occur- 

 rence of a cambium is not uncommon in the main veins; and, in 

 some large leaves, there may be a considerable amount of secondary 

 vascular tissue. Associated with the main veins as a bundle 

 sheath, or occurring as strands which parallel the veins along the 

 adaxial and abaxial surface, are the mechanical tissues which 

 strengthen the framework of the petiole and blade. These scleren- 

 chymatous elements are similar to those described for the stem, 

 consisting of elongated cells with thickened walls. 



In addition to the principal tissues noted, special glandular 

 structures may develop in the mesophyll (Gossypium), and lactif- 

 erous cells and ducts may be differentiated (Lactuca). Cysto- 

 liths (Cannabis) and cells containing crystalline substances and 

 mucilages also occur. 



Ontogeny of the Leaf. — Ontogenetic studies of the leaf have 

 been few, and it is only within recent years that interest in this 

 neglected phase of developmental anatomy has been renewed. 

 Following the classical studies of Trecul (39), Eichler (13), Han- 

 stein (x5), and others of that period, little significant work on leaf 

 ontogeny was published until the first part of this century, except 

 for the contribution by Lund Qlj) to which Foster (18) recently 



