124 THE LEAF 



In llii.s exercise, a bi-iet' study will l)e made of a few represen- 

 tative leaf tyi)es, with particular emphasis upon the anatomy of 

 the leaf-blade or lamina. It is beyond the scope of this book to 

 discuss the process of leaf origin and the differentiation of the 

 various leaf tissues in seed plants. For information on these 

 matters, reference should be made to Foster (1936), Ilaywa'rd 

 (Ch. Ill, pp. 77-8.-)), Troll (1938), and Cross (194{). 1941). 



II. Material for the Study of Leaf Anatomy. — 



1. Tliv himhia of the foliage leaf of lilac {Syriiuja viilgari.'^). 

 Obtain a stained trans-section of the lamina and study its histology 

 mider low magnification. Note first of all the clear distinction 

 between tlie midrib and the two thin latei'al flaps of tissue. An 

 examination of the midrib reveals a large collateral vascidar 

 bundle in which the phloem is directed towards the ahaxial or 

 lower leaf-surface while the xylem is situated beneath the adaxial 

 or upper leaf surface. With this orientation in mind, it will now 

 be clear that the lamina exhibits a typical dorsiventral character, 

 shown not only by the relative positions of xylem and jihloem in 

 the larger veins but also by the differentiation of the mesophyll, 

 into palisade and spongy parenchyma. Since the anatomy of the 

 lamina in the region of the veins differs somewhat from the iiiter- 

 veinal areas, it will be more convenient to describe briefly the 

 various tissues and then to point out their topographical varia- 

 tions. In the lamina of this leaf, three principal tissues are 

 present, viz. : 



{a) The epidermis. The adaxial epidermis consists of some- 

 what oval cells, the outer walls of which are covered by a thin 

 cuticle. Although exact measurements are lacking, there seems 

 to be relatively little difference in the thickness of the inner, outer 

 and radial walls of the epidermis. Observe that many of the 

 epidermal cells possess a protoplast which is peripheral in i)osi- 

 tion. Stoma fa ai'e not unconnnon in the adaxial epidermis. Note 

 partieularl.N' the I'elatively small size of the guard cells and the air 

 chamber pi-esent beneath each stomate. Trichomes, represented 

 by capitate <nid simple hairs, occasionally devc-lop but these struc- 

 tures are more abundant on the abaxial surface of the lamina. 

 The abaxial epidermis is fundamentally similai- to the adaxial 

 epidermis except that the cells are somewhat smaller and thinner- 



