MATERIAL FOB STUDY OF LEAF ANATOMY 



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call be rather clearly seen. In general, cambial activity is never 

 prolonged in the veins of most leaves. In the smaller veins of 

 the lamina, the vascular tissue is considerably reduced in amount 

 and secondary growth may be entirely lacking. In certain regions 

 of the blade, the diverging bundles may be cut more or less 

 obliquely so that the characteristic type of primary xylem ele- 

 ments may be recognized. The very small leaf veins may consist 

 of several parenchyma cells and a few primary xylem elements ; 

 a bundle of this character is usually surrounded by a jacket of 

 parenchyma cells containing chloroplasts (i.e., the so-called "bor- 

 der parenchyma"). 



2. The leaf blade of corn {Zea Mays) . Obtain a transverse sec- 

 tion of a corn leaf and examine it under low power. The adaxial 

 epidermis is readily identified by the presence of groups (3-5 

 cells) of somewhat lens-shaped, apparently empty cells. These 

 cells are known as bulliform cells and by changes in their turgor 

 allow the leaf-blade to curl or uncurl, a phenomenon which may 

 be advantageous in restricting the loss of water from the leaf 

 under arid conditions. The typical epidermal cells of both the 

 abaxial and adaxial epidermis are somewhat oval in transverse 

 section (actually they are rather elongated cells) and are provided 

 with a definite cuticle. Stomafa, with conspicuous air-chambers 

 beneath them, are present in both epidermal layers. Occasional 

 unicellular, sharp-pointed hairs occur on the adaxial epidermis. 

 The mesophyll tissue of the leaf shows no clear differentiation 

 into palisade and spongy parenchyma but instead is composed of 

 several layers of rather compact parenchyma cells. 



The vascular system of the leaf consists of a parallel series of 

 collateral bundles. The majority of the bundles are rather small ; 

 at intervals, fairly large bundles occur. Examining one of the 

 small bundles under high power, note that it is completely sur- 

 rounded by a bundle sheath of rather large "isodiametric" par- 

 enchyma cells which contain chloroplasts; the bundle sheath, 

 sometimes termed the "mestome sheath,'' may act as a conducting 

 layer which presumably transports the products of photosynthe- 

 sis directly to the vascular system, i.e., the phloem. The xylem of 

 each bundle is directed towards the adaxial surface of the leaf 

 and consists only of small tracheae. The phloem of the bundle 



