TRITICUM 169 



while the cells of the adaxial epidermis are thin-walled with few 

 stomata. 



The chlorenchyma of the mesophyll is very uniform, although 

 the cells abutting both epidermal surfaces may be somewhat 

 elongated in a manner suggesting palisade tissue. There are 

 large intercellular spaces subtending the stomata and projecting 

 into the central portion of the mesophyll. In the sheath, chloren- 

 chyma strands, three or four cell layers in thickness, occur between 

 the vascular bundles; and in the mature sheath, large lacunae 

 develop centrad to them. 



Vascular bundles of two sizes extend parallel with the long 

 axis of the lamina, and small transverse veins interconnect the 



I'-mech 



^-bu sh 



Fig. 76. Transection of small portion of foliage leaf: ac c, accessory cell; bush, bundle 

 sheath ; ep, epidermis ; g c, guard cell ; m c, motor cells ; meci>, mechanical tissue ; msl, 

 mesophyll; ^^, phloem; xj, xylem. (After Avery.) 



longitudinal strands which may occasionally branch, or anasto- 

 mose toward the apex of the blade. All the bundles are collateral 

 and the larger ones resemble those of the stem in their organiza- 

 tion, having one or two annular or spiral elements and two large 

 pitted vessels. The small longitudinal bundles have no large ves- 

 sels, and the xylem of the transverse bundles usually consists of 

 short tracheids. In all cases, phloem is present, but in the trans- 

 verse bundles it is limited to a few somewhat elongated parenchym- 

 atous cells. The bundles are surrounded by a double sheath, the 

 inner layer consisting of elongated thick-walled cells, and the 

 outer of parenchymatous cells which are usually devoid of chloro- 

 plasts at maturity. In some cases, the outer layer of the sheath 

 does not completely encircle the bundle, being restricted to a 

 sector over its adaxial or xylem portion. 



Longitudinal strands of thick-walled mechanical tissue are 

 located above and below the bundles, and there is also a strand 

 of this type extending along each margin of the leaf blade immedi- 



