LEAVES 



639 



(leptome), mechanical elements (stereome), and undifferentiated parenchyma. The 

 position of the hadrome cells in the upper portion of the vein just above the leptome 

 (fig. 760) and near the palisade cells may be advantageous, since the amount of 



water transported greatly exceeds the combined 

 amount of other substances. Near the ends of 

 the veinlets there are no tracheae, but chiefly 

 tracheids which frequently diverge in such a 

 way as to increase the area of the diffusing sur- 

 face (fig. 936) ; com- 

 monly they are sur- 

 rounded by a sheath 

 of mesophyll cells. 



Mechanical tis- 

 sues in leaves. — 

 Mechanical leaf tis- 

 sues, while mostly 

 lacking in hydro- 

 phytes and not espe- 

 cially well developed 

 in mesophytes, are 

 developed promi- 

 nently in many xero- 

 phytes, particularly in those with stiff evergreen leaves, 



Fig. 936. — A longitudinal radial 

 section of a leaf of the hop tree 

 (Ftelea trifoliata) near the end of 

 a vein,^showing a bundle terminus 

 with its tracheids (<), surrounded 

 by a bundle sheath (b) ; note that 

 the upper chlorenchyma consists of 

 palisade cells (^), which at the leaf 

 margin grade into the cells char- 

 acterizing the lower chlorenchyma 

 (5); highly magnified. 



the so-called sclerophylls. 



Fig. 937. — A cross sec- 

 tion through a leaf of the 

 fragrant olive {Osmanthus 

 fragrans), showing a T- 

 shaped sclereid {s), which to- 

 gether with the thick cuticle 

 (c) accounts for the stiffness 

 of the leaf; note the three 

 palisade layers (p), indicat- 

 ing relative xerophytism ; 

 highly magnified. 



As noted elsewhere, the 

 cutinized outer epidermal 

 wall is an important 

 means of stiffening in 

 these and other leaves, 



supplemented in some cases by a thickening of the 

 lateral epidermal walls (as in Ficus, fig. 801) or even 

 of the hypodermal walls (as in Pinus, fig. 1039). In 

 the leaves of xerophytic grasses and sedges, patches of 

 thick-walled bast fibers and other mechanical cells may 

 occur just beneath the epidermis (fig. 835), and festoons 

 of such cells often surround the conductive bundles, 

 giving a great amount of strength to the leaves. Some- 

 times (as in Osmanthus, fig. 937) evergreen leaves con- 

 tain isolated mechanical cells (sdereids) extending from 

 the lower to the upper epidermis, apparently acting as 

 supports or braces; occasionally these stiff cells are 

 Y- or T-shaped. The outer cortical layers of most 

 petioles have the collenchymatic thickening charac- 

 teristic of young stems (p. 697). The tearing of leaves 

 at their margins is prevented largely by the presence 

 there of an extra amount of cutinization, while similar protection is afforded in 

 many leaves by marginal veins. Lobed leaves would seem especially subject to 

 tearing at their sinuses ; in some cases (as in Ribes, fig. 938) strong arcuate veins 



Fig. 938. — A portion of 

 a leaf of the golden currant 

 (Ribes aureum), showing ar- 

 cuate veins (v) just below 

 the leaf sinuses, where the 

 danger of tearing otherwise 

 would be considerable. 



