LEAVES 



639 



(leptome), jnerhamVa,! elpimprjjs (slereome), and undifferentiated parenrhymji The 

 position of the hmf Jome"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 



FIG. 936. A longitudinal radial 

 section of a leaf of the hop tree 

 (Ptelea trifoliate) near the end of 

 a vein, showing a bundle terminus 

 with its tracheids (t), surrounded 

 by a bundle sheath (Z>) ; note that 

 the upper chlorenchyma consists of 

 palisade cells (/>), which at^the leaf 

 margin grade into the cells char- 

 acterizing the lower chlorenchyma 

 (j); highly magnified. 



lacking in hydro- y 



phytes and not espe- 

 cially well developed 

 in mesophytes, are 

 d&veloped promi- 

 nently in many xero- 



of the leaf; note the three 

 palisade layers (/>), indicat- 

 ing relative xerophytism ; 

 highly magnified. 



FIG. 937. A cross sec- 

 tion through a leaf of the 

 fragrant olive (Osmanthus 

 fragrans), showing a T- 



- shaped sclereid (s), which to- 



particularly in those with stiff evergreen leaves J ge ther with the thick cuticle 

 the so-called sclerophylls. As noted elsewhere, the (c) accounts for the stiffness 

 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) er even 

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

 the leaves of xerophvtir grasses and sciigS_ paJuhes of 

 thick-walled bast fibers and other mechanical cells may 



OCCUr just henpath fhp ppirWmjt; (fifr 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. 93 7 J evergreen leaves con- 

 tain ij^laledjmechamcaicetisCsclerelds) 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 I 

 petioles have the collenchymatic thickening charac- \ 

 teristic of voung stems {D. 697}. The tearing of leaves 

 at their margins is prevented largely by the presence 

 there of an extra amount of cut?nfzation, 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. 



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