2IO BOTANY AND THARMACOGNOSY. 



in the lower surface, and if they are much reticulated in addition, 

 the surface is described as reticulate. 



STRUCTURE OF THE LEAF. 



In all green leaves the typical structure is as follows : A cuticle 

 covers the outer cell-wall of the epidermis, while the epidermis 

 itself shows much of the same modifications as exist in the stem; 

 frequently the lumen of the cells of the epidermis is wider on the 

 ventral face than on the dorsal. Hairs abound on the leaves in 

 many plants and stomata are especially frequent on the dorsal 

 surface. The upper epidermis may further be characterized by 

 the presence of water-pores, the origin and function of which have 

 already been described (p. 193). 



The green chlorophyl-bearing tissue is called chlorenchyma 

 (frequently spoken of as mesophyll), and is frequently differen- 

 tiated into a ventral palisade tissue, composed of long cells 

 which are placed vertically to those of the epidermis ; and a 

 DORSAL pneumatic TISSUE, made up of irregularly branched or 

 lobed cells with very large intercellular spaces. Secreting ducts 

 or cells occur in the chlorenchyma of many plants and correspond 

 with those found in the cortex of the stem. When the palisade 

 tissue is distributed on both faces of the leaf blade, the pneu- 

 matic tissue is thus located in the center, the leaf is called 

 "bifacial," otherwise the leaf is said to be " uni facial " or 

 " dorsi ventral." (See Figs. 117, 139, 141, 158, 175, 257, 261, etc.) 



Mechanical tissues, as coUenchyma and stereome, are frequent 

 and these accompany the veins as hypodermal strands, being best 

 developed usually on the dorsal face of the latter, as underneath 

 the leptome. The mestome-strands of the leaf blade generally 

 lie in a single plane. They are collateral and have the leptome 

 situated towards the dorsal face. They are nearly always sur- 

 rounded by thin- walled parenchyma-sheaths, or as in several 

 grasses and sedges by thick-walled mestome-sheaths. In some 

 plants of various families, the midrib is not only stronger devel- 

 oped than the lateral veins, but it may be composed of several, 

 instead of only one, mestome-strand, sometimes representing a 

 true stele. 



