366 A TEXT-BOOK OF BOTANY. 



The green chlorophyll-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. Secretory cells or 

 receptacles occur in the chlorenchyma of many plants and corre- 

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

 palisade tissue occurs on both faces of the leaf blade with the pneu- 



FiG, 211. Transverse section of midrib of leaf of stramonium: EU, upper epidermis; 

 CO, collenchyma; PA, palisade cells; O, layer of cells containing rosette aggregates of 

 calcium oxalate; M, loose mesophyll; EL, lower epidermis; OP, prisms of calcium oxalate; 

 OS, sphenoidal micro-crystals of calcium oxalate; ST, stoma; T, tracheae; SU, leptome or 

 sieve on upper side of tracheae or vessels; SL, sieve on lower side of tracheae, this arrange- 

 ment of leptome or sieve and tracheae forming bicollateral fibrovascular bundles. 



matic tissue in the center, the leaf is called *' unifacial " or " iso- 

 lateral " (Figs. 211, 215) ; othervv'ise the leaf is said to be ^'bi- 

 facial " or *' dorsiventral," i.e., with two distinct surfaces. 



Mechanical tissues, as collenchyma 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- 



