LEA VES: STRUCTURE 9 1 



are either smooth, or hairy, or rough from outgrowths of the 

 epidermal cells. ^ These outgrowths are in the form of hairs, 

 glands and scales (as in Shepherdia). The hairs are simple or 

 bra.nched (in the mullein), some of the latter being star-shaped 

 (as in some oaks). These hairs and scales aid the leaf in retain- 

 ing moisture since evaporation of moisture from the surface is 

 hindered by them. 



146. Structure of the leaf in cross section. The epider- 

 mis and guard cells as seen in a cross section of a leaf have been 

 described above. The interior portion of the leaf consists of the 

 mesophyll and the fibro-vascular bundles. These may be studied 

 in cross and longitudinal section according to the way the veins 

 run in the portion of the leaf sectioned; and the parts in general, 

 the wood portion with its vessels and wood fibers, and the bast 

 portion with its bast, can be made out 



by consulting paragraphs 100-104. 

 The mesophyll usually consists of two 

 kinds of parenchyma cells, the pali- 

 sade layer of cells and the loose, 

 spongy tissue (fig. 76). The palisade 

 layer of cells is found usually just Fig. 76. 



Hpnpnfh fhp iinnpr prnVWrm's Tt rrm Section of ivy leaf (seen under 



Deneatn me upper epidermis, it con- the microscope)) pa iisade ceils 

 sists of elongated cells lying closely 

 side by side and perpendicular to 

 the epidermis. Sometimes there are 



two layers of palisade cells; in the compass plants, one layer on 

 each side of the leaf. The remaining part of the mesophyll is 

 the loose parenchyma, so-called because the intercellular spaces 

 are large, thus giving the cells a loose arrangement (fig. 76). 

 These intercellular spaces connect throughout the leaf and also 

 with the stomates. They thus provide for aeration of the leaf, 

 for the entrance and escape of gases in photosynthesis and 

 respiration, and for the escape of moisture. 



147. The chlorophyll bodies. The green color of plants 

 resides in definite bodies called chlorophyll bodies (except in some 

 rare cases) . These are minute, more or less oval, flattened bodies, 



