LEAVES: STRUCTURE 85 



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

 (long slender cells or rows of cells), glands (special cells for excret- 

 ing various substances) and scales (as in Shepherdia}. The hairs 

 are simple or branched (in the mullein), some of the latter being 

 star-shaped (as in some oaks). These hairs and scales aid the 

 leaf in retaining moisture since evaporation of moisture from the 

 surface is hindered. 



144. 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 

 94-98. The mesophyll usually consists of two kinds of paren- 

 chyma cells, the palisade layer of cells , . ., 

 and the loose, spongy tissue. The pali- 

 sade layer of cells is found usually just 



beneath the upper epidermis. It consists 

 of elongated cells lying closely side by 

 side and perpendicular to the epidermis. 

 Sometimes there are two layers of pali- 



Section of ivy leaf, palisade cells 

 Sade Cells; in the COmpaSS plants, One above, loose parenchyma, with 



large intercellular spaces in center, 

 layer On each Side 01 the leaf. The Epidermal cells on either edge. 



with no chlorophyll bodies. 



remaining part of the mesophyll is the 



loose parenchyma, so-called because the intercellular spaces are 

 large, thus giving the cells a loose arrangement. These inter- 

 cellular 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. 



145. The chlorophyll bodies. The green color of leaves 

 (as well as of other parts of plants except in some rare cases) 

 resides in definite bodies called chlorophyll bodies. These are 

 minute, more or less oval, flattened bodies, of a soft and plasmic 





