188 THE ELEMENTS OF VEGETABLE HISTOLOGY 



thin, and the cavities are filled with chloroplasts. The 

 palisade cells may be so grouped that several appear 

 to be in contact with one of the leaf parenchyma cells. 

 It is probable that the assimilation starch formed by 

 the chloroplasts is removed through these leaf par- 

 enchyma cells. Oil or resin cells occasionally occur 

 in the palisade layers. On surface view the palisade 

 cells may be seen through the epidermis, and appear 

 in small polygonal forms within the larger epidermal 

 cells. In powdered materials the palisade cells may 

 be seen in transverse section or on surface view. The 

 green masses frequently apparent in powdered leaves 

 consist of epidermal, palisade and perhaps leaf par- 

 enchyma cells, which hold together during powdering, 

 and which are so thick that the cellular structure 

 of each element in the mass is obscured. 



Leaf Parenchyma. — The mesophyll or leaf par- 

 enchyma elements represent the original tissues of 

 the leaf, or those which correspond to the tissues 

 present in the plerom zone of the stem. The mesophyll 

 consists of several layers of irregularly circular, thin- 

 walled cells, loosely connected and showing large 

 intercellular spaces (Plates 62, 63). This tissue fills 

 in the space between the upper and lower palisade 

 layers, or is the tissue between the epidermal layers 

 when the palisade is lacking. The cells contain 

 chloroplasts, and function partly as tissues for the 

 manufacture of starch and partly as tissues for the 

 transportation of the assimilation starch produced 

 in the palisade cells. Stone cells of the branched 

 type are occasionally present in the mesophyll layer. 

 Crystals may occur in the leaf parenchyma cells. 

 Starch grains in process of formation may be identified 

 by special methods. In powdered materials the leaf 



