158 HISTOLOGY OF MEDICINAL PLANTS 



plants. In acer spicatium they are brown; in witch-hazel they 

 are gray; in xanthoxylium they are yellowish; and lastly, the 

 number of lenticels occurring in a given surface of the bark 

 should always be considered. 



On cross-sections the lenticel (Plate 57, Fig. 2) is seen to 

 have a central depressed portion maole up of loosely arranged 

 cells. Bordering the cavity are typical cork cells. The cork 

 cells immediately surrounding the lenticels are usually darker 

 in color, and many of the cells are partly broken down. 



The size of lenticels will vary according to the type of the 

 lenticel. In studying sections more attention should be paid 

 to the character of the cells forming the lenticels than to the 

 size of the lenticel. 



On cross-section the intercellular spaces (Plate 58) are tri- 

 angular, quadrangular, or irregular. The spaces between equal 

 diameter parenchyma cells is triangular if three cells surround 

 the space, and quadrangular if four cells surround the space, 

 etc. These spaces are in direct contact with similar spaces that 

 traverse the tissue at right angles to its long axis. 



The branched mesophyll cells of the leaf and aquatic plant 

 parenchyma (Plate 59) are arranged around irregular cavities. 

 In leaves and aquatic plants these spaces run parallel to the 

 long axis of the organ. 



In each of the above cases the cavity is formed by the sepa- 

 ration of the cell walls. There is still another type of irregular 

 cavities which is formed by the dissolution or tearing apart of 

 the cell walls. Such cavities are found in the stems and roots 

 of many herbs. 



The pith cells in the stems of many herbs become torn 

 apart during the growth of the stem, with the result that large 

 irregular cavities are formed. These cavities are usually filled 

 with circulatory air. 



In the stems of conium, cicuta, angelica, and other larger 

 herbaceous stems the pith separates into layers. When a 

 longitudinal section is made of such a stem it is seen to be com- 

 posed of alternating air spaces and masses of pith parenchyma. 



The intercellular spaces are very large in leaves where 

 enormous quantities of carbon dioxide are vitalized in photo- 

 synthesis. 



