LEAVES. 177 



tubes have very delicate walls, 0.045 niicrons in diameter, 

 and in dried specimens are made out with difficulty. 

 The sieve plates are horizontal. 



LEAVES. 



For the purposes of the general student of pharmacog- 

 nosy little or nothing need be said regarding the general 

 shape, size, margins, apices, and bases of leaves, but 

 attention is here given to the microscopical character- 

 istics that present themselves in the determination of the 

 leaves that are broken or powdered. Leaf structures are 

 very characteristic. 



In order to correctly understand this microscopical 

 structure it will be necessary to recall that in the leaf 

 there are to be found parts of three types of tissues; 

 the Epidermal, the Respiratory, and the Conducting 

 Systems. The surface is mainly made up of the epider- 

 mal tissues; the leaf is built up, and around the veins of 

 the leaf, that is, about the conducting tissue, and in 

 between are to be found the tissues of respiration and 

 assimilation. 



The Epidermis. — From a diagnostic standpoint this is 

 the most imp>ortant of the tissues in broken or powdered 

 leaves. The epidermis of leaves is a continuation of the 

 epidermis of the stem, and is fotmd to consist generally 

 of one layer of tissue completely surrounding the leaf 

 surfaces. It consists of isodiametric cells that fit into 

 one another without intercellular spaces. Usually the 

 outer wall of the cells is strongly cutinized, the degree 

 of cutinization varying, as a rule, according to the amount 

 of heat or cold the leaves are called upon to stand. In 

 leaves that are normally horizontal the upper or more 

 exposed side has epidermal cells whose walls are generally 

 more markedly cutinized than are those on the lower or 

 less exposed side. 



