l8o PLANT ORGANS OR PARTS OF PLANTS. 



oblique at the base, entire, thin, yellowish-green or dull 

 green, nearly smooth; odor peculiar, somewhat tea-like, 

 taste mucilaginous, bitter, and nauseous. 



Histology. — The cross-section of the midrib shows a 

 woody pith, convex downward, whose vessels are placed 

 in radial, fan-shaped rows. In Cassia acutifolia they 

 are separated by broad medullary rows; in Cassia an- 

 gustifolia they are closer together. A small layer of 

 sieve-tissue surrounds the convex margin; this is bor- 

 dered by several layers of polygonal, lignified cells in the 

 form of a semicircle. 



The parenchymatous cells pass over into a collenchyma, 

 which is covered by epidermis and cuticle. Along the 

 upper side of the leaf the woody portion is covered by a 

 plano-convex or elliptical plate, consisting of several 

 layers of polygonal, lignified cells. Following this is a 

 layer of chlorophyll, containing palisade cells, which lie 

 close to the epidermis. In the mesophyll, a loose tissue, 

 containing vascular bundles, crystal sacs, and calcium 

 oxalate crystals, separates the broad upper palisade layer 

 from the narrow lower one. The cells of the epidermis 

 are polygonal, and both upper and lower surfaces arc 

 supplied with stomata and warty hairs. 



Powder. — This is brownish-green and has a leaf -like and 

 also a peculiar characteristic odor. The following elements 

 enter into the composition of the powder: Hairs, crystals, 

 parenchyma, chlorophyll grains, stomata, and the con- 

 stituents of the midribs and veinlets, fibres and spiral or 

 annular vessels, and occasionally pitted ducts. 



The hairs are characteristic ; they vary greatly in size, 

 reaching a length of 300 microns and usually averaging 

 about 25 microns at the base; they are usually sharply 

 bent either at the base or, more often, below the centre ; 

 the upper third is minutely roughened. 



The cells clustered about the scars and the number of 

 leaf scars found upon a fragment are characters used by 



