1 98 PLANT ORGANS OR PARTS OF PLANTS. 



delicate, and are either annular ly or spirally thickened; 

 pitted ducts may be found in specimens which have an 

 appreciable quantity of the stout stems present. 



Fibres are also few in number in the powder. Bast 

 fibres are apt to be isolated. Wood fibres are in groups. 

 The walls of each type of fibre are but slightly thickened. 



Fragments of delicate, fiat tissue, derived from the 

 plant corolla, may be found; these are slightly colored, 

 usually brownish, and readily recognized. 



Pollen grains are also not infrequent. The grains are 

 echinate, roughened with minute spines, are globular, 

 sometimes pointed at one side, to ovoid triangular. 

 The surface is undulate, and they average in size, exam- 

 ined in oil, 18.5 to 21.5 by 20 to 21 microns, being almost 

 double the size of the pollen grains of Mentha viridis, 

 which, moreover, are distinctly triangular. 



Collenchymatic tissues of no particular characters 

 are present ; these are derived from the structures of the 

 midrib and the angles of the stem. 



Chemistry. — The plant contains resin, tannin, gum, and 

 from I to 1 . 2 5 per cent, of ethereal oil. The perfectly fresh 

 leaves contain 0.3 per cent. If the leaves are carefully 

 dried, there is little or no loss, as the cuticle of the epi- 

 dermis and glands does not permit of much evaporation. 

 The ethereal oil is extremely complex, it is greenish 

 yellow, thickens on standing, is soluble in equal volumes 

 of alcohol, specific gravity 0.910, and consists of numer- 

 ous terpenes, at least fifteen in number, with the general 

 formulaB CjoH^g and CjgHjg; also menthon, C^^^fi, in 

 which the crystallizable stearopten menthol, CjoH^gOH, 

 is contained. 



ERYTHROXYLON. COCA. 



Erythroxylon is the leaves of Erythroxylon coca. 

 This is a shrub, three to six feet in height, a native of 

 western South America, growing in and aBout the moun- 



