146 BROWN: Woops FOR MICROSCOPIC STUDY 
D. ESSENTIAL OILS, RESINS, AND GUM-RESINS 
Substances of this description occur abundantly in woody 
tissues of tropical dicotyledons, in the'cell lumina or in intercellular 
canals (Guignard, ’92; Record, ’18), in the form of globules, 
transverse plates or irregular masses. As before indicated, such 
substances may readily be distinguished from the gums by the 
fact that after drying they do not swell in water; also, by their 
solubility in chloroform. The gum-resins, however, may have 
some of the properties of each component. To determine the 
solubility in chloroform, sections (particularly those from seasoned 
blocks) should remain twenty-four hours in the solvent. 
The essential oils, which often possess an aromatic odor, 
resemble the resins except that such bodies readily dissolve when 
absolute alcohol is drawn beneath the cover glass. 
E. FATS : 
The fatty oils resemble, in appearance, the essential oils with 
which they often occur in the same cell. Both are stained by 
Sudan III. The fatty oi’s are distinguished from essential oils 
‘from the fact that absolute alcohol dissolves few fatty oils, but 
does dissolve the essential oils; strong potassic hydrate saponifies 
fatty oils, but not essential oils, although some complex resins 
which break down into a fatty component may appear to be 
saponified; a temperature of 130° C. volatilizes essential oils but 
not the fats. 
F. TANNIN 
The presence of tannin in tissues is readily demonstrated by 
familiar tests. A bluish or greenish black is caused by a neutral- 
ized solution of ferric chloride, and a reddish yellow by ammonium 
molybdate. 
G. MINERAL CRYSTALS 
Vertical strands of short cells, each with a single crystal of 
calcium oxalate nearly filling the cavity, are of frequent occurrence 
in tropical woods, being found in over 26 per cent of the Hawaiian 
woods studied by writer. Less frequently they occur in ray-cells 
and other tissues. Their composition is determined from the 
fact that calcium oxalate dissolves without effervescence in 
