“Parenchyma is optically inactive, and is not stained by iodine solution, except in so far as its contents are concerned. Sclerenchyma, the stone cell, is optically active, and in the dark field of the microscope, with crossed Nicols, appears as shining silvery cells, displaying their real structure. The fibro-vascular bundles are stained yellowish brown by iodine, and are thus differentiated from the surrounding tissue.
“Starch is stained a deep blue, or blue black, by iodine solution, and since the contents of the parenchyma cells often consist of much starch, the parenchyma in these cases seems to assume this colour.
“To distinguish some of the peculiarities of structure which have been mentioned requires some little practice and skill, but not more than is readily acquired with a short experience. There are however some aids which should not be neglected.
“In the ground spices it will be found more difficult to recognise the anatomy of the parts than in a carefully prepared section. The hardest parts are often the largest particles, and scarcely at all transparent. The mounting of the material in water or glycerine will render them more so, but it is necessary to employ some other means of which two are available. A solution of chloral hydrate in water, 8 to 5, serves after 24 hours to make the particles less obscure. In many instances also, it has been found advisable to bleach the deep colour by Schulze’s method, using nitric acid of 1·1 sp. gr. and chlorate of potash. When this is done, hard tissue is broken down and rendered transparent where otherwise nothing could be seen. As examples, olive stones and cocoa-nut shells will serve. Without treatment little can be made out of their structure.
“Of course, it is plain that the detection of starch must be in a portion of the material which has received no treatment, and that progress must be made from the least to the most violent reagents.
“For this work an elaborate microscope is unnecessary. It should, for work with starches, have objectives of ½ and 1⁄5 inch equivalent focus, arrangements for polarising light, and if possible, a condenser system. Many good stands are to-day made at reasonable prices which will serve the purpose.”
The microscopical appearance of various starches in polarised light is shown in Plate IX. Plate XII. exhibits several spices, under polarised light, in a pure and adulterated state. Those represented are:—
Ginger, pure, and adulterated with foreign starch.
Cinnamon and Cassia; the pure barks, ground, showing the relative greater frequency of fibro-vascular bundles in the former.
Cayenne, pure, and adulterated with rice starch.