Fig. 319.—Pollen Grains.
A. Pollen-grain of Clove-pink; B. Poppy; C. Passion-flower (Passiflora cœrulea); D. Cobœa scandens.
Starch Granules.—One of the most universally distributed materials found in plants is starch composed of two substances, granulose, which constitutes by far the largest part, and a skeleton of farinose. It is only the former of these that stains blue with iodine solutions; the latter partially assumes a brownish colour. The structure of starch granules is not of equal density throughout; the hilum or nuclear portion is most conspicuous, around which the rest of the material is deposited in layers, indicative of stratification. The several layers next to the hilum are less dense than those farthest from it. The position of the hilum determines the form of the grain, a few being rounded, others oval or elongated. The grain also contains different proportions of water; this conveys the appearance of concentric lines or curves about the nucleus. The latter is more conspicuous in the potato starches, as seen in [Plate XIII]., Nos. 6-15. Starch grains, in nearly all cases, are formed by the agency of proteid bodies, either chloroplasts or amyloplasts, and under the action of sunlight are gradually broken up and employed in the process of growth. There are some plants, however, notably the Compositeæ, in which another carbohydrate, inulin, takes the place of starch from the first, and is used as a reserve food material. For this reason we look in vain for starch in the cells of Inula, Taraxacum, &c. From the whole group of fungi starch is absent; this seems to explain the fact that chlorophyll, or colouring matter, is rarely met with in the fungi, hence their inability to utilize, like green plants, carbon-dioxide as food.
Fig. 320.—Swollen Potato Starch, after the application of potassium hydrate. (Magnified 210 diameters.)
The tissues which most commonly contain starch, or which contain it in largest quantity, are those of the parenchymatous series, though it sometimes occurs in the latex of laticiferous tissues, and even in ducts and tracheids. In the stems of Dicotyledons it occurs chiefly in the parenchyma of the middle and inner bark, in the medullary ray cells, and in the cells of the pith. In the roots of these plants it has a similar distribution, being for the most part confined to the middle or inner bark and the medullary rays, pith not being present in these organs. In succulent stems and roots, of course, it also commonly occurs in the xylem tissues of the fibro-vascular bundles.
A study of the various kinds of starches is important, since this material is very largely used as an adulterant. Other than microscopical means of detecting frauds are practically useless; assaying is tedious and expensive, while the microscope is always available and at hand. The limits of variation should be studied in starches from the same species of plants; the variations are not very wide, but in most cases characteristic, so that the discrimination is at all times an easy task. The reagents required are simply iodine and dilute potassium hydrate, aided by polarised light.
Fig. 321.