a a a. Granules and cells of cocoa; b b b. Arrowroot, Tous-les-mois; c c c. Tapioca starch. (Magnified 300 diameters.)
The starch grains of the potato are the best to study in the first instance on account of their large size ([Fig. 320]).
In arrowroot starch ([Fig. 321]) the stratification is almost as distinct as in that of the potato; the grains much resemble each other. Although somewhat smaller, the grains of arrowroot are more uniform in size. The starches are much used as an adulterant of drugs and various articles sold as cocoas.
Wheat-starch ([Fig. 322]) consists of circular flattened grains varying much in size, the central nucleus and stratification of which are very difficult to distinguish.
In the smaller starches the hilum becomes more indistinct, and without stratification, as in rice-starch, the latter being angular in shape. The hilum in other leguminous plants forms a longitudinal cleft; white rye-starch exhibits distinct cracks. Compound grains are occasionally met with, as in the oat. In [Plate XIII]. will be found small groups of starches taken under the same medium power for the sake of comparison. In the microscopical examination of starches first use a 2⁄3-inch or a ½-inch and then a 1⁄6-inch objective.
Fig. 322.
a. Husks of Wheat-starch, swollen by reagents and heat; b. A portion of cellulose; c. Rice-starch, magnified 420 diameters.
The bran of the husk of wheat when broken by grinding is seen to be composed of two coats of hexagonal cells, the outer of which is detached by the roasting process. The hexagonal cell layer is, however, so little altered as to be perfectly distinguishable under the microscope. Thus even a small admixture of roasted corn with coffee or chicory can be detected without much difficulty. As to whether starch granules should be regarded as crystalline or colloid bodies, a difference of opinion still prevails. There are, however, reasons for believing that the polarisation effects produced by starch grains are not due to crystalline structure but to stress or strain, of the same nature as the polarisation of glass when it is subject to strain. The polarising phenomena are precisely such as would be induced in any transparent solid composed of layers, the inner of which being kept in a state of stress by the compression exerted by the outer layers. Moreover, when by use of a swelling reagent, such as caustic potash solution, the outer wall of the starch is made to expand by the imbibition of water, the polarisation effects immediately disappear. Were the solid particles of crystal thus forced apart by water each particle would still exhibit polarisation phenomena.
Want of space will not permit me to further enlarge upon other micro-chemical substances that enter into the composition of plants; as, for example, the oil secreting glands. These when present take the place of starch. There is, however, one product among the cell contents of plants of some interest to the microscopist—those extremely fine crystals known as raphides, composed of calcium-phosphate and oxalate. Mr. Gulliver insisted upon the value of raphides as characteristic of several families of plants. Schleiden states that “needle-formed crystals, in bundles of from twenty to thirty in a cell, are present in almost all plants,” and that so really practical is the presence or absence of raphides, that by studying them he has been able to pick out pots of seedling Onagraceæ, which had been accidentally mixed with pots of other seedlings of the same age, and at that period of growth when no other botanical character would have been so readily sufficient.