Common objects of the microscope

by J. G. Wood

In the earliest stages of their growth the starch granules appear to be destitute of these markings, or at all events they are so few and so delicate as not to be visible even with the most perfect instruments, and it is not until the granules assume a comparatively large size that the external markings become distinctly perceptible.

We will now glance at the examples of starch which are given in the Plate, and which are a very few out of the many that might be figured. Fig. [2] represents the starch of wheat, the upper grain being seen in front, the one immediately below it in profile, and the two others being examples of smaller grains. Fig. [6] is a specimen of a very minute form of starch, where the granules do not seem to advance beyond their earliest stage. This specimen is obtained from the parsnip; and although the magnifying power is very great, the dimensions of the granules are exceedingly small, and except by a very practised eye they would not be recognisable as starch grains.

Fig. [3] is a good example of a starch grain of wheat, exemplifying the change that takes place by the combined effects of heat and moisture. It has already been observed that cold water exercises little, if any, perceptible influence upon starch; but it will be seen from the illustration that hot water has a very powerful effect. When subjected to the action of water at a temperature over 140° Fahr., the granule swells rapidly, and at last bursts, the contents escaping in a gelatinous mass, and the external membrane collapsing into the form which is shown in Fig. [3], which was taken out of a piece of hot pudding. A similar form of wheat starch may also be detected in bread, accompanied, unfortunately, by several other substances not generally presumed to be component parts of the “staff of life.”

In Fig. [7] are represented some grains of starch from West Indian arrowroot, and Fig. [8] exhibits the largest kind of starch grain known, obtained from the tuber of a species of canna, supposed to be C. edúlis, a plant similar in characteristics to the arrowroot. The popular name of this starch is “Tous les Mois,” and under that title it may be obtained from the opticians, or chemists.

Fig. [10] shows the starch granules from Indian corn, as they appear before they are compressed into the honeycomb-like structure which has already been mentioned. Even in that state, however, if they are treated with iodine, they exhibit the characteristics of starch in a very perfect manner. Fig. [11] is starch from sago, and Fig. [12] from tapioca, and in both these instances the several grains have been injured by the heat employed in preparing the respective substances for the market.

Fig. [13] exhibits the granules obtained from the root of the water-lily, and Fig. [14] is a good example of the manner in which the starch granules of rice are pressed together so as to alter the shape and puzzle a novice. Fig. [16] is the compound granule of the oat, which has already been mentioned, together with some of the simple granules separated from the mass; and Fig. [15] is an example of the starch grains obtained from the underground stem of the horse-bean. It is worthy of mention that the close adhesion of the rice starch into those masses is the cause of the peculiar grittiness which distinguishes rice flour to the touch.

Whilst very easily acted on by heat, starch-granules are very resistent to certain other reagents. Weak alkalies, in watery solution, readily attack them, but by treating portions of plants with caustic potash dissolved in strong spirit, the woody and other parts may be dissolved away; and after repeated washing with spirit the starch may be mounted. This, however, must never be in any glycerine medium, except that given on p. [172].

In Plate III. Fig. [1], may be seen a curious little drawing, which is a sketch of the laurel-leaf cut transversely, and showing the entire thickness of the leaf. Along the top may be seen the delicate layer of “varnish” with which the surface of the leaf is covered, and which serves to give to the foliage its peculiar polish. This varnish is nothing more than the translucent matter which binds all the cells together, and which is poured out very liberally upon the surface of the leaf. The lower part of this section exhibits the cells of which the leaf is built, and towards the left hand may be seen a cut end of one of the veins of the leaf, more rightly called a wood-cell.

We will now examine a few examples of surface cells.

Fig. [5] is a portion of epidermis stripped from a Capsicum pod, exhibiting the remains of the nuclei in the centre of each cell, together with the great thickening of the wall-cells and the numerous pores for the transmission of fluid. This is a very pretty specimen for the microscope, as it retains its bright red colour, and even in old and dried pods exhibits the characteristic markings.