In many flowers, the stamens, as they ripen, move so as to place themselves directly before the entrance to the nectary, where they are necessarily struck by any insect searching for honey; after the pollen is shed, they move aside or bend downward, and their place is taken by the pistil, so that an insect which has come from a younger flower will strike the part of the body previously dusted with pollen against the stigma, and deposit the pollen upon it. This arrangement is very beautifully seen in the nasturtium and larkspur ([Fig. 99], J).
The tubular flowers of the Sympetalæ are especially adapted for pollination by insects with long tongues, like the bees and butterflies, and in most of these flowers the relative position of the stamens and pistil is such as to ensure cross-fertilization, which in the majority of them appears to be absolutely dependent upon insect aid.
The great orchid family is well known on account of the singular form and brilliant colors of the flowers which have no equals in these respects in the whole vegetable kingdom. As might be expected, there are numerous contrivances for cross-fertilization among them, some of which are so extraordinary as to be scarcely credible. With few exceptions the pollen is so placed as to render its removal by insects necessary. One of the simpler contrivances is readily studied in the little spring-orchis ([Fig. 89]) or one of the Habenarias ([Fig. 90], G). In the first, the two pollen masses taper below where each is attached to a viscid disc which is covered by a delicate membrane. These discs are so placed that when an insect enters the flower and thrusts its tongue into the spur of the flower, its head is brought against the membrane covering the discs, rupturing it so as to expose the disc which adheres firmly to the head or tongue of the insect, the substance composing the disc hardening like cement on exposure to the air. As the insect withdraws its tongue, one or both of the pollen masses are dragged out and carried away. The action of the insect may be imitated by thrusting a small grass-stalk or some similar body into the spur of the flower, when on withdrawing it, the two pollen masses will be removed from the flower. If we now examine these carefully, we shall see that they change position, being nearly upright at first, but quickly bending downward and forward ([Fig. 89], D, ii, iii), so that on thrusting the stem into another flower the pollen masses strike against the sticky stigmatic surfaces, and a part of the pollen is left adhering to them.
The last arrangement that will be mentioned here is one discovered by Darwin in a number of very widely separated plants, and to which he gave the name “heterostylism.” Examples of this are the primroses (Primula), loosestrife (Lythrum), partridge-berry (Mitchella), pickerel-weed (Pontederia), ([Fig. 84], I), and others. In these there are two, sometimes three, sets of flowers differing very much in the relative lengths of stamens and pistil, those with long pistils having short stamens and vice versa. When an insect visits a flower with short stamens, that part is covered with pollen which in the short-styled (but long-stamened) flower will strike the stigma, as the pistil in one flower is almost exactly of the length of the stamens in the other form. In such flowers as have three forms, e.g. Pontederia, each flower has two different lengths of stamens, both differing from the style of the same flower. Microscopic examination has shown that there is great variation in the size of the pollen spores in these plants, the large pollen from the long stamens being adapted to the long style of the proper flower.
It will be found that the character of the color of the flower is related to the insects visiting it. Brilliantly colored flowers are usually visited by butterflies, bees, and similar day-flying insects. Flowers opening at night are usually white or pale yellow, colors best seen at night, and in addition usually are very strongly scented so as to attract the night-flying moths which usually fertilize them. Sometimes dull-colored flowers, which frequently have a very offensive odor, are visited by flies and other carrion-loving insects, which serve to convey pollen to them.
Occasionally, flowers in themselves inconspicuous are surrounded by showy leaves or bracts which take the place of the petals of the showier flowers in attracting insect visitors. The large dogwood ([Fig. 110], J), the calla, and Jack-in-the-pulpit ([Fig. 86], A) are illustrations of this.
CHAPTER XXI.
HISTOLOGICAL METHODS.
In the more exact investigations of the tissues, it is often necessary to have recourse to other reagents than those we have used hitherto, in order to bring out plainly the more obscure points of structure. This is especially the case in studies in cell division in the higher plants, where the changes in the dividing nucleus are very complicated.
For studying these the most favorable examples for ready demonstration are found in the final division of the pollen spores, especially of some monocotyledons. An extremely good subject is offered by the common wild onion (Allium Canadense), which flowers about the last of May. The buds, which are generally partially replaced by small bulbs, are enclosed in a spathe or sheath which entirely conceals them. Buds two to three millimetres in length should be selected, and these opened so as to expose the anthers. The latter should now be removed to a slide, and carefully crushed in a drop of dilute acetic acid (one-half acid to one-half distilled water). This at once fixes the nuclei, and by examining with a low power, we can determine at once whether or not we have the right stages. The spore mother cells are recognizable by their thick transparent walls, and if the desired dividing stages are present, a drop of staining fluid should be added and allowed to act for about a minute, the preparation being covered with a cover glass. After the stain is sufficiently deep, it should be carefully withdrawn with blotting paper, and pure water run under the cover glass.
The best stain for acetic acid preparations is, perhaps, gentian violet. This is an aniline dye readily soluble in water. For our purpose, however, it is best to make a concentrated, alcoholic solution from the dry powder, and dilute this as it is wanted. A drop of the alcoholic solution is diluted with several times its volume of weak acetic acid (about two parts of distilled water to one of the acid), and a drop of this mixture added to the preparation. In this way the nucleus alone is stained and is rendered very distinct, appearing of a beautiful violet-blue color.
If the preparation is to be kept permanently, the acid must all be washed out, and dilute glycerine run under the cover glass. The preparation should then be sealed with Canada balsam or some other cement, but previously all trace of glycerine must be removed from the slide and upper surface of the cover glass. It is generally best to gently wipe the edge of the cover glass with a small brush moistened with alcohol before applying the cement.