If the pollen is exposed to the wind it will be liable to be wetted and injured. Catkins—such as those of the walnut or hazel—give some protection to the pollen, since the stamens are covered in by tile-like scales; but where—as in the grasses and plantains—the anthers hang far out of the flowers, the pollen is easily injured. Some of the cereals protect themselves against injury by means of a remarkably rapid growth of the filaments; thus the anthers remain hidden within the flowers until the last moment, and, under the influence of a warm sunny morning, rapidly protrude themselves. If the scales of the flower are artificially separated, the growth can be produced by warmth and moisture; Askenasy describes a trick of country children, who put ears of rye in their mouths and thus produce a miraculous growth of stamens. The growth or rapid turgescence takes place, according to the same
writer, at the pace of one millimetre in three minutes.
The explosive male flowers of the nettle have a somewhat similar meaning. The young stamen is bent so that the upper end of the anther touches the base of the filament. On the inner concave side of the stamen are large cells, whose turgescence tends to unfold the filament: I do not know by what means the unfolding is prevented, but whatever the cause may be, it is at last overcome and the stamen uncurls with a jerk, and scatters forth the pollen. Here, as in the rye, the pollen is protected until the actual moment when it starts on its voyage through the air.
Another of the Nettle tribe, Pilea serpyllifolia—a plant often cultivated in our greenhouses—is also explosive, and its little puffs of smoke-like pollen have gained for it the popular name of the artillery plant. Its power of explosion must be of value to it as counterbalancing the disadvantage, to a wind-fertilised plant, of such a lowly habit.
The adaptations found in the female organs are chiefly such as increase the surface capable of receiving the pollen, and therefore increase the chance of fertilisation. A big stigmatic surface is common: not only is the receptive part of the style large, but it usually bears very large stigmatic papillæ, which gives a velvety hoary look to this type of stigma. In the grasses the three divisions of the stigma are always more or less conspicuous; and reach a climax, in this respect, in the huge beard-like tangle of the maize.
Some of the most interesting cases of wind fertilisation are those in which an isolated instance occurs in a Natural Order otherwise served by insects. Thus in the Rosaceæ, Poterium sanguisorba is wind fertilised, and has long pendent stamens, and a tufted stigma; while the closely allied Sanguisorba officinalis, although it secretes nectar (and this can only mean that it hopes to attract insects), retains the tufted stigma of its anemophilous relatives.
In the case of the Kerguelen cabbage (Pringlea antiscorbutica), the cause of its degeneration seems to be the want of winged insects on the wind-blown shores on which it grows. It has acquired some anemophilous characters—e.g., increased stigmatic surface and exserted anthers. Its flowers are inconspicuous like those of wind-fertilised plants in general, and it seems in fair way to lose its petals altogether—many flowers only retaining a single one. The entomophilous ancestry of Pringlea is clearly shown by the occasional remnants of coloured markings in the petals, like those which in other flowers serve as finger-posts to visiting-insects, and are called nectar-guides.
But these are digressions—sidepaths of tempting detail which have lured me from the straight highway. However, they have brought me back to the main road.
In Blomefield’s Observations in Natural History (p. 332), he points out that “however much the seasons may differ in different years, the phenomena generally follow one another in the same order. And it follows that those which occur together any
one year, will occur at or nearly [at] the same time every other.” This indeed is what we might expect, from the circumstances of any interruption in the time of their occurrence, due to seasonal influence, necessarily affecting them all equally. One of the examples by which he supports his view is the parallel behaviour of the ground-ivy (Nepeta Glechoma) and the box-tree, whose flowers appear simultaneously on 3rd April, as an average date; while in a certain backward year they flowered later, but still close together—namely, 20th April and 19th April. There is to me an especial charm in these duets. Thus I like to imagine that the larch is waiting to put on its new green clothes till it hears the black-cap. Or is it that the larch rules the orchestra, and with his green baton signals to the songster to strike into the symphony? [11]