Page:Popular Science Monthly Volume 19.djvu/171

Rh, the arrow-root, a West Indian species allied to Canna, that if the plant has had a severe shock it can not get to sleep for the next two or three nights.

The sleep of flowers is also probably a case of the same kind, though, as I have elsewhere attempted to show, it has now, I believe, special reference to the visits of insects; those flowers which are fertilized by bees, butterflies, and other day insects, sleep by night, if at all; while those which are dependent on moths rouse themselves toward evening, as already mentioned, and sleep by day. These motions, indeed, have but an indirect reference to our present subject. On the other hand, in the dandelion (Leontodon), the flower-stalk is upright while the flower is expanded, a period which lasts for three or four days; it then lowers itself and lies close to the ground for about twelve days, while the fruits are ripening, and then rises again when they are mature. In the Cyclamen the stalk curls itself up into a beautiful spiral after the flower has faded.

The flower of the little Linaria of our walls (L. cymbalaria) pushes out into the light and sunshine, but as soon as it is fertilized it turns round and endeavors to find some hole or cranny in which it may remain safely ensconced until the seed is ripe.

In some water-plants the flower expands at the surface, but after it is faded retreats again to the bottom. This is the case, for instance, with the water-lilies, some species of the Potamogeton (Trapa natans). In Valisneria, again, the female flowers (Fig. 1, a) are borne on long stalks, which reach to the surface of the water, on which the flowers float. The male flowers (Fig. 1, b), on the contrary, have short, straight stalks, from which, when mature, the pollen (Fig. 1, c) detaches itself, rises to the surface, and, floating freely on it, is wafted about, so that it comes in contact with the female flowers. After fertilization, however, the long stalk coils up spirally, and thus carries the ovary down to the bottom, where the seeds can ripen in greater safety.

The next points to which I will direct your attention are the means of dispersion possessed by many seeds. Farmers have found by experience that it is not desirable to grow the same crop in the same field year after year, because the soil becomes more or less exhausted. In this respect, therefore, the powers of dispersion possessed by many seeds are a great advantage to the species. Moreover, they are also advantageous in giving the seed a chance of germinating in new localities suitable to the requirements of the species. Thus a common European species, Xanthium spinosum, has rapidly spread over the whole of South Africa, the seeds being carried in the wool of sheep. From various considerations, however, it seems probable that in most cases the provision does not contemplate a dispersion for more than a short distance.

There are a great many cases in which plants possess powers of movement directed to the dissemination of the seed. Thus, in