Page:Popular Science Monthly Volume 19.djvu/179

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167
ON FRUITS AND SEEDS.

thus hurls the seeds to some distance, being even itself sometimes also torn away from its attachment.

Other cases of projected seeds are afforded by Hura, one of the Euphorbiœ, Collomia, Oxalis, some species allied to Acanthus, and by Arceuthobium, a plant allied to the mistletoe, and parasitic on junipers, which ejects its seeds to a distance of several feet, throwing them thus from one tree to another.

Even those species which do not eject their seeds often have them PSM V19 D179 Seed head of poppy.jpgFig. 11.—Seed-head of Poppy(Papver.) so placed with reference to the capsule that they only leave it if swung or jerked by a high wind. In the case of trees, even seeds with no special adaptation for dispersion must in this manner be often carried to no little distance; and to a certain, though less extent, this must hold good even with herbaceous plants. It throws light on the (at first sight) curious fact that in so many plants with small, heavy seeds, the capsules open not at the bottom, as one might perhaps have been disposed to expect, but at the top. A good illustration is afforded by the well-known case of the common poppy (Fig. 11), in which the upper part of the capsule presents a series of little doors (Fig. 11, a), through which, when the plant is swung by the wind, the seeds come out one by one. The little doors are protected from rain by overhanging eaves, and are even said to shut of themselves in wet weather. The genus Campanula is also interesting from this point of of view, because some species have the capsules pendent, some upright, and those which are upright open at the top, while those which are pendent do so at the base.

In other cases the dispersion is mainly the work of the seed itself. In some of the lower plants, as, for instance, in many sea-weeds, and in some allied fresh-water plants, such as Vaucheria, the spores[1] are covered by vibratile cilia, and actually swim about in the water, like infusoria, till they have found a suitable spot on which to grow. Nay, so much do the spores of some sea-weeds resemble animals, that they are provided with a red "eye-spot" as it has been called, which, at any rate, seems so far to deserve the name that it appears to be sensitive to light. This mode of progression is, however, only suitable to water-plants. One group of small, low-organized plants {Marchantia) develop among the spores a number of cells with spirally thickened walls, which, by their contractility, are supposed to disseminate the spores. In the common horse-tails {Equisetum), again, the spores are provided with curious filaments, terminating in expansions, and known

  1. I need hardly observe that, botanically, these are not true seeds, but rather motile buds.