IV.] ERODIUM. 89 heat. Now if the awn were fixed instead of the seed, it is obvious that during the process of untwisting, the seed itself would be pressed downwards, and, as M. Roux has shown, this mechanism thus serves actually to bury the seed. His observations were made on an allied species, Erodium cincoium, which he chose on account of its size. He found that if a seed of this plant is laid on the ground, it remains quiet as long as it is dry; but as soon as it is moistened —¿.e. as soon as the earth becomes in a condition to permit growththe outer side of the awn contracts, and the hairs surrounding the seed commence to move outwards, the result of which is gradually to raise the seed into an upright position with its point on the soil. The awn then commences to unroll and consequently to elongate itself upwards, and he suggests that as it is covered with reversed hairs, it will probably press against some blade of grass or other obstacle, which will prevent its moving up, and will therefore tend to drive the seed into the ground. If then the air becomes drier, the awn will again roll up, in which action M. Roux thought it would tend to draw up the seed, but from the position of the hairs the feathery awn can easily slip downwards, and would therefore not affect the seed. When moistened once more, it would again force the seed further FIG. 54.-Eurodium downwards, and so on until the proper depth was obtained. A species of Anemone (A. montana) again has essentially the same arrangement, though belonging to a widely separated order. A still more remarkable instance is afforded by a beautiful South European grass, Stipa pennata (Fig. 55), the structure of which has been described by Vaucher, and more recently, as well as more com pletely, by Frank Darwin. The actual seed is small, with a sharp point, and stiff, short hairs pointing backwards. The posterior end of the seed is produced into a fine twisted corkscrew-like rod, which is followed by a plain cylindrical portion, attached at an angle to the corkscrew, and ending in a long and beautiful feather, the whole being more than a foot in length. The long feather, no doubt, facilitates the dispersion of the seeds by wind; eventually, however, they sink to the ground, which they tend to reach, the seed being the heaviest portion, point downwards. So the seed remains as long as it is dry, but if a shower comes on, or when the dew falls, the spiral unwinds, and if, as is most probable, the surrounding herbage or any other obstacle prevents the feathers from rising, the seed itself is forced down and so driven by degrees into the ground. I have already mentioned several cases in which plants produce two kinds of seeds, or at least of pods, the one being adapted to burying itself in the ground. Heterocarpism, if I may term it so, or the power of producing two kinds of reproductive bodies, is not confined to these species. There is, for instance, a North African species of Corydalis (C. heterocarpa of 92 PLANTS WITH TWO KINDS OF SEEDS. [CHAP. Durieu) which produces two kinds of seed (Fig. 56), one somewhat flattened, short and broad, with rounded angles; the other elongated, hooked, and shaped like a shepherd's crook with a thickened staff. In this case the hook in the latter form perhaps serves for dispersion. Our common Thrincia hirta (Fig. 486) also possesses, besides the fruits with the well-known feathery crown, others which are destitute of such a provision, and which probably therefore are intended to take root at home. Mr. Drummond, in the volume of Hooker's Journal of Botany for 1842, has described a species of Alismacea which has two sorts of seed-vessels; the one produced from large floating flowers, the other at the end of short submerged stalks. He does not, however, describe either the seeds or seed-vessels in detail. Before concluding I will say a few words as to the very curious forms presented by certain seeds and IV.] SEEDS WHICH MIMIC ANIMALS. 93 fruits. The pods of Lotus, for instance, quaintly resemble a bird's foot, even to the toes; whence the specific name of one species, ornithopodioides; those of Hippocrepis remind one of a horse-shoe; those of Trapa bicornis have an absurd resemblance to the skeleton of a bull's head. These likenesses appear to be accidental, but there are some which probably are of use to the plant. For instance there are two species of Scorpiurus, Fig. 57, the pods of which lie on the ground, and so curiously resemble the one. (S. subvillosa, Fig. 57, a) a centipede, the other (S. vermiculata, Fig. 57, b) a worm or caterpillar, that it is almost impossible not to suppose that the likeness must be of some use to the plant. May it not be possible that in these cases birds carry the seeds some little distance before they find out that they are not really insects? The pod of Biserrula pelecinus (Fig. 58) also has a striking resemblance to a flattened centipede; while the seeds of Abrus precatorius, both in size and in their very striking colour, mimic a small beetle, Artemis circumusta. Mr. Moore has recently called attention to other cases of this kind. Thus the seed of Martynia diandra much resembles a beetle with long antennæ; several species of Lupins have seeds much like spiders, and those of Dimorphochlamys, a gourdlike plant, mimic a piece of dry twig. In the common Castor Oil plants (Fig. 58a), though the resemblance is not so close, still at a first glance the seeds might readily be taken for beetles or ticks. In many Euphorbiaceous plants, as for instance in Jatropha (Fig. 586), |