Again, a species of the allied genus Lathyrus (Fig. 19), L. amphicarpos affords us another case of the same phenomenon.
Other species possessing the same faculty of burying their seeds are Okenla hypogæa several species of Commelyna, and of Amphicarpæa, Voandzeia subterranea, Scrophularia arguta, etc.; and it is very remarkable that these species are by no means nearly related, but belong to distinct families, namely, the Cruciferœ, Leguminosœ, Commelynaceæ Violaceæ, and Scrophulariaceæ.
Moreover, it is interesting that in L. amphicarpos as in Vicia amphicarpa and Cardamine chenopodifolium, the subterranean pods differ from the usual and aërial form in being shorter and containing fewer seeds. The reason of this is, I think, obvious. In the ordinary pods the number of seeds of course increases the chance that some will find a suitable place. On the other hand, the subterranean ones are carefully sown, as it were, by the plant itself. Several seeds together would only jostle one another, and it is therefore better that one or two only should be produced.
In the Erodiums, or cranesbills, the fruit is a capsule which openFig. 20.—Erodium glaucophyllum .(after Sweet). elastically, in some species throwing the seeds to some little distance. The seeds themselves are more or less spindle-shaped, hairy, and produced into a twisted hairy awn as shown in Fig. 20, representing a seed of E. glaucophyllum. The number of spiral turns in the awn depends upon the amount of moisture; and the seed may thus be made into a very delicate hygrometer, for, if it be fixed in an upright position, the awn twists or un-twists according to the degree of moisture, and its extremity thus may be so arranged as to move up) and down like a needle on a register. It is also affected by 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 downward, and, as M. Roux has shown, this mechanism thus serves actually to bury the seed. His observations were made on an allied species, Erodium ciconium, 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—i. e., as soon as the earth becomes in a condition to permit growth—the outer side of the awn contracts, and the hairs surrounding the seed commence to move outward, 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 upward, and it is obvious 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