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Show 556 SUMMARY AND CHAP. XII. the upper part of the hypocotyls of at least some plants. As the arch grows upwards the cotyledons are dragged out of the ground. The seed-coats are either left behind buried, or are retained for a time still enclosing the coty lodons. These are afterwards cast off merely by the swelling of the coty lodons. But with most of the Oucurbitacere there is a curious special contrivance for bursting the seed-coats whilst beneath the ground, namely, a peg at the base of the hypocoty I, projecting at right angles, which holds down the lower half of the seed-coats, whilst the growth of the arched part of the hypocotyllifts up the upper half, and thus splits them in twain. A somewhat analogous structure occurs in Mimosa pudica and some other plants. Before the cotyledons are fully expanded and have diverged, the hypocotyl generally straightens itself by increased growth along the concave side, thus reversing the process which caused the arching. Ultimately not a trace of the former curvature is left, except in the case of the leaf-like cotyledons of the onion. The cotyledons can now assume the function of leaves, and decompose carbonic acid; they also yield up to other parts of the plant the nutriment which they often contain. When they contain a large stock of nutriment they generally remain buried beneath the ground, owing to the small development of the hypocotyl; and thus they have a better chance of escaping destruction by animals. From unknown causes, nutriment is sometimes stored in the hypocotyl or in the radicle, and then one of the cotyledons or both become rudimentary, of which several instanc~s have been given. It is probable that the ex.traor?I· nary manner of germination of Megarrhiza Oalifornwa, CHAP. XII. CONCLUDING REMARKS. 557 Jpomma leptophylla ~nd pandurata, and of Quercus virens, is connected with the burying of the tuber-like roots, which at an early ago arc stot:ked with nutriment; for in those plants it is the petioles of the cotyledons which first protrudo from tho seeds and they are then merely tipped with a minute radicl~ and hypocotyl. These petioles bend down geotropically like a root and penetrate the ground, so that the true root, which aft~rwards becomes greatly enlarged, is buried at some httle depth beneath the surface. Gradations of structure are always interesting, and Asa Gray informs us that with Ipomcea Jalappa, which likewise forms huge tubers, the hypocoty 1 is still of considerable length, and the petioles of the eoty ledons are only moderately elongated. But in addition to the advantage gained by the concealment of the nutritious matter stored within the tubers, the plumule, at least in the case of Megarrhiza, is protected from tho frosts of winter by being buried. With many dicotyledonous seedlings, as has lately been described by De Vries, the contraction of the parenchyma of the upper part of the radicle drags the hypocotyl downwards into the earth; sometimes (it is said) until even the cotyledons are buried. The hypocotyl itself of some species contracts in a like manner. It is believed that this burying process serves to protect the seedlings against the frosts of winter. O?r imaginary seedling is now mature as a seedling, for Its hypocotyl is straight and its cotyledons arc fully expanded. In this state the upper part of the hypocotyl and the cotyledons continue for some tjme ro circu~nutate, generally to a wide extent relatively to th~ size of the parts, and at a rapid rate. But ~ee~hngs profit by this power of movement only when It IS modified, especially by the action of light and |