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Show 102 RUP'rURE OF THE SEED-COATS. CHAP. II. Cucurbitacere, the seed-coats are ruptured by a curious contrivance, described by M. Flahault.* A heel or peg is developed on one side of the summit of the radicle or base of the hypocotyl; and this holds clown the lower half of the seed-coats (the radicle being fixed into the ground) whilst the continued growth of the arched hypocotyl forces upwards the upper half, and totu·~ asunder the seed-coats at one end, and the cotyledons are then easily withdrawn. The accompanying figure (Fig. 62) will render this description intelligible. Fortyone seeds of Oucurbita ovifera were laid on friable peat and were covered by a layer about an inch in thickness, not much prossed down, so that the cotyledons in being dragged up were subjected to very little friction, yet forty of them came up naked, the seedcoats being left buried in the peat. 'fhis was certainly dneto the action of the peg, for when it was prevented from acting, the cotyledons, as we shall presently see, were lifted up still enclosed in their seed-coats. 'fhey were, however, Fig. 62. Oucurbita ovifera : germinating seed, ohowiug the heel or peg projecting on one side from summit of radicle and holding down lower tip of seedcoats, which have been partially ruptured by the growth oft he arched hypocotyl. east off in the course of two or three days Ly tLe swelling of the cotyledons. Until this occurs light is excluded, and the cotyledons cannot decompose car· honic acid; but no one probably would have thought that the advantage thus gained by a little earlier cast- * 'Bull. Soc. Bot, do Frn.nce,' tom. xxiv. 1877, p. 201. CHAP. II. RUPTURE OF TilE SEED-COA'l'S. 103 ing off of the seefl-coats wonlcl be sufficient to account for the development of the peg. Yet, according to U. Flahault, seedlings which have been prevented from casting their seed-coats whilst beneath the ground, are inferior to those which have emerged with their cotyledons naked and ready to act. The peg is developed with extraordinary rapidity; for it could only just be distinguished in two seedlings, having radicles ·35 inch in length, but after an interval of only 24 hours was well developed in both. It is formed, according to Flahault, by the enlargement of the layers of the cortical parenchyma at the base of the hypocotyl. If, however, we judge by the effects of a solution of permanganate of potassium, it is developed on the exact line of junction between the h ypocoty I and radicle ; for the flat lower surface, as well as the edges, were coloured brown like the radicle ; whilst the upper slightly inclined surface was left uncoloured like the hypocotyl, excepting indeed in one out of 33 immersed seedlings in which a large part of the upper surface was coloured brown. Secondary roots sometimes spring from the lower surface of the peg, which thus seems in all respects to partake of the nature of the radicle. The peg is always developed on the side which beco'~es concave by the arching of the hypocotyl; and It would be of no service if it were formed on any other side. It is also always developed with the flat low?r side, which, as just stated, forms a part of the radicle, at right angles to it and in a horizontal plane. 'fJ~is fact was clearly show~ by burying some of the thm fl_at seeds in the same position as in Fig. 62, e~ceptmg that they were not laid on their flat broad Sides, but with one edge downwards. Nine seeds were thus planted, and the peg was developed in the |