102 



RUPTURE OF THE SEED-COATS. Chap. IL 



Fig. 62. 



Cucurbitaceae, 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 down 

 the lower half of the seed-coats (the radicle being 

 fixed into the ground) whilst the continued growth of 

 the arched hypocotyl forces up- 

 wards the upper half, and tears 

 asunder the seed-coats at one end, 

 and the cotyledons are then easily 

 withdrawn. The accompanying 

 figure (Fig. 62) will render this 

 description intelligible. Forty- 

 one seeds of Cucurbita ovifera 

 were laid on friable peat and were 

 covered by a layer about an inch 

 in thickness, not much pressed 

 down, so that the cotyledons in 

 being dragged up were subjected 

 to very little friction, yet forty of 

 them came up naked, the seed- 

 nating seed, showing the coats being left buried in the peat. 

 heel or peg projecting TMs was certainly due to the action 



on one side from summit „ , <• i 



of radicle and holding 01 the peg, lOr When it Was prC- 



down lower tip of seed- ygnted from acting, the cotyledons, 



coats, which have been in i 



partially ruptured by as we shall presently see, were 

 the growth of the arched ^f^g^ y„ ^^^ enclosed in their 



hypocotyl. ^ 



seed-coats. Ihey were, however, 

 cast off in tne course of two or three days by the 

 swelling of the cotyledons. Until this occurs light is 

 excluded, and the cotyledons cannot decompose car- 

 bonic acid ; but no one probably would have thought 

 that the advantage thus gained by a little earlier cast« 



* 'Bull. Soc. Bot. de France,' torn. xxiv. 1S77, p. 201. 



