CHAP. II. RUPTURE OF THE SEED-COATS. 105 



With Mimosa pudica the radicle protrudes from a 

 small hole in the sharp edge of the seed ; and on its 

 summit, where united with the hypocotyl, a transverse 

 ridge is developed at an early age, which clearly aids 

 in splitting the tough seed-coats ; but it does not aid 

 in casting them off, as this is subsequently effected by 

 the swelling of the cotyledons after they have been 

 raised above the ground. The ridge or heel therefore 

 acts rather differently from that of Cucurbita. Its 

 lower surface and the edges were coloured brown by 

 the permanganate of potassium, but not the upper 

 surface. It is a singular fact that after the ridge has 

 done its work and has escaped from the seed-coats, 

 it is developed into a frill all round the summit of the 

 radicle.* 



At the base of the enlarged hypocotyl of Abronia 

 umbellata, where it blends into the radicle, there is a 

 projection or heel which varies in shape, but its out- 

 line is too angular in our former figure (Fig. 61). The 

 radicle first protrudes from a small hole at one end of 

 the tough, leathery, winged fruit. At this period the 

 upper part of the radicle is packed within the fruit 

 parallel to the hypocotyl, and the single cotyledon is 

 doubled back parallel to the latter. The swelling of 

 these three parts, and especially the rapid development 

 of the thick heel between the hypocotyl and radicle 

 at the point where they are doubled, ruptures the 

 tough fruit at the upper end and allows the arched 

 hypocotyl to emerge ; and this seems to be the function 

 of the heel. A seed was cut out of the fruit and 



* Our attention was called to at the junction of the radicle and 



this case by a brief statement by hypocotyl. This seed possesses a 



Nobbe in his ' Handbuch der very hard and tough coat, and 



Samenkunde,' 1876. p. 215, where would be likely to require aid in 



a figure is also given of a seedling bursting and freeing the cotyle- 



of Marty nia with a heel or ridge dons. 



