Lastly, nature shows us the use of the peg; for in the one Cucurbitaceous genus known to us, in which the cotyledons are hypogean and do not cast their seed-coats, namely, Megarrhiza, there is no vestige of a peg. This structure seems to be present in most of the other genera in the family, judging from Flahault's statements' we found it well-developed and properly acting in Trichosanthes anguina, in which we hardly expected to find it, as the cotyledons are somewhat thick and fleshy. Few cases can be advanced of a structure better adapted for a special purpose than the present one. [page 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 outline 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 this case by a brief statement by Nobbe in his 'Handbuch der Samenkunde,' 1876, p. 215, where a figure is also given of a seedling of Martynia with a heel or ridge at the junction of the radicle and hypocotyl. This seed possesses a very hard and tough coat, and would be likely to require aid in bursting and freeing the cotyledons. [page 106]
allowed to germinate in damp air, and now a thin flat disc was developed all round the base of the hypocotyl and grew to an extraordinary breadth, like the frill described under Mimosa, but somewhat broader. Flahault says that with Mirabilis, a member of the same family with Abronia, a heel or collar is developed all round the base of the hypocotyl, but more on one side than on the other; and that it frees the cotyledons from their seed-coats. We observed only old seeds, and these were ruptured by the absorption of moisture, independently of any aid from the heel and before the protrusion of the radicle; but it does not follow from our experience that fresh and tough fruits would behave in a like manner.
In concluding this section of the present chapter it may be convenient to summarise, under the form of an illustration, the usual movements of the hypocotyls and epicotyls of seedlings, whilst breaking through the ground and immediately afterwards. We may suppose a man to be thrown down on his hands and knees, and at the same time to one side, by a load of hay falling on him. He would first endeavour to get his arched back upright, wriggling at the same time in all directions to free himself a little from the surrounding pressure; and this may represent the combined effects of apogeotropism and circumnutation, when a seed is so buried that the arched hypocotyl or epicotyl protrudes at first in a horizontal or inclined plane. The man, still wriggling, would then raise his arched back as high as he could; and this may represent the growth and continued circumnutation of an arched hypocotyl or epicotyl, before it has reached the surface of the ground.