Fig. 58, A. Megarrhiza Californica:  sketch of seedling, copied from Asa Gray, reduced to one-half scale:  c, cotyledons within seed-coats; p, the two confluent petioles; h and r, hypocotyl and radicle; p1, plumule; G..........G, surface of soil.

The germination of the seeds in their native Californian home proceeds in a rather different manner, as we infer from an interesting letter from Mr. Rattan, sent to us by Prof.  Asa Gray.  The petioles protrude from the seeds soon after the autumnal rains, and penetrate the ground, generally in a vertical direction, to a depth of from 4 to even 6 inches. they were found in this state by Mr. Rattan during the Christmas vacation, with the plu-[page 83] mules still enclosed within the tubes; and he remarks that if the plumules had been at once developed and had reached the surface (as occurred with our seeds which were exposed to a high temperature), they would surely have been killed by the frost.  As it is, they lie dormant at some depth beneath the surface, and are thus protected from the cold; and the root-hairs on the petioles would supply them with sufficient moisture.  We shall hereafter see that many seedlings are protected from frost, but by a widely different process, namely, by being drawn beneath the surface by the contraction of their radicles.  We may, however, believe that the extraordinary manner of germination of Megarrhiza has another and secondary advantage.  The radicle begins in a few weeks to enlarge into a little tuber, which then abounds with starch and is only slightly bitter.  It would therefore be very liable to be devoured by animals, were it not protected by being buried whilst young and tender, at a depth of some inches beneath the surface.  Ultimately it grows to a huge size.

Ipomoea leptophylla.—­In most of the species of this genus the hypocotyl is well developed, and breaks through the ground as an arch.  But the seeds of the present species in germinating behave like those of Megarrhiza, excepting that the elongated petioles of the cotyledons are not confluent.  After they have protruded from the seed, they are united at their lower ends with the undeveloped hypocotyl and undeveloped radicle, which together form a point only about .1 inch in length.  They are at first highly geotropic, and penetrate the ground to a depth of rather above half an inch.  The radicle then begins to grow.  On four occasions after the petioles had grown for a short distance vertically downwards, they [page 84] were placed in a horizontal position in damp air in the dark, and in the course of 4 hours they again became curved vertically downwards, having passed through 90o in this time.  But their sensitiveness to geotropism lasts for only 2 or 3 days; and the terminal part alone, for a length of between .2 and .4 inch, is thus sensitive.  Although the petioles of our specimens did not penetrate the ground to a greater depth than about ½ inch, yet they continued for some time to grow rapidly, and finally attained