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Show I I : 82 HYPOCOTYLS, EPICOTYLS, ETC., CHAP. II. ·3 inch in length, and was quite straight ; but from having increased in thickness it had just begun to split open the lower part of the petioles on one si.cle, along the line of their confluence. By the followmg mornino- the upper part of the plumule had arched b itself into a right angle, and the Fig. 58• A. convex side or elbow had thus been ~ forced out through the slit. Here f~\-\\ then the arching of the plumule ~~ .!~.~) G--- ---~~-- '-\\_------ G plays the same part as in the case of · b the petioles of the Delphinium. As i~~-"-,~) '1 1 (( the plumule continued to grow, the l ~ tip became more arched, and in 1 the course of six days it emerged ) .. ~ ,:li pt through the 2~ inches of superin- 1\ /, cumbent soil, still retaining its ·"·.- /J'~ arched form. After reaching the '•,' .:~> 1 "' "' surface it straightened itself in the \_, ~~ usual manner. In the accompany- /\, \1 ) h 1 ' inbo- figure (Fig. 58, A we ave a ( ') atr~ Ji ' sketch of a seedling in this ad- ~nJT ,· ... ("'"!\ 't vanced state of development; the ' ~ 1 1 • f\ surface of the ground being re- .ltfeqan·Mza CuW01·n~ca: presented by the line G-·······G. . ske~ch of seedling, The germination of the seeds lll cop1ed from A sa Gray, . • . . . reduced to one-half theu native Cahforman home pro-sc~ le_: c, cotyledons ceeds in a rather different manner, w1thm seed-coats; p, . the t·wo confluent as we infer from an interestmg petioles; h and ?'•. hy- l tt . f lVI . Rattan sent to us poco! yl and radicle j e er rom r. ' . pl, plumule; G ...... G, by Prof. Asa Gray. The petwles surface of soil. protru d e f rom t h e see d s soon af tcr 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. Hattan during the Christmas vacation, with the plu· CHAP. II. BREAKING THROUGH THE GROUND. 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 roothairs on the petioles would supply them with sufficient moisture. We shall hereafter see that many seedlings are protected from frost, but by a wi<lely 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 ~epth of some inches beneath the surface. Ultimately It grows to a huge size. Ipomcea 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 ·~ 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 G 2 |