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Show 80 HYPOCOTYLS, EPICOTYLS, ETC., 0IIAP. II. either had proved decidedly advantageous or disadvantageous, one of them no doubt would soon have prevailed. . Asa Gray has described* the pecuhar m~nner.of ger-mination of three widely different plants, 1n whiCh the hypocotyl is hardly at all developed. rrhese were t~erefore observed by us in relation to our present s?bJect. Delphinium nudicaule.-The elongated petwle~ of the two cotyledons are confluent (as are somet1mos their blades at the base), and they break through the ground as an arch. They thus resemble in a most deceptive manner a hypocotyl. At first they are solid, but after a time become tubular; and the basal part beneath the ground is enlarged into a hollow chamber, within which the young leaves are developed without any prominent plumule. Externally roothairs are formed on the confluent petioles, either a little above, or on a level with, the plumule. The first leaf at an early period of its growth and whilst within the chamber is quite straight, but the petiole soon becomes arched ; and the swelling of this part (and probably of the blade) splits open one siCle ~f the chamber, and the leaf then emerges. The slit was found in one case to be 3 · 2 mm. in length, ancl it is seated on the line of confluence of the two petioles. The leaf when it first escapes from tho chamber is buried beneath the ground, and now an upper part of the petiole near the blade becomes arched in the usual manner. The second leaf comes out of the slit either straight or somewhat arched, b~t afterwards the upper part of the petiole,-certainly Ill some, and we believe in all cases,-arches itself whilst forcing a passage through the soil. * 1 Botanical Text-Book,' 1879, p. 22. CuAP. II. BREAKING 'rHROUGH THE GROUND. 81 Megarrhiza Californica.-The cotyledons of this Gourd never free themselves from the seed-coats and are hypogean. Their petioles are completely confluent, forming a tube which terminates downwards in a little solid point, consisting of a minute radicle and hypocotyl, with the likewise minute plumule enclosed within the base of the tube. ~f1his structure was well exhibited in an abnormal specimen, in which on~ of the two cotyledons failed to produce a petiole, wh1lst the other produced one consisting of an open semicylinder ending in a sharp point, formed of the parts just described. As soon as the confluent petioles protrude from the seed they bend down: as they are strongly geotropic, and penetrate the ground. The seed itself retains its original position, either on the surface or buried at some depth, as the case may be. If, however, the point of the confluent petioles meets with some obstacle in the soil, as appears to have occurred with the seedlings described and figured by Asa Gray,* the cotyledons are lifted up above the ground. ~rhe petioles arc clothed with r?ot-~airs like tho3e on a true radicle, and they ~1kew1se resemble ra,clicles in becoming brown wheu Immersed in a solution of permanganate of potassium. ?nrhseecls were subjected to a high temperature, and m t e course of three or four days the petioles penetrated th~ soil perpendicularly to a depth of from 2 to 2~ mches ; and not until then did the true radicle begin to grow. In one specimen which was closely observed, the petioles in 7 days after their first protrusion attained a length of 2.1.. inches and the ~·~dicle by this time had also becom: well d~veloped. Ihe plumule, still enclosed within the tube, was now * 'American Jourual of Science,' vol. xiv. 1877, p. 21. u |
