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Show 92 HYPOCOTYLS, EPICOTYLS, ETC., CHAP. II. wise hypogean. Some seedlings with well-developed radicles were first immersed in a solution of permanganate of potassium; and, judging from the changes of colour (though these were not very clearly defined), the hypocotyl is about ·3 inch in length. Straight, thin, black lines of this length were now drawn from the bases of the short petioles along the hypocotyls Fig. 5~. B Jl \( '' ~ J (I ,. Vicia fuba: germinating seerls, suspended in damp air: A, with radicle growing perpendicularly downwards; B, the same bean after 24 hour~ and after the radicle has curved itself; r, radicle; h, short hypo~otyl f e, epicotyi appearing as a knob in A and as an arch in B; p, pet10le 0 the cotyledon, the latter enclosed within the seed-coats. of 23 germinating seeds, which were pinned to the lids of jars, generally with the hilum downwards, and with their radicles pointing to the centre of the earth. After an interval of from 24 to 48 hours the black lines on the hypocotyls of 16 out of the 23 seedlings became distinctly curved, but in very various degrees (namely, with radii between 20 and CHAP. II. BREAKING 'l'HROUGH THE GROUND. 93 80 mm. on Sachs' cyclometer) in the same relative direction as shown at B in :Fig. 59. As geotropism will obviously tend to check this curvature, seven seeds were allowed to germinate with proper precautions for their growth in a klinostat,* by which means geotropism was eliminated. The position of the hypocotyls was. observed during four successive days, and they continued to bend towards the hilum and lower surface of the seed. On the fourth day they were deflected by an average angle of 63° from a line perpendicular to the lower surface, and were therefore con~ide1:ably more curved than the hypocotyl and radicle 1n the bean at B (Fig. 59), thouo·h in the same relative direction. 0 It will, we presume, be admitted that all leo-uminous plants with hypogean cotyledons are descen°ded from forms which once raised their cotyledons above the grou~d in the o.rdinary manner; and in doing so, it is certam that. then hypocotyls would have been abruptly arched, as 1n the case of every other dicotyledonous plant. This is especially clear in the case of Phaseolus for out of five species, the seedlings of which w~ observed, ~amely, P. multiflorus, caracalla, vulga'ris, Ile?·nandesn and Roxbtw,qh'ii (inhabitants of the Old and New W oriels), the three last-named species have well-developed hypocotyls which break throuo·h tho ground as arches. Now, if we imagine a seeclling of the co~mon bean or of P. multijlorus, to behave as its progemtors once did, the hypocotyl (h, Fig. 59) in whatever position the see d may h ave b een bun'.e cl would become so much arched that the upper par~ would be doubled dowu parallel to the lower part ; and , * An instrument clevised bv Soc!Js, consi~;ting e~sentinlly of a Rlowly revolving horizontal axis. on which the plant un1ler observAtion is supported : sec 'Wiirzburg At·beiten,' 1879, p. 200. |