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Show 512 MODIFIED CIRCUMNU'rA'l'ION. CUAP. X. cut off for a length in three cases of ·2 inch and in the four other cases of ·14, ·12, ·1, and ·07 inch. But these cotyledons, after being extended horizontally, bowed themselves upwards as effectually as the unmutilated specimens in the same pots, showing that sensitiveness to gravitation is not confincu to their tips. GE0'1.'ROPISM. rrhis movement is directly the reverse of upogeotropism. Many organs bend downwards through epinasty or apheliotropism or from their own weight; but we have met with very few cases of a downward movement in sub-aerial organs due to geotropism. We shall, however, give one good instance in the following section, in the case of Trifolium subterrane'um, and probably in that of Arachis hypogma. On the other hand, all roots which penetrate the ground (including the modified root-like petioles of Megarrhiza and Ipomcea leptophylla) are guideu in their down ward course by geotropism ; and so are many aerial roots, whilst others, as those of the Ivy, appear to be indifferent to its action. In our first chapter the movements of the radicles of several seedlings were described. We may there see (Fig. 1) how a radicle of the cabbage, when pointing vertically upwards so as to be very little acted on by geotropism, circumnutated; and how another (Fig. 2) which was at first placed in an inclined position bowed itself downwards in a zigzag line, sometimes remaining stationary for a time. Two other radicles of the cabbage travelled <lownwards in almost rectilinear courses. A radicle of the bean placed upright (Fig. 20) made a great sweep and zigzao-ged; but as it sank downwards an~ was more stronog ly acted on by geotropiHm, 1. t move d m an CnAP. X GEO'rUOPISl\1. 513 tllmost straight course. A radicle of Cucurbita, directed upwards (Fig. 26), also zigzagged at first, and described small loops; it then moved in a straight line. Nearly the same result was observed with the radicle:; of Zea mays. But the best evidence of the intimate connection between circumnutation aiHl geotropism was afforded by the nulicles of Phascolns, Vicia, an<l Querens, and in a less degree by those of Zea an(l ~sculus (see Figs. 18, 19, 21, Lll, and 52); for when these were compelled to grow aJHl sli(1e clown highly inclinerl surfaces of smoked glass, they left distinctly serpentine tracks. The Buryi11.1 of Seed-crtpsules : T1·~[olium subterranewn.-Thc flower-heads of this plant arc remarkable from producing only 3 or 4 perfect flowers, which are Rituated exteriorly. All the other many flowers abort, and nrc modified into rigid points, with a bundle of vessels running up theh· centres. After a time 5long, elastic, claw-like projections, which represent the divisions of the calyx, arc developed on their summits. As soon as the perfect flowers wither they ben l downwards, supposing tlw peduncle to stand upright, and they then closely surround jtH upper part. This movement is due to epinaRty, as is likewise tbe case with tho flowers of 'l'. repens. 'l'hc imperfect centra! flowers ultimately follow, one afte1· the other, the same course. Whilst tho perfect flowers arc thus bending clown, the whole peduncle curves downwards and increases much in lenn-th until the flower-head reaches the ground. Vaucher * says that when the plant is so placed thn.t the heads cannot soon reach the ground, the peduncles grow to the extraordinary length of from 6 to 9 inches. In . whatever position the branches may be placed, the upper part of the peduncle at first bends vertjcally up~ards through heliotropism; but as soon as the flowe1:s hegm to wither the downward curvature of the whole peduncle commences. As this latter movement occurred in complete darkness, and with peduncles nrising from upright and from dependent branches, it cannot be clue to apheliotropism or to epmasty, but must be attributed to geotropism. Nineteen • 'Hist. Phys. des Plantcs ll' Europe,' tom. ii. 1841, p. lOG. 2 L |