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Show 508 MODIFIED CIRCUMNUTATION. CIIAP. X. tinually circumnutating, we may conclude that eve a rectilinear course is merely an extremal y modifie~ form of circumnutation. It is remarkable that a stem or o~her organ ~hich is highly sensitive to apogeo~ ropism,. and ~hiC~l has bowec~ itself rapidly upwards In a straight hne, Is often earned beyond the vertical as if by momentum. It then bends a little backward~ to a point round which it finally circumnutates. Two instances of this were observed with the hypocotyls of Beta vulgaris, one of which is shown in Fig. 183, and two other instances with the hypocotyls of Brassica. This momentum-like movement probably results from the accumulated effects of apogeotropism. For the sake of observing how long such after-effects lasted, a pot with seedlings of Beta was laid on its side in the dark, and the hypocotyls in 3 h. 15m. became highly inclined. The pot, still in the dark, was then placed upright, and the movements of the two hypocotyls were traced ; one continued to bend in its former direction, now in opposition to apogcotropism, for about 37m., perhaps for 48 m.; but after 61 m. it moved in an opposite direction. The other hypocotyl continued to move in its former course, after being placed upright, for at least 37 m. Different species and different parts of the same species are acted on by apogeotropism in very different degrees. Young seedlings, most of which circumnutate quickly and largely, bend upwards and become vertical in much less time than do any older plants observed by us; but whether this is due to their greater sensitiveness to apogeotropism, or merely to their greater flexibility we do not know. A hypo· cotyl of Beta traversed an angle of 109° in 3 h. 8 m., and a cotyledon of Phalaris an angle of 130° in 4 h. 30 m. On the other hand the stem of a herbaceous ' CHAP· x. APOGEOTllOriSM. 509 Verbena rose 90° in about 24 h.; that of Rubus 67°, in 70 h.; that of Uytisus 70°, in 72 h. ; that of a young American Oak only 37°, in 72 h. The stem of a young O!fperus. alternijolius rose o~ly 11 o in 96 h.; the bending bemg confined to near Its base. Though the sheath-like cotyledons of Phalaris are so extremely sensitive to apogeotropism, the first true leaves which protrude from them exhibited only a trace of this action. Two fronds of a fern, Nephrodium molle, both of them young and one with the tip still inwardly curled, were kept in a horizontal position for 46 h., and during this time they rose so little that it was doubtful whether there was any true apogeotropic movement. The most curious case known to us of a difference in sensitiveness to gravitation, and consequently of movement, in different parts of the same organ, is that offered by the petioles of the cotyledons of Ipomcea leptophylla. The basal part for a short length where united to the undeveloped hypocotyl and radicle is strongly geotropic, whilst the whole upper part is strongly apogeotropic. But a portion near the blades of the cotyledons is after a time acted on by epinasty and curves downwards, for the sake of emerging in the form of an arch from the ground; it subsequently straightens itself, and is then again acted on by apogeotropism. A branch of Oucurbita ovifera, placed horizontally, moved upwards during 7 h. in a straight line, until it stood at 40° above the horizon; it then began to circumnutate, as if owing to its trailing nature it had no tendency to rise any higher. Another upright branch was secured to a stick, close to the base of a tendril, and the pot was then laid horizontally in the dark. In this position the tendril circumnutated and made |