Ohap. V. EPINASTY AND HYPONASTY. 271 



well-marked zigzags (one of them forming an angle of 112°), 

 and this indicates circumnutation. 



The still closed lobes of a young leaf of Dionrea projected at 

 right angles to the petiole, and were in the act of slowly rising. 

 A glass filament was attached to the under side of the midrib, 

 and its movements were traced on a vertical glass. It circum- 

 nutated once in the evening, and on the next day rose, as already 

 described (see Fig. 106, p. 240), by a number of acutely zigzag 

 lines, closely approaching in character to ellipses. This move- 

 ment no doubt was due to epinasty, aided by apogeotropism, 

 for the closed lobes of a very young leaf on a plant which had 

 been placed horizontally, moved into nearly the same line with 

 the petiole, as if the plant had stood upright ; but at the same 

 time the lobes curved laterally upwards, and thus occupied an 

 unnatural position, obliquely to the plane of the foliaceous petiole. 



As the hypocotyls and epicotyls of some plants protrude from 

 the seed-coats in an arched form, it is doubtful whether the 

 arching of these parts, which is invariably present when they 

 break through the ground, ought always to be attributed to 

 epinasty; but when they are at first straight and afterwards 

 become arched, as often happens, the arching is certainly due to 

 epinasty. As long as the arch is surrounded by compact earth 

 it must retain its form; but as soon as it rises above the 

 surface, or even before this period if artificially freed from the 

 surrounding pressure, it begins to straighten itself, and this no 

 doubt is mainly due to hyponasty. The movement of the 

 upper and lower half of the arch, and of the crown, was occa- 

 sionally traced ; and the course was more or less zigzag, showing 

 modified circumnutation. 



With not a few plants, especially climbers, the summit of the 

 shoot is hooked, so that the apex points vertically downwards. 

 In seven genera of twining plants * the hooking, or as it has been 

 called by Sachs, the nutation of the tip, is mainly due to an 

 exaggerated form of circumnutation. That is, the growth is so 

 great along one side that it bends the shoot completely over to 

 the opposite side, thus forming a hook ; the longitudinal line or 

 zone of growth then travels a little laterally round the shoot, 

 and the hook points in a slightly different direction, and so 

 onwards until the hook is completely reversed. Ultimately it 



' The Movements and Habits of Climbing Plants,' 2n(l edit. o. X3. 



