770 MECHANICAL LAWS OF GROWTH. 



Corylus At^ellana, and some other plants were found on the other hand to be hyponastic ; 

 when laid horizontally in their natural position they curved upwards, but downwards 

 if reversed, because their hyponasty was stronger than their geotropism. 



Similar experiments to those made on petioles with respect to heliotropism, 

 showed in many cases the absence of this phenomenon, especially in the case of stolons; 

 and that in other cases it was always positive, but too feeble to overcome the influence 

 of their epinasty. In the case of branches, especially such as are long and slender, 

 more account must be taken of weight in modifying the direction of growth than in 

 that of leaves. The removal of the leaves (<?. g. in Corylus) is in this case followed by a 

 sudden curving upward, the result of elasticity ; but this is subsequently intensified by 

 geotropism and in many cases (as in Abies) also by hyponasty. 



It may be left to the ingenuity of the student to determine the directions of organs 

 by his own observation in any particular case, from the points of view stated above. 



Sect. 23. — Torsion^ Organs of any considerable length very commonly dis- 

 play torsions about their axis of growth ; the striations on the surface of the organ 

 are not parallel to its axis of growth, but run round it in the form of more or less 

 obhque spiral lines, as if the organ were fastened at one end, and then twisted at the 

 other. Torsions of this kind occur in the unicellular internodes of Nitella ; they are 

 common in the elongated multicellular internodes of the erect stems of Dicoty- 

 ledons, universal in climbing internodes ; the pedicels of the thecae of Mosses are 

 generally very strongly twisted. Even in flat leaves, as Wichura has shown, torsions 

 of the lamina occur very commonly ; they behave like strips of paper fastened at 

 one end and twisted by the other round their median line. These torsions are 

 particularly conspicuous in the leaves of many Grasses, of Allium ursinum, species 

 of Alstroemeria, &c., causing the under side of the lamina to lie uppermost towards 

 the apex I 



Since the striae on a twisted organ run spirally round the axis, they must exceed 

 the axis in length ; if therefore the torsion is the result of growth, the growth of the 

 outer layers of cylindrical, conical, or prismatic organs (internodes, roots, &c.) must 

 be more rapid or must last longer than that of the inner layers ; and in twisted 

 leaves there must be the same difference as respects the growth of the mid-rib in 

 comparison to that of the margins. The fact that at the time of most rapid growth 

 the inner layers generally grow more rapidly than the outer ones (Sect. 13), thus 

 preventing the possibility of torsion, the additional fact that torsion does not gener- 

 ally take place until growth is ceasing, and lastly, the circumstance that etiolated 

 internodes, which in a normal state do not exhibit torsion, usually manifest this 

 phenomenon at the close of their growth, lead to the conclusion that torsion is the 

 result of growth continuing in the outer layers after it has ceased or begun to cease 

 in the inner layers. In twisted leaves, especially those of Alstroemeria, the torsion 

 however begins earher. If the growth of the outer layers, besides being greater, 



^ H. de Vries in the second Heft of the Proceedings of the Wiirzburg Botanic Institute 1 871, 

 p. 272. — Wichura in Flora 1852, No. 3, and Jahrbuch fiir wissensch. Bot. vol. II, i860. — Braun in 

 Bot. Zeitg. 1870, p. 158. 



^ [Similar torsions occur in petals as Cyclamen, fruits as Aila7ithus malabarica, and not unfre- 

 quently in pedicels or inferior ovaries as Orchidece, causing the anterior part of the flower to become 

 apparently posterior, and vice versa. — Ed.] 



