20 ME. DARWIN ON CLIMBING PLANTS. 



course, and, consequently, the majority, as is well known, ascend 

 their supports from left to right. Occasionally, though rarely, 

 plants of the same order twine in opposite directions, of which 

 Mohl (S. 125) gives a case in the Leguminosrc, and we have in 

 the table another in the Acanthacese. At present no instance is 

 known of two species of the same genus twining in opposite di- 

 rections ; and this is a singular fact, because different individuals 

 of Solatium dulcamara (Dutrochet, torn. xix. p. 299) revolve and 

 twine in both directions : this plant, however, is a most feeble 

 twiner. Loasa aurantiaca (Leon, p. 351) offers a much more 

 striking case : I raised seventeen plants : of these eight revolved 

 in opposition to the sun, and ascended from left to right ; five 

 followed the sun, and ascended from right to left ; and four re- 

 volved and twined first in one direction, and then reversed their 

 course*, the petioles of the opposite leaves affording a point 

 d'appui for the reversal of the spire. One of these four plants 

 made seven spiral turns from right to left, and five turns from 

 left to right. These individuals of the Loasa are interesting, as 

 showing how almost every change is effected most gradually. 

 For another plant in the same family, the Scypliantlius elegans, 

 habitually twines in this manner. I raised many plants of it, and 

 the stems of all took one turn, or occasionally two or even three 

 turns in one direction, and then, ascending for a short space straight, 

 reversed their course and took one or two turns in an opposite 

 direction. The reversal of the curvature occurred at any point in 

 the stem, even in the middle of an intcrnode. Had I not seen 

 this case, I should have thought its occurrence most improbable. 

 It could hardly occur with any plant which ascended above a few 

 feet in height, or which lived in an exposed situation ; for the 

 stem could be easily pulled from its support with little unwinding ; 

 nor could it have adhered at all, had not the internodes soon be- 

 come moderately rigid. With leaf-climbers, as we shall soon see, 

 analogous cases frequently occur ; but these present no difficulty, 

 as the stem is secured by the clasping petioles. 



In the many other revolving and twining plants observed by 

 me, I never but twice saw the movement reversed ; once, and only 

 for a short space, in Ipomcea jucunda ; but frequently with Hib- 

 bertia dentata. This plant at first much perplexed me, for I con- 

 tinually observed its long and flexible shoots, evidently well fitted 

 for twining, make a whole or half or quarter circle in one direction 



* I raised nine plants of the hybrid Loasa Herbertii, and six of these re- 

 versed their spire in ascending their supports. 



