Tendrils, which consist of leaves or of other organs modified, and the stems of twining plants, are, as Mohl long ago remarked, rarely heliotropic; and here again we can see the reason why, for if they had moved towards a lateral light they would have been drawn away from their supports.  But some tendrils are apheliotropic, for instance those of Bignonia capreolata

* According to F. Kurtz (’Verhandl. des Bot.  Vereins der Provinz Brandenburg,’ Bd. xx. 1878) the leaves or pitchers of Darlingtonia Californica are strongly apheliotropic.  We failed to detect this movement in a plant which we possessed for a short time. [page 451]

and of Smilax aspera; and the stems of some plants which climb by rootlets, as those of the Ivy and Tecoma radicans, are likewise apheliotropic, and they thus find a support.  The leaves, on the other hand, of most climbing plants are heliotropic; but we could detect no signs of any such movement in those of Mutisia clematis.

As heliotropism is so widely prevalent, and as twining plants are distributed throughout the whole vascular series, the apparent absence of any tendency in their stems to bend towards the light, seemed to us so remarkable a fact as to deserve further investigation, for it implies that heliotropism can be readily eliminated.  When twining plants are exposed to a lateral light, their stems go on revolving or circumnutating about the same spot, without any evident deflection towards the light; but we thought that we might detect some trace of heliotropism by comparing the average rate at which the stems moved to and from the light during their successive revolutions.* Three young plants (about a foot in height) of Ipomoea caerulea and four of I. purpurea, growing in separate pots, were placed on a bright day before a north-east window in a room otherwise darkened, with the tips of their revolving stems fronting the window.  When the tip of each plant pointed directly from the window, and when again towards it, the times were recorded.  This was continued from 6.45 A.M. till a little after 2 P.M. on June 17th.  After a few observations we concluded that we could safely estimate the time

* Some erroneous statements are unfortunately given on this subject, in ‘The Movements and Habits of Climbing Plants,’ 1875, pp. 28, 32, 40, and 53.  Conclusions were drawn from an insufficient number of observations, for we did not then know at how unequal a rate the stems and tendrils of climbing plants sometimes travel in different parts of the same revolution. [page 452]

taken by each semicircle, within a limit of error of at most 5 minutes.  Although the rate of movement in different parts of the same revolution varied greatly, yet 22 semicircles to the light were completed, each on an average in 73.95 minutes; and 22 semicircles from the light each in 73.5 minutes.  It may, therefore, be said that they travelled to and from the light at exactly the same average rate; though