16 JOURNAL OF THE EOYAL HORTICULTURAL SOCIETY. 



In intensely cold winters the leaves of evergreens, such as holly, may 

 be seen to hang vertically downwards as long as the cold lasts. • 



An analogous orthotropism is seen in the sleep or hypnotic condition 

 of plants fully described by Darwin, in his Movements of Plants. 



CIRCUMNUTATION. 



As soon as the root and stem have grown long enough, having been 

 vertical at first, they both begin to move approximately in circles or 

 ellipses. This is presumably due to the apical growth of the stem not 

 being strictly simultaneous at all points of the circumference. If the tip 

 of the stem of some cryptogams — say, Equisetum* — which is simpler 

 than that of an Angiosperm, be examined, it will be found to possess a 

 conspicuous terminal cell. From this, new cells are cut off in a spiral 

 order, the centre of each cell being at 120° from the preceding, as 

 measured on radii from the centre of the apical cell. 



Though it has not been clearly demonstrated that the apices of 

 Angiospermous stems start with a single cell, as this apex is so soon 

 composed of a mass of merismatic tissue, still, judging by the " bowing 

 around," or " circumnutation " as it is called, in a continuously spiral 

 manner (allowing for irregularities), one is inclined to assume that a 

 batch of cells at one point precedes in its formation that of another, 

 about 120° from the former, just as is the case with single cells in the 

 apex of the stem of Equisetum or horse-tail. 



Darwin has given us many diagrams of traces, both of roots and 

 stems, circumnutating, so the reader can consult them in his work on The 

 Movements of Plants. 



Circumnutation of a stem, therefore, may be assumed to result from 

 the process of growth in a definite manner. Perhaps the most con- 

 spicuous case is seen in the movements of climbing or twining stems. 

 Darwin attributed this to the elongated shoot bending or nodding around, 

 in search, as it were, of a support. As soon as the stem touches another 

 all motion is arrested below the point of contact, while the upper part 

 continues to nutate, and so it winds itself round the support. Sachs calls 

 attention to an omission, that the apex in consequence of the continued 

 elongation of the stem, really moves in a spiral or corkscrew-like manner. 

 This may perhaps facilitate the twining process.t 



Tendrils also circumnutate until the tip has coiled round some object 

 (by haptotropism) ; having acquired a sensitiveness to touch, the previously 

 straight tendril then coils itself into spirals, as many times in one 

 direction as in the opposite. This is effected by means of crank-like 

 straight lengths rotating. I have described this in detail elsewhere.? 



The circumnutation of more or less completely formed organs, such 

 as leaves and tendrils, seems to require some other interpretation than 

 apical growth ; and at present there does not appear to be any satisfactory 

 explanation of this phenomenon. 



* See Fig. Ill in Sach's Text Book of Botany, p. 143. 



f Darwin, however, appears to recognize this fact, for he says : " The belief that 

 twiners have a natural tendency to grow spirally, probably arose from their assuming 

 a spiral form when wound round a support, and from the extremity, even whilst 

 remaining free, sometimes assuming this form."— Climbing Plants, p. 17. 



J Jour. B. Hort. Soc, vol. xxxiv. p. 417. 



