226 



SCIENCE- G OS SIP. 



SPIRALS IN PLANTS. 



By J. A. Wheldox. 



THE subject of the torsion of the trunks of trees 

 raised by Mr. Mott is a most interesting one 

 (ante, page 219), and it seems very desirable, as he 

 suggests, that further observations on the subject 

 should be recorded. With a greater amount of 

 evidence before us, it might be possible to pro- 

 pound some theory that would more fully account 

 for all the phenomena than any already offered. 

 It would be advantageous also to know more than 

 we do about other spiral vegetable organs, in 

 addition to those exhibited in the trunks of trees — 

 e.g. the twining of the stems of climbers, the coil- 

 ing of tendrils and petioles, and the twisting of 

 pods and other fruits. 



In recording any such observations it is abso- 

 lutely necessary that everyone should use uniform 

 terms in describing the direction of the spirals, 

 otherwise errors may easily be caused. We must, 

 therefore, always bear in mind that in determining 

 the direction of a spiral the observer should 

 imagine himself to stand within its coil. Then, in 

 a right spiral, the ascending lines will be seen to 

 run from left to right across the breast of the 

 observer : if viewed from the outside, the ascending 

 curves appear to run from right to left. In a left 

 spiral, of course, the reverse of this is seen. - 



It has been suggested that the twisting of stems 

 is due to the growing apes following the light, but 

 Darwin's observations (ride " Climbing Plants ") 

 disprove that ; nor could this explanation be 

 reconciled with the fact that some stems twist in 

 a direction opposite to that of the sun's apparent 

 course. According to Darwin (op. ait. page 7), 

 there is in twining plants a distinction between 

 the revolving motion of the growing apex of a 

 shoot and its axial twisting. By his carefully 

 conducted experiments and patient observation he 

 discovered that a growing shoot revolved at its 

 apex many times more often than was required to 

 produce the number of twists subsequently deve- 

 loped in its axis ; and, further, that in some in- 

 stances the apex revolved in a direction opposite 

 to that in which the stem was found to be 

 twisted. 



In the " Sagacity and Morality of Plants " Dr. 

 Taylor repeats the theory — originally. I believe, 

 propounded by Sachs — that the twiuing: of stems 

 " is due to the weak and rapidly developing stem 

 growing a trifle faster on one side than on the 

 other, just as a carpenter produces any degree of 

 curl in his shavings according as he presses his 

 plane a little more on one side than the other." 

 Even this demands explanation as to why the 

 growth on the two sides should be unequal in 

 some plants, and not in others -. and why the stems 



of some plants, such as Hibbertia. should twist 

 both ways, and of others, as Solanum dulcamara, 

 to the right or left indifferently. 



Professor Asa Gray remarked in a letter to Mr. 

 Darwin that '• in Thuja occidentalism the twisting 

 of the bark is very conspicuous. The twist is 

 generally to the right of the observer ; but in 

 noticing about one hundred trunks four or five 

 were observed to be twisted in the opposite direc- 

 tion. The Spanish chestnut is often much twisted." 

 The same author suggested that the stem gained 

 rigidity by being twisted, on the same principle 

 that a much-twisted rope is stiffer than a slack 

 one. 



In my own neighbourhood there are few oppor- 

 tunities for observing trees ; but I have noticed 

 that in some laburnums and elders the base of the 

 stem shows a tendency to twist to the right. 



Amongst Cryptogams spirals are of frequent 

 occurrence, as in some of the climbing ferns, the 

 stems of Cliara, the cells of Sphagnum, the capsules, 

 leaves, and setae of mosses, and the elaters of 

 liverworts. 



The twisting of the setae of mosses has occasion- 

 ally been used as a character of diagnostic im- 

 portance in the separation of species. Mr. Dixon, 

 in his " Handbook of British Mosses," says: — "It 

 is rarely, however, I believe, that the direction is 

 sufficiently constant in any species to afford a good 

 character of distinction, and it is only in a few 

 cases that I have relied upon it." 



It would be interesting to compare a large 

 number of observations, and thus ascertain if the 

 twistings so markedly exhibited when dry by the 

 setae of some mosses were constant enough to 

 be of specific value. With regard to the stems 

 of Phaenoganis. Darwin says : — " I have seen no 

 instance of two species of the same genus twining 

 in the opposite direction, and such cases must be 

 rare." 



' In mosses, on the contrary, the converse of this 

 is very commonly observed in the directions 

 assumed by the gyration of the seta, but of course 

 this is hardly comparable, not being homologous 

 with the stems of Phaenogams, nor even with 

 then- peduncles. I select a few examples from 

 the Bryales. commencing with the Xematodonteae. 

 In the Polytrichaceae the setae are not con- 

 spicuously twisted in the majority of the species, 

 but in some there is occasionally a tendency to 

 gyrate feebly to the right — e.g.PolytricJiu/n nanum. 

 P. urnigerUm, P. alpinum. P.fornwsum, P. gracile, 

 P. piliferum, and P. aloide*. In a few there is a 

 much more decided twisting in the same direction 

 — viz. OligotricJiumincurvum. Catharinea undulata. 



