TEANSACTIONS OF SECTION D. 675 



mechanical strength is gained by turgor (as in Mucor), instead of by brute strength 

 of material, as in a tree-trunk, a great economy in cellulose is effected. If turgor 

 played our hypothetical part of smoothing out the membrane and insuring that 

 it shall occupy as large a space as possible, it would effect the same kind of 

 economy. 



It is not necessary to inquire how far this hypothesis accords with our know- 

 ledge of cell mechanics. It is only put forth as an example of the difficulties in 

 which we land if we seek for a new function for tm-gor. We are, indeed, surrounded 

 by diffiiculties ; for, though the theories which are classed together as protoplasmic 

 have much in their favour, they, too, lead us into an imimsse. 



Circumnutation. 



I shall conclude by saying a few words about the theory of growth-curvatures 

 put forward in the ' Power of Movement in Plants.' I can here do no more than 

 discuss the relation of circumnutation to curvature, which is the thesis of the book 

 in question, without attempting to enter the arena with regard to the many 

 objections which have been raised to other parts of our work. 



A distinguished botanist, Professor Wiesner, of Vienna, published in 1881 a 

 book, ' Das Bewegungsvermogen der Pflanzen,' entirely devoted to a criticism of 

 the ' Power of Movement.' It is founded on a long series of experiments, and is 

 written throughout in a spirit of fairness and candour which gives it value, apart 

 from its scientific excellence, as a model of scientific criticism. The words 

 wi'itten on the title-page of the copy presented to my father are character- 

 istic of the tone of the book : — •' In getreuer Opposition, aber in imwandelbarer 

 Verehrung.' A letter printed among my father's correspondence shows how 

 warmly he appreciated his opponent's attack both as to matter and manner. 

 "Wiesner's opposition is far-reaching, and includes the chief theoretical conclusion 

 of the book, namely, that movements such as heliotropism and geotropism are 

 modifications of circumnutation. Neither will he allow that this revolving nuta- 

 tion is the widely-spread phenomenon we held it to be. According to "Wiesner, 

 many parts of plants which do not ciicumnutate are capable of curving geo- 

 tropicaUy, &c. ; he is, therefore, perfectly justified, from his own point of view, 

 in refusing to believe that such curvatures are derivations from circumnutation. 

 He points out that our method of observing circumnutation is inaccurate, inasmuch 

 as the movement is recorded in oblique projection. This we were aware of,' and 

 I cannot but think that Wiesner has unintentionally exaggerated its inaccuracy ; 

 and that, if used with reasonable discretion, it cannot lead to .anything like such 

 faulty records as in the supposititious cases given by our critic. However this may 

 be, Wiesner's results are perhaps more trustworthy than ours, and should receive 

 the most careful consideration. 



Wiesner's conclusions, taken from his own summaries, are as follows : — 



The movement described as circumnutation is not a wide-spread phenomenon in 

 plants. Stems, leaves, and acellular fungi are to be found which grow in a per- 

 fectly straight line. Some roots grow for considerable periods of time without 

 deviating from the vertical. When circumnutation does occur, it cannot be 

 considered to have the significance given to it in the ' Power of Movement.' The 

 movements observed by Wiesner are explained by him in three different ways: — 



i. As the expression of a certain irregularity in growth depending on the want 

 of absolute symmetry in structure, and on the fact that the component cells of the 

 organ have not absolutely similar powei-s of growth. 



ii. As the expression of opposing growth -tendencies. Thus certain organs have 

 inherent tendencies to curve in definite planes^for instance, the bending of the 

 hypocotyl in the plane of the cotyledons. Wiesner believes that such tendencies, 

 when combined with others — heliotropic, geotropic, &c.— lead to alternate bendings 

 in opposite directions, according as one or other of the components is temporarily 

 the stronger, 



iii. Wiesner allows that circumnutation does exist in some cases. This last 



' PoKer of Movement, p. S. 



