414 



NA TURE 



[August 27, 1891 



Such structural differences do, of course, exist, but whether they 

 are sufficient to account for the phenomena is a different question. 

 StrasburgerC Zellhaute," p. 194) supposes that the elasticity of 

 a cell-wall depends on the last-formed layers, and as in these 

 the microsomes are seen arranging themselves in lines or pat- 

 terns, we have a heterogeneity of structure which may or may 

 not be sufficient. 



We have now seen that it is difficult to believe, although it is 

 not inconceivable, that the extending force of cell-turgor, com- 

 bined with differences in extensibility of the membranes 

 (depending on structural characters), may account for the 

 phenomena of rectilinear growth. But, even if we allow that 

 this is. so, how are we to apply the same explanation to growth- 

 curvatures ? How are we to account for the rapid changes in 

 extensibility necessary to produce geotropic or heliotropic curva- 

 tures? The influences which Strasburger and Noll suppose to 

 act on the cell-walls and render them ductile cannot account for 

 extensibility in one direction only. Nor does Wortmann';; theory, 

 that difference in extensibility depends on difference in thick- 

 ness^, meet the case completely. What we need is an increase 

 in longitudinal, not in general extensibility. I presume that 

 these writers mi^ht say that the excess in longitudinal extensi- 

 bility is always present whether general extensibility is grea'er 

 or less. In the meanwhile we must pass on to more recent 

 researches on surface-growth by apposition. 



In Strasburger's later work (" Histologische Beitrage," 1889), 

 his views on growth have undergone c )nsiderable modification. 

 The study of certain epidermic cells, of the folds in membranes, 

 and the repetition of Krabbe's work on certain bast fibres, have 

 convinced him that apposition does not account for all forms of. 

 growth. Krabbe (Pringsheim's Jahrb., xviii. ) showed that in 

 full-grown sclerenchyma [e.g. in Oleander) local widenings occur 

 without any such amount of thinning in the membrane as would 

 occur if the bulging were due to stretching. The only possible 

 explanation seems to be that there is a migration of new 

 material into the cell-wall. Such intussusception might be, 

 as Nageli supposed, a flow of fluid out of which new micellae 

 crystallize ; but it is now established that cellulose arises as a 

 modification of protoplasm, so that it would harmonize with our 

 knowledge of the origin of cellulose if we assume that intussus- 

 ception was preceded by a wandering of protoplasm into the 

 cell-wall. Such a state of things would render possible the 

 •regulation of longitudinal growth in the case of Nitella and 

 Spirogyra, already alluded to, as well as in growth- curvatures. 

 This view might also harmonize with Wiesner's theory {Sitz. 

 Wien. Akad., 1886, vol. xciii. p. 17) that the cell- wall contains 

 ^protoplasm as long as it continues to grow. 



For the sake of brevity I content myself with the above 

 -examples : I think it will be allowed that there is a focussing of 

 speculation from many sides in favour of " active " surface-growth 

 — or, what is perhaps a better way of putting it, in favour of a 

 belief that the extension of cell membranes depends on physio- 

 logical rather than physical properties:, that it is in some way 

 under the immediate control of the protoplasm. We may take 

 our choice between Wiesner's wall-protoplasm (dermatoplasm), 

 protoplasmic intussusception as conceived by Strasburger, or the 

 action of the ectoplasm in the manner suggested by Vines.^ who 

 supposes that the crucial point is a change in the motility of the 

 protoplasm, not of the cell membrane. The latter theory would 

 undoubtedly meet the difficulties — if we could believe that so 

 yieldi ig a substance as protoplasm could resist the force of 

 -turgor. 



The great difficulty is, as it seems tc me, that since, e.g. in 

 Caulerpa, surface-growth is clearly due to stretching, as Noll has 

 demonstrated, and since in osmotic cell-pressure a stretching 

 force does exist, it cannot be doubted that turgor, and ordinary 

 physical extensibility are conditions of the problem. This 

 remains true in spite of Klebs's {Tubingen. Uniersuchungen, ii. p. 

 489) curious observations on the growth of plasmolyzedAlgK, or 

 in spite of the fact that pollen tubes may grow without turgor, 

 in spite of the same being perhaps true of young cells filled with 

 protoplasm (see Noll, Wilrzburg. Arbeiten, iii, p. 530). In the 

 face of all these facts, osmotic pressure in the cell must remain a 

 vera causa tending to surface-growth. 



If we accept some form of " active" surface-growth, we must 



' Sachs's Arbeiten, 1878, and " Physiology," 1886. See also Gardiner, on 

 protoplasmic contractility, in the Annals of Botany, i p. 366. Pfeffer has, 

 I think, shown that Vines's and Gardiner'.-; theories assume the existence of 

 too great strength in the ectoplasm. See Pfeffer in Abhandl. der k. Sachs. 

 • Gesellsck. xvi. 1890, p. 329. 



NO. II 39, VOL. 44] 



deal with turgor in another way, although to do so may require 

 a violent exercise of the imagination. Are we to believe, for 

 instance, that the function of turgescence is the attaining of 

 mechanical strength ^ If we hold that cell-walls increase in area 

 independently of turgor, we shall be forced to invent a hypothesis 

 such as the following — which I am far from intending to uphold. 

 It is possible to imagine that the function of the force of turgor is 

 merely to spread out the growing membrane to its full extent, 

 and, as it were, to make the most of it. Turgor would in this 

 respect play the part occupied by the frame used in embroidery, 

 making it easier to carry on the work satisfactorily, but not 

 being absolutely necessary. When 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 mem- 

 brane 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 knowledge 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 turgor. We are, indeed, surrounded by 

 difficulties ; for, though the theories which are classed together 

 as protoplasmic have much in their favour, they, too, lead us 

 into an impasse. 



Circumnutalion. 

 I shall conclude by saying a few words about the theory o 

 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. Prof. Wiesner, of Vienna, published 

 in 1881 a book, "Das Bewegungsvermogen der Pflanzen," 

 entirely devoted to a criticism of the " Power of Movement " 

 (p. 8). 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 written on the title-page of the 

 copy presented to my father are characteristic of the tone of 

 the book : "In gelreuer Opposition, aber in unwandelbarer 

 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 nutation is the widely-spread phenomenon we 

 held it to be. According to Wiesner, many parts of plants 

 which do not circumnutate are capable of curving geotropically, 

 &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 observ- 

 ing circumnutation is inaccurate, inasmuch as the movement is 

 1 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 

 suppo'^ititious 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 phenomena in plants. Stems, leaves, and acellular 

 fungi are to be found which grow in a perfectly 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 powers of growth. 



ii. As the expression of opposing growth-tendencies. Thus 



certain organs have inherent tendencies to curve in definite 



I planes — for instance, the bending of the hypocotyl in the plane 



of the cotyledons. Wiesner believes that such tendencies, when 



1 combined with others — heliotropic, geotropic, &c. — lead to 



