August 27, 1891] 



NATURE 



A similar argument may be drawn from Elfving's experiments, 

 lie found that the pulvini of grass halms placed on the klinostat 

 increase in length. This experiment .shows incidentally that the 

 klinostat does not remove but merely distribute equally the geo- 

 tropic stimulus : also that geotropic stimulus leads to increased, 

 not to diminished growth. The same thing is proved by the 

 simple fact that a grass halm shows no growth in its pulvinus 

 while it is vertical, so that when curvature begins (on its being 

 placed horizontal) it must be due to acceleration on the convex, 

 since there is no growth on the concave side in which retardation 

 could occur. Noll's view is that the primary change is an in- 

 crease in extensibility of the tissues on the convex side. This 

 view he proceeded to test experimentally. A growing shoot was 

 fixed in a vertical position, and a certain bending force was ap- 

 plied to make it curve out of the vertical, first to the right and 

 then to the left. If the cortical tissues are, at the beginning of 

 the experiment, equally resisting all round, it is clear that the 

 excursions from the vertical to the right and left will be equal. 

 As a matter of fact the excursions to the right and left were 

 nearly the same, and the difference was applied as a correction 

 to the subsequent result. The shoot was then placed horizon- 

 tally until geotropic or other curvature was just beginning, when 

 the above bending experiment was repeated. It was then found 

 that when it was bent so that the lower side was made convex, 

 the excursion was greater than it had been. In the few experi- 

 ments given by Noll the excursion in the opposite direction 

 (stretching of ttie concave side) was less than it had been, and 

 lie states that all the other experiments showed a similar result 

 The increased extensibility of the convex side is clearly the most 

 >triking part of the phenomenon, but I fail to see why Noll takes 

 so little notice of the diminution in the extensibility of the con- 

 cave side, which is only mentioned towards the end of his paper 

 {loc. cit. p. 529). Yet such a diminution is a necessary factor 

 in the mechanism of curvature. It should be noted that 

 results like Noll's might be obtained under other conditions 

 of growth curvatures. Thus if De Vries's view were the true one, 

 and the curvature were due to difference in osmotic force on the 

 convex and concave sides, the shoot would react differently in 

 the two directions ; for instance, the concave side would be the 

 more easily compressed. Noll and Wortmann's explanations 

 differ in this : the former lays the greater stress on the increased 

 extensibility of the convex side, the latter on the diminution of 

 that of the concave side. Again, Wortmann explains the dif- 

 ference in extensibility as due to differences in thickness of the 

 cell-walls. Noll gives no mechanical explanation, but assumes 

 that the ectoplasm has the power of producing changes in the 

 quality of the cell-wall in some unknown way. 



In the early stages of curvature, a phenomenon takes place to 

 which Noll attaches great importance as supporting his view. 

 ^Vhen a curved organ is plasmolyzed, it suffers a diminution of 

 curvature, as De Vries showed, but Noll ^ has proved that in the 

 early stages of curvature a contrary movement occurs — that is to 

 -ay, the curvature is increased. This seems to show that the 

 yielding of the convex side is owing to a ductility, which pre- 

 vents its holding its own against the more perfect elasticity of 

 the concave side. But this is only the beginning of the phe- 

 nomenon ; as the plasmolyzing agent continues to act, a reverse 

 movement takes place, the well-known flattening of the curva- 

 ture described by De Varies. It is to me incomprehensible how 

 in a given condition of cell-walls these results can occur in dif- 

 ferent stages of plasmolysis. I can understand one occurring 

 when the curvature is recent, and the other, the flattening of the 

 curve, occurring when the ductile convex parts have reacquired 

 elasticity. The fact undoubtedly is as Noll describes it : his 

 explanation seems to me inadequate. 



We have now seen that the most acceptable theory of the 

 machinery of these curvatures is in its main features akin to 

 Ilofmeister's, the power of elongation supplying the motive force, 

 while the varying extensibility of the membranes determines the 

 nature and direction of the bend. 



The question now arises : Is it possible by these means to 

 account for all the facts that must be explained? Taking the 

 theory for which there is most to be said on experimental 

 grounds — viz. Noll's — it will be noted that it is essentially con- 

 nected with the doctrine of growth by apposition. The question, 

 therefore, whether the apposition-theory is sufficient to account 

 for the phenomena of ordinary growth, may be applied mutatis 

 viutandis to growth curvature. This doctrine in its original 

 purity absolutely requires turgescence to account for the elonga- 

 ' The similar results obtained by Wicsn;r are noticed aSove. 



NO. II 39, VOL. 44] 



tion of growth. The older layers, separated from the ec'.oplasm 

 by the younger layers of cell-wall, can only be elongated by 

 traction. Growth by intussusception does not absolutely re- 

 quire this force ; the theory that the micellre are separated by 

 traction, and thus allow intercalation of fresh micellae, is a view 

 for which Sachs is chiefly responsible. 



Since surface- growth by apposition is absolutely dependent on 

 the traction exercised by cell-pressure, it is a fair question — how 

 far growth is influenced by forcible elongation. Baranetzky 

 {Mem. Acad. St. /\t., v. vol xxvii. p. 20) states that when a 

 plant is subject to traction, as by even a small weight attached 

 to the free end, the rate of growth is lowered. Ambronn 

 (Pringsheim's ^rt//r(^., xii.), as Zimmermann points out in the 

 same connection, found no increased elongation of collenchyma 

 when stretched for some days by means of a weight. A greater 

 difficulty is that growth may be absolutely and at once stopped 

 by placing the growing organ in an atmosphere free from oxygen 

 (Wieler, Pfeffer's Unlcrsuch., Bd. i. p. 189). Such treatment 

 apparently does not dimini^h turgescence, yet growth stops. 

 If the cell-walls are increasing in length by mechanical stretch- 

 ing, and if the turgor is not interfered with, increase in length 

 ought to continue. The same thing applies to curvatures. 

 Wortmann has shown {Bot. Zeit., 1884, p. 705) that in an atmo- 

 sphere of pure hydrogen a geotropic curvature which has begun 

 in ordinary air cannot continue ; in other words, after-effect 

 ceases. 1 his seems to me inexplicable on Noll's or Wortmann's 

 theories ; the convex side has become more extensible than the 

 concave, turgescence, as far as we know, continues, yet no after- 

 effect is observed. The same result may be gathered from 

 Askenasy's^ interesting experiments on the growth of roots. 

 He showed that lowering the temperature has an almost instan- 

 taneous inhibitive effect on growth. Thus maize roots (at a 

 temperature of 26°6) growing at the rate of 33 divisions of the 

 micrometer per hour, were placed in water at 5°, and absolutely 

 no growth occurred during the following ten minutes, in which 

 the thermometer rose to 6°-$. This result is all the more valuable 

 because we know from Askenasy's- other results that the turgor, 

 as estimated by plasmolytic shortening, is about the same 

 whether the root is in full growth or not growing at all. This is 

 not conclusive, for if the growing cell-walls were ductile they 

 might shorten but little although under great pressure, whereas 

 the non-growing cells might shorten a good deal, owing to their 

 more perfect elasticity ; ^ therefore Askenasy's plasmolytic 

 results are not in this particular connection of great importance, 

 except as showing that the non-growing roots were certainly to 

 some extent turgescent. 



There are other facts which make it extremely diflicult to 

 understand how surface-growth can depend on cell-pressure. 

 Nageli (" Stiirkekorner," p. 279) pointed out that the growth of 

 cylindrical cells which elongate enormously without bulging out- 

 wards laterally, is not explicable by simple internal pressure. 

 An internodal cell of Nitella increases to 2000 times its original 

 length, while it only becomes ten times as wide as it was at 

 first. The filaments of Spirogyra become very long, and keep 

 their original width. Nageli found that in Spirogyra the 

 shortening produced by plasmolysis was practically the same in 

 the longitudinal and in the transverse direction. He therefore 

 concluded that the growth of Spirogyra cannot be accounted for 

 by the cell-wall being differently extensible along different axes. 

 But it must once more be pointed out that this type of plasmo- 

 lytic experiment has not the force which Nageli ascribes to it. 

 If the cell-wall stretched like putty in one direction and like 

 india-rubber in the other, there might be no plasmolytic shorten- 

 ing in the line of greatest growth. Neverthele-^s, in spite of 

 this flaw in Niigeli's argument, great elongation in a single direc- 

 tion remains a problem for those who believe in surface-growth 

 by apposition. 



The point of special interest is that differences in extensibility 

 in different directions cannot be supposed to exist in a homo- 

 geneous membrane. If any purely physical characters can 

 explain the facts, they must be architectural characters. That 

 is to say, we must be able to appeal to remarkable structural 

 differences along different axes if we are to explain the facts. 



' Dfiitsch. Bot. Cfs., 1890, p. 61. This paper contains an excellent dis- 

 cussion on the mechanics of growth, to which I am much indebted. 



^ Loc. cit. p. 71. 



3Wiesner (Sitz. ll'kn. Akad., 1884, vol. Ixxxix.-xc. Abth. i. p. 223) 

 showed that under certain conditions decapitated roots grow much more 

 quickly than normal ones, yet the amount of plasmolytic shortening is less. 

 Decapitated: growth 79 per cent.; plas.nolytic shortening, 8 per cent.; 

 normal : growth, 39 p«r cent. ; shortening, 13 per cent. 



