672 EEPom— 1891. 



of growth-curvatures. Thus if De Vries' 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. Apain, Wortmann 

 explains the difference 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 



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

 attaches great" importance as supporting his view. When a curved organ is 

 plasmolysed, it sutlers 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 say, the curvature is increased. This seems to show that the yielding of the 

 convex side is owing to a ductility, which prevents its holding its own against the 

 more perfect elasticity of the concave side. But this is only the beginning of the 

 phenomenon ; as the plasmolysing agent continues to act, a reverse movement takes 

 place, the well-known flattening of the curvature described by De Vries. It is to 

 me incomprehensible how in a given condition of cell-walls these results can occur 

 in different stages of plasmolysis. I can understand one occurring when the curva- 

 ture 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. 



"NVe have now seen that the most acceptfible 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 tlie 

 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 

 connected with the doctrine of growth by apposition. The question, therefore, 

 whether the apposition-theory is suflicient to account for the phenomena of 

 ordinary growth, may be applied mutatis mutandis to growth curvature. This 

 doctrine in its original purity absolutely requires turgescence to account for the 

 elon"-ation of growth. The 'older layers, separated from the ectoplasm by the 

 younger layers of cell-wall, can only be elongated by traction, (irowth by intus- 

 susception does not absolutely require this force ; the theory that the micelkx) are 

 separated by traction, and thus allow intercalation of fresh micella^, 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 - 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,^ as Zimmermanu points out in the same connection, found no increased 

 elongation of collenchyma when stretched for some days by means of a weight. 

 A o-reater difficulty is that growth may be absolutely and at once stopped by 

 placing the growing organ in an atmosphere free from oxygen.'' Such treatment 

 apparently does not diminish turgescence, yet growth stops. If the cell-walls are 

 increasing in length by mechanical stretching, and if the turgor is not interfered 

 with, increase in length ought to continue. The same thing applies to curvatures. 

 "Wortmann has shown ^ that in an atmosphere of pure hydrogen a geotropic 

 curvature which has begun in ordinary air cannot continue ; in other words, after- 

 effect ceases. This 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 



' The similar results obtained by Wiesner are noticed above. 



* Mem. Acad. St. PH. v. vol. xxvii. p. 20. ' Pringsheim's Jahrh. xii. 



« Wieler, Pfeffer's UntersucJt. Bd. i. p. 189. » Bot. Zeit., 1884, p. 705. 



