TKANSACTIONS OF SECTION D. 673 



be gathered from Askenasy's ' interesting experiments on the growth of roots. 

 He showed that lowering the temperature has an almost instantaneous inhibitive 

 eft'eet 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 6-5. This result is to some extent 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, however, 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 turgesceut. 



There are other facts which make it extremely difficult to understand how 

 surface-growth can depend on cell-pressure. Nageli * pointed out that the growth 

 of cylindrical cells which elongate enormously without bulging outwards laterally, 

 is not explicable by simple internal pressure. An internodal cell of Nitella 

 increases to 2,000 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 orginal 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 plasmolytic 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 

 shortening in the line of greatest growth. Nevertheless, in spite of this flaw in 

 Niigeli's argument, great elongation in a single direction 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 homogeneous 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. Such structural differences do, of 

 course, exist, but whether they are sufficient to account for the phenomena is 

 a different question. Strasburger '" 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 patterns, 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 incon- 

 ceivable, that the extending force of cell-turgor, combined 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. 

 Low 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 curvatures ? 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's theory, that difference in extensibility depends 

 on difference in thickness, meet the case completely. What we need is an increase 



' JDeutsch. Bot. Ges., 1890, p. 61. This paper contains an excellent discussion on 

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



- Loc. cit. p. 71. 



' Wiesner (<S?te. Wien. Aliad., 188-1, 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.; plasmolytic shortening, 8 per cent. ; normal: growth, 39 per cent. ; shortening, 

 13 per cent. 



< StdrlteMrner, p. 279. . ' Zellhdnte, p. 194. 



1891. XI 



