Host and Parasite in certain Diseases of Plants. 413 



is larger or smaller depends on the resultant action of these processes. 

 Anything which interferes with oxidation promotes the accumulation 

 of organic acids, whereas those changes which lead to increased 

 oxidation in the cells are followed by a decrease of acids. 



Now a few words as to growth, and its dependence on external 

 factors. Apart from the thickening of the cell-walls, which comes 

 afterwards and depends on the addition of materials formed by the 

 protoplasm,* the principal phenomenon that concerns us is the exten- 

 sion of the cellulose membranes. This process is promoted by 

 moderately high temperatures, and retarded by low ones and by very 

 high ones, | in accordance with respiration and the general metabolism 

 of the cell; the curves are not quite the same, because respiration 

 begins at temperatures too low for growth, and goes on rising in 

 intensity to temperatures at which growth begins to decline; still the 

 connexion is very close, and the dependence of growth on respiration 

 and metabolism implies this. 



Light is usually considered to have a retarding effect on the growth 

 of the cells. Apart from the possibility that there may be a more 

 direct action of light on the extensibility of cell-walls or of cells 

 generally, by its effects on the protoplasm at the spot, one way in 

 which this retarding effect may be brought about is in connexion 

 with the turgidity of the cells. Without concerning ourselves with 

 the general discussion of the whole subject, which would be a very 

 long one, it seems, at least, clear that in the ordinary course of 

 events light exercises some retarding action on growth by exten- 

 sion ; what, if any, connexion exists between this phenomenon and 

 the observed diminution of the organic acids in the cells (and we 

 have seen that their turgidity depends on these acids and their salts) 

 in daylight still needs investigation, and the same may be said as re- 

 gards the influence of temperature in relation to growth and the 

 production of acids. It is customary to regard the retarding action 

 of light on the extensibility of the cell- wall as a complex phenomenon 

 of irritability,! and it is by no means certain that such is not the 

 case; meanwhile we simply accept the facts that in ordinary bright 

 light the extension of the growing parts is retarded, that this is 

 connected with diminished turgidity, which in its turn is depen- 

 dent on the pressure in the sap of substances capable of retaining the 



carbon assimilation, and not on any direct action of the rays of light, can hardly be 

 doubted (see Warburg, op. cit., pp. 77 — 92). 



* For details see Strasburger, ' Ubsr den Bau und Wachsthum der Zellhaute ' and 

 " Ueber das Wachsthum vegetabilischer Zellhaute " (' Histologische Beitrage,' 

 No. 2, 1889). 



f Sachs, ' Physiology,' pp. 194 and 553. 



X See Yines, ' Physiology of Plants/ p. 398 ; but cf. also Wortmann, ' Bot. Zeitg.,' 

 1887, Nos. 48—51, especially col. 808—810. 



