Host and Parasite in certain Diseases of Plants. 419 



the cell- walls bounding the inter-cellular passages and the air in the 

 passages themselves are thoroughly saturated with water and aqueous 

 vapour respectively, and the movements of gases must be retarded 

 accordingly, turgescence is promoted, and the water contents accumu- 

 late to a maximum, owing to the disturbance of equilibrium between 

 the amounts absorbed by the active roots in the relatively warm soil 

 and those passing off into the cold damp air; much more water is 

 absorbed by the roots in the relatively warm soil than passes off as 

 vapour in equal periods of time. An enhanced wateriness of the 

 whole plant, then, is one result. 



But the low temperature, feeble light, and partially blocked 

 ventilation system have for a consequence a depression of respiratory 

 activity and the absorption of oxygen generally. Enough oxygen 

 gas finds its way slowly into the cells to keep the life-processes going, 

 of course, but not enough to complete the oxidations and decompose 

 the organic acids, at the prevailing low temperature, so rapidly as 

 before,* and thus another consequence is a tendency to the accumula- 

 tion of organic acids. According to de Vries, however, the increase 

 of organic acids mnst make itself effective in enhancing the turgidity 

 of the cells, and no doubt it does so to a certain extent; beyond a 

 certain point, however, it is more likely to increase the permeability 

 of the protoplasm, f and we may even suppose small quantities of the 

 acids to filter out even to the watery cell- walls.! 



Partly due to the low temperature and the depressed gas-inter- 

 change, but far more owing to the feeble light, the process of 

 assimilation will be less active than previously. This will not be 

 immediately felt if, as will probably be the case, there are large 

 quantities of temporary reserves in the leaves and internodes ; but it 

 may react indirectly on the processes of oxidation and respiration, 

 inasmuch as less free oxygen is evolved in the cells than would be 

 the case in bright weather. As the temporary stores of starch dis- 

 appear, however, the cells become more and more surcharged with 

 glucose, together with organic acids, and it depends on several circum- 

 stances, especially on how rapidly growth is going on (e.g., in the 

 parts below ground), whether this glucose in solution passes away, or 

 is nsed np slowly or rapidly ; if it cannot move, or only extremely 

 slowly, then we have the case of tissues surcharged with water con- 

 taining organic acids and glucose in solution. It may be surmised 



* See Warburg, op. cit., especially pp. 73 — 77, and 126. 



f See Pf effer, " Ueber Aufnahme von Anilinfarben in lebenden Zellen," in ' Unters. 

 a. d. Bot. Inst, zu Tubingen,' vol. 2, 1886, pp. 296 and 329, for proof that dilute acids 

 can traverse without permanently injuring the protoplasm. 



X Pfeffer showed, for instance, that methyl-orange, after being taken up in the 

 living cell and held there by the protoplasm, can be made to diffuse out again if a 

 little citric acid is imbibed (" Ub. Aufnahme," &c, op. cit., p. 293). 



