Host and Parasite in certain Diseases of Plants. 



417 



"brings us to note that the key to the condition of affairs is the fact 

 that the dry weight of the etiolated shoot is decreasing : every molecule 

 of carbon dioxide which conies away lessens the dry organic substance 

 of the plant, and no restoration of such substauce is possible in the 

 absence of light. 



In other words, then, the etiolated plant is growing to death, at the 

 expense of what organic carbon-compounds it possessed at the begin- 

 ning. 



Assimilation is, of course, profoundly affected, like every metabolic 

 process, according to the relative amounts of oxygen and carbon 

 dioxide in the air, and although it never happens in nature that the 

 extremes are approached, nevertheless experiments on this subject 

 have led to interesting results.* 



The quantity of water present in the plant and its atmosphere and 

 the rapidity of the transpiration current undoubtedly affect the 

 process of assimilation in a high degree. Not only is water needed 

 for the molecular processes concerned in the act of assimilation, and 

 not only does the supply of materials to the protoplasm depend 

 mainly on the transpiration current, but, as we have seen, the 

 aeration of the intercellular passages, and consequently the move- 

 ments of gases generally are affected. 



It has long been known that the quantities of carbon dioxide 

 absorbed, and of oxygen evolved, in the process of assimilation, vary 

 with the age of the leaf or other organ concerned, and Kreussler has 

 shown that one reason for these variations is the quantity of water 

 present in the tissues at the time. In fact, an essential cause of 

 variations in assimilation exists in the differences in the water 

 contents of the tissues, f and it is no doubt largely due to the want of 

 water that older leaves assimilate so unequally — they are unable to 

 rapidly restore the equilibrium between losses and gains when it is 

 seriously disturbed. 



As for transpiration itself, and all the movements of water cor- 

 related with it, it is well known that the various factors of the 

 environment affect it profoundly. 



Apart from the more obvious relations^ between transpiration and 

 the temperature of the atmosphere, and the quantity of aqueous 



* See Godlewski, " Abhangigkeit der Stark ebildung in den Cbloroplxyllkornern 

 ron dem Koblensauregebalt der Luft " (' Flora,' 1873, p. 378) ; also in ' Arb. des 

 Bot. Inst, in Wurzburg,' 1873, vol. 1, p. 343. Furtber, Pringsbeim, " Ueber die 

 Abbangigkeit der Assimilation gruner Zellen von ibrer Sauerstoffatbmung," &c. 

 (' Sitzungsber. d. Kgl. Preuss. Akad. der Wiss. zu Berlin,' 1887, No. 38, pp. 763 — 

 111). 



f Kreussler, " Beobacbtungen iiber die Koblensaure-Aufnabme und -Ausgabe 

 der Pflanzen," II (' Landwirtbscb. Jabrb.,' vol. 16, 1887, especially pp. 728—30). 



X See tbe text-books referred to, especially Pfeffer, 4 Pnanzenpbysiologie,' vol. l r 

 pp. 146—150. 



