Host and Parasite in certain Diseases of Plants. 



403 



up with them into the more complex bodies, to be again broken 

 down, and repeat the process,* and so on. 



If, however, for any reason a lack of these carbohydrates occurs,, 

 then these amide-bodies increase for the time being, and the proto- 

 plasm suffers accordingly ; in fact, it undergoes further decompositions 

 as a result of starvation. 



The evidence also goes to show that organic acids (such as malic, 

 citric, tartaric, oxalic, &c.) are formed in the protoplasm, and accu- 

 mulate in the cell-sap during these metabolic processes, as products 

 of incomplete oxidation, and their variations in quantity depend 

 greatly on the activity of these metabolic processes, and, therefore, on 

 the intensity of respiration. f A fact of primary importance for us is 

 that these organic acids increase considerably in amount under con- 

 ditions which lead to less complete oxidation, and, conversely, they 

 decrease when certain oxidation-processes in the cell are promoted. 

 In other words, they are continually being formed and destroyed in 

 metabolic changes, and sometimes one process, sometimes another, 

 predominates. 



As a third group of life-processes which our selected cells would 

 exhibit, we may regard the phenomena of growth; processes which 

 are intimately dependent upon respiration and metabolism, and, indeed, 

 inseparable from them in life. 



For our present purpose, it suffices to regard growth J as consisting 

 in an extension of the still soft cellulose cell-walls, which tends to 

 increase the area of the membrane at the expense of their thickness, 

 and in a compensating increment in their thickness due to the 

 activity of the protoplasmic lining in secreting and laying on cellulose 

 on the inside of, or even in the structure of, the wall. Passing over 

 the fact that the secretion of this cellulose is another manifestation of 

 metabolic activity on the part of the protoplasm, it is important to 

 notice that growth is only possible so long as respiration is proceeding, 

 and so long as the cell is turgid. § Now turgidity depends on the 



* See E. Schulze, ' Landwirtbschaftliche Jahrbucher,' 1876, vol. 5, p. 848 ;. 

 Borodin, 'Bot. Zeitg.,' 1878, col. 801; Pallaclin, " Ueber Eiweisszersetzung in d. 

 Pflanzen," &c.' (' Ber. d. Deutsch. Bot. Gesellscb.,' 1888, p. 205, see p. 212). 

 Eurther literature in Pfeffer, ' Pnanzenpbys.,' p. 301. 



f See especially Warburg, " Ueber die Bedeutung der organiscben Sauren fur den 

 Lebens-process der Pflanzen" (' Unters. aus d. Bot. Inst, zu Tubingen,' 1886 — 88, 

 vol. 2, pp. 53 — 152, where the literature is collected up to date; and Pallaclin, 

 " Athmung und Wacbsthum" (' Ber. d. Deutsch. Bot. Gresellsch.,' 1886, pp. 322 — 

 328), and tbe same on " Bildung der organiscben Sauren in den waclisenden Pnan- 

 zentheilen" (ibid., 1887, pp. 325—326). 



X Eor a general account of growth, cf. Sachs, ' Lectures,' pp. 411 — 424 and 567 

 —569. 



§ See de Tries, 1 Unters. uber die mechaniscben Ursacben der Zellstreckung,' 

 Leipzig, 1877, and the literature tbere quoted. 



