Host and Parasite in certain Diseases of Plants. 



411 



later on, there are some other remarkable changes going on in the 

 cell, and connected indirectly with the action of light and respiration, 

 but these do not probably affect the general conclnsions just advanced, 

 close as is the connexion between respiration and metabolism gene- 

 rally. 



The qnestion now arises as to the quantity of oxygen necessary for 

 respiration, and as to the effects of undue accumulation of the carbon 

 dioxide : it is too long a subject and it is unnecessary to discuss it in 

 detail.* I need only remind you that in the absence of oxygen 

 respiration ceases,f while it is interfered with when the amount of 

 oxygen is much greater per unit of volume than in ordinary air, i.e., 

 when the oxygen is condensed ; under ordinary circumstances, how- 

 ever, the free oxygen of the atmosphere amply suffices, provided it 

 can pass readily into the cells and be renewed. Anything of the 

 nature of stagnation must be assumed to impede respiration, whether 

 simply from the accumulation of carbon dioxide, or other products of 

 respiratory activity and consequent metabolism, or because sufficient 

 oxygen-molecules do not pass into the protoplasm in a given time. 

 Since the extremes are not nearly attained in nature, however, I pass 

 by this subject with the remark that in proportion as the intercellular 

 passages or other communications with the atmosphere J become 

 blocked by condensed water, for instance, the ventilation of the 

 plant — and therefore its respiration — may su±fer§ for the time being 

 simply on account of the slower diffusion of the gases, carbon dioxide 

 and oxygen, from one part of the plant to another. 



Coming now to the subject of destructive metabolism, we find that 

 it is affected by external factors ; in the first place, by whatever affects 

 respiration, and therefore the foregoing remarks apply to metabolism 

 generally. This is especially so in the case of temperature, and the 

 statements already given may serve broadly with respect to metabol- 

 ism as a whole. A few details are of importance, however. We have 



* For further details, of. the text-books already cited, e.g., Pfeffer, vol. 1, p. 377. 



f We are of course not concerned with so-called " intra-molecular " respiration 

 (cf. Pfeffer, vol. 1, pp. 370—374). 



X See Eussow, " Zur Kenntniss der Holzer," &c, in 'Bot. Centralbl.,' 1883 

 vol. 13, p. 136. 



§ It is no uncommon event, even in England, to see the intercellular passages of 

 leaves blocked with suffused water after a cold night, but the phenomenon is much 

 commoner in the tropics, and occurs quite generally in the hill country in Ceylon, 

 for example. As the temperature rises during the morning, the water quietly 

 evaporates and the leaf loses its dark, suffused, limp appearance, and becomes 

 normal. Of course, the phenomenon is due to proportionally more water being 

 absorbed from the relatively warm soil than the cool air can take up. See also 

 Pfeffer, ' Pflanzenphys.,' vol. 1, p. 172, and the literature concerning the ascent of 

 water in plants (collected in Marshall Ward, ' Timber and some of its Diseases,' 

 1889, pp. 59—141). 



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