Host and Parasite in certain Diseases of Plants. 401 



scription of the histological peculiarities of the cell, and it will 

 probably suffice to remind you that great differences occur in detail 

 as to the size of the cell, the thickness of the wall, number and sizes 

 of the chlorophyll corpuscles, and the presence or absence of colouring 

 matters, crystals, various organised bodies, and so forth. Finally, it 

 will be remembered that all the parts — cell wall, nucleus, chlorophyll 

 corpuscles, and protoplasm generally* — are more or less thoroughly 

 saturated with water, and that aqueous vapour and gases will be found 

 in varying proportions in the passages between the cells, and con- 

 tinuous with the atmosphere, on the one hand, and with the water in 

 the roots and soil, on the other. 



Let us now inquire what these normal living cells are doing when 

 they still form an integral part of the tissues of the healthy plant. 



In the first place, they ai-e respiring. That is to say, the protoplasm 

 absorbs oxygen gasf brought to it in the water from the roots, from the 

 intercellular spaces which communicate with the atmosphere by means 

 of stomata and lenticels, and from the chlorophyll corpuscles when 

 they are assimilating in bright light. This oxygen enters in solution 

 into the protoplasm, and combines with some of the bodies which for 

 the time being enter into the composition of this complicated 

 structure. The effects of these unions of the oxygen are expressed in 

 molecular disturbances in the protoplasm : some bodies are broken 

 down, others enter into new unions. Finally, the disturbing actions 

 of the energetic oxygen result in the combustion of certain carbon- 

 compounds to carbon dioxide and water, and these escape from the 

 field of action : such combustion implies the liberation of energy, 

 and we recognise this in the complicated movements and life-processes 

 set up in the protoplasm and in the rise of temperature, which can be 

 proved to take place.J One point of importance should be insisted 

 on from the first. "When the oxygen- molecule enters the protoplasm, 

 it must be pictured as coming into a busy arena, where numerous 

 but definite possibilities are presented to it, and although we are not 

 in a position to trace its movements, § and the intermediate effects of 

 these, in detail, the evidence shows that while the quantities produced 

 accord with the general view that it is such substances as glucose, 



# For particulars as to t.hese, cf., e.g., Zimnierniann, " Die Morph. imd Physiol, 

 der Pflanzenzelle ; " Schenk's ' Handbuch,' vol. 3, Heft 2, pp. 497—700; and Noll, 

 " Die "vrichtigsten Ergebnisse der botaniscben Zellenforschung in den letzten 

 15 Jahren" ('Flora,' 1889, pp. 155—168). 



t Cf. Sachs, ' Lectures on the Physiology of Plants,' pp. 395—408 ; Vines, 

 'Physiology of Plants,' pp. 195—202; Pfeffer, ' Pflanzenphysiologie,' vol. 1, pp. 

 346—363. 



X Kodewald, ' Pringsheim, Jahrb. f . wiss. Bot.,' vol. 17, 1886, p. 338 ; vol. 19, 

 1888, p. 221. 



§ It may be regarded as certain that for respiration it does not suffice for a body 

 to be merely in the protoplasm (see Pfeffer, ' Oxydationsvorgange,' pp. 489 — 490). 



