Host and Parasite, in certain Diseases of Plants. 407 



ments of water, chiefly dne to transpiration, but also incidentally 

 caused by local decomposition, osmotic absorptions, &c, are effective 

 in bringing about aeration of the tissues ; of course this aeration (or 

 ventilation) is not to be confounded with the movements of free 

 gases, due to diffusion or to expansions or contractions due to changes 

 of temperature.* 



These, then, are some of the changes which are continually and con- 

 tinuously going on in the living cells of the normal plant. Of course 

 I have not attempted any exhaustive list, or even a complete sketch 

 of the structures and processes met with in living cells, the purpose 

 being simply to bring prominently into view certain features of im- 

 portance to the matter in hand. 



The Death of the Cell 



The next point to consider is, what changes are observed when 

 such cells as the above are killed.f lb appears to be of little moment 



Fig. 5. A thin-walled parenchymatous cell killed by a few seconds' immersion in 

 water at 75° C. The protoplast contracts from the cell-wall, carrying with it 

 the nucleus and chlorophyll-corpuscles, and allowing the cell-sap to escape ; the 

 thin cellulose wall consequently becomes lax, and suffused with cell-sap. A 

 similar result is brought about by longer immersion in water at lower tempera- 

 tures (above 50° C), or by very low temperatures, the action of poisons, &c. 

 (Highly magnified.) 



* For further discussion, see Pfeffer, 8 Pflanzenphysiologie,' vol. 1, pp. 112 — 113, 

 and the literature on transpiration. 



f On this subject cf. Frank, ' Krankheiten der Pnanzen,' 1880, pp. 12 — 15 ; 

 Detmer, in ' Bot. Zeitung,' 1886, No. 30 ; Pringsheim, in ' Jahrb. f . wiss. Bot.,' 

 vol. 12, 1880 (pp. 47 — -50 of the separate copy) ; also de Vries, ' TJntersuch. 

 mechan. Ursachen d. Zellstreckung,' pp. 17 — 21. 



