Host and Parasite in certain Diseases of Plants. 40^ 



the protoplasm of membranes of living cells,* and if the weather 

 becomes dry the dead tissues rapidly desiccate and shrivel. 



In attaining the above described extreme, commonly called death, 

 the normal living cell, in the condition commonly called health, 

 passes through a series of vicissitudes which affect every part of 

 it ; but it is necessary to admit that the state called death and that 

 called life, in the above discussion, are by no means definite and 

 utterly distinct from one another — on the contrary, the very essence 

 of life consists in its mobility, and the living cell is continually 

 approaching and receding from the state termed death. In a certain 

 sense, no doubt, death may be regarded as the cessation of life, bat 

 this does not help us, because the crux resides in determining when 

 life ceases in the protoplasm. Of course we can lay oar hands, as it 

 were, on given cells or tissues of cells, and say these are " living," 

 whereas others are " dead," but the difficulty is to decide when the 

 one state passes into the other. 



Between normal life, i.e., the condition of affairs where the life- 

 processes are going on actively, and the state of permanent death, 

 then, there are all possible gradations : many of these gradations 

 •coincide with the phenomena of disease — pathological conditions — and 

 it is towards this difficult domain that I have now to carry the 

 discussion. 



Variations in the Environment as affecting the Physiological Processes in 



the Host. 



In describing the phenomena going on in what was termed the 

 normal, living cell, I only hinted at the fact that variations, more or less 

 periodic in nature, occur in the intensity of the processes, a trath which 

 at once shows the difficulty of deciding what a normally living cell 

 really is. But it is of the utmost importance to recognise that all the 

 life-processes, and the changes dependent on them, are in their very 

 nature variable. One set of factors which bring about the variations 

 are internal and inherited, and very little is known of them beyond 

 the fact of their existence, which is usually formally expressed by the 

 admission that different plants differ in " constitution ; " fortunately, 

 this series of factors does not concern us at present, and it does not 

 vitiate our general conclusions to assume that on the whole the 

 differences in constitution between plants of the same species are so 

 minute that they may be neglected. 



The second set of factors is of much greater importance, because 

 they give rise to pronounced and easily recognisable changes in the 



* See Sachs, ' Physiologie Vegetale,' pp. 253, for proof that water evaporates less 

 rapidly from living cell-surfaces than from dead ones, and for literature (my edition 

 is the French one of 1868) . 



VOL. XLVTI. 2 E 4 



