Ward . — Recent Researches on the Parasitism of Fungi. 31 
External influences undoubtedly exert important effects. I have given 
evidence (190, p. 391) to show that too high a temperature during incuba- 
tion may ruin the Fungus. Internal influences may result in the death of 
both hypha and host-cell, and then comes ruin to the former. 
Klebahn ( 92 , p. 262) shows that sporidia of P. Conv altar ia-digraphidis 
if sown on Polygonatum, and that the sporidia of Gy mno sporangium 
clavariaeforme if sown on Pyrus Aucuparia ( 91 , p. 150), form poor spots, &c. 
Also Bolley ( 80 , p. 893) got mere specks on cereals with uredospores ; 
and I have given similar examples ( 191 , p. 298). 
Here we have several cases possible: (1) It is possible that the en- 
vironment stops the growth of the Fungus ; (2) too many competing infections 
may be present ; (3) the Fungus may be too weak to overcome the host- 
cells ; (4) the host-plant may be too rich in antitoxins; (5) the Fungus 
may be too strong, and so kills the cells destructively ; or (6) the host is 
too weak and succumbs too rapidly at the spot attacked. 
In this connexion it is instructive to find that Mr. Evans and myself 
have satisfied ourselves (1) that so-called immune plants furnished us by 
the agriculturists may be really badly inoculated, bearing innumerable minute 
yellow and brown spots, each of which is an abortive infection area ; (2) that 
experimental inoculation results in just such spots: the germ-tube goes in, 
but the walls of the cells in contact with it turn brown and die, and the 
hyphae are starved ; and (3) we can stop the infection-tube, even when well 
in the leaf, by various modes of interference with nutrition, especially by 
starving the leaf of carbon dioxide : also by floating the leaf on water and 
starving it of salts, or by heating or cooling the roots. 
That chemotactic actions are factors in the complex fight which results 
from the antagonism between the Fungus — with its attacking weapons such 
as enzymes and toxins — on the one hand ; and the cells of the host — with 
their defending armoury of anti-toxins and other enzymes — on the other, 
cannot be denied. But that it is only one of several sets of factors in an 
extremely complex phenomenon, seems clear when we see that inoculation — 
the process of entry of the germ-tube and development of the sub-stomatal 
vesicle — and infection- — the attack of the cells by the infecting tube and 
haustoria — are two distinct events ; that external influences such as tempera- 
ture, and internal factors such as the age and ripeness of the spore, and the 
condition of nutrition, starvation, &c., of the host-cells, affect the matter ; and 
that a certain species of host A, though perfectly capable of being infected 
with, say, Puccinia dispersa , will refuse to be infected by spores of that 
Fungus taken from a host B, while it will take it from a host C, and will 
even take it from C when the latter has received it from B. 
It seems justifiable, therefore, to speak of predisposition and immunity 
in the light of the above facts. 
