128 Salmon . — Further Cultural Experiments with 
Through the work of Janczewski (7) and of Sorauer (8) proof has 
been given that the action of frost causes injuries to plants which render 
them susceptible to attacks by certain saprophytic Fungi which are not 
able to infect them under normal circumstances. Injuries caused by 
hail, &c., affecting plants in the same way, have been described by 
Hartig (10) and Sorauer (9)> 
In the hypothesis advanced in my previous paper (1, p. 1 13) to 
account for the susceptibility shown by leaves injured by a cut or burn, it 
was assumed that in consequence of the vitality 1 of the leaf-cells being 
affected by the injury, either the protective enzymes or similar substances 
normally present are destroyed or become weakened, or the production of 
them by the protoplasm is interfered with, in the cells in the neighbourhood 
of the injury. This hypothesis may still be advanced to account for the 
same susceptibility being shown when leaves are injured by the action of 
alcohol, ether, chloroform, or heat, since it is known that protoplasmic 
functions are temporarily inhibited by anaesthetization, and that a high 
temperature may partially or entirely inhibit the process of the secretion 
of an enzyme (3). 
Attention may be directed to the fact that these cases of the loss of 
immunity, brought about by causes which affect the vitality of the leaf, find 
their exact parallel in the recorded instances of induced susceptibility in 
animals to certain diseases caused by bacteria. The decrease of vitality 
caused by fatigue, action of drugs, abnormal food, or environment has been 
proved to induce susceptibility to certain bacteria in the case of animals 
which are immune under normal circumstances (4, 5). 
In a recent paper published by Ray (6) the statement is made that 
plants of maize exposed to ether vapour or to heat were rendered by this 
treatment more susceptible to the attacks of a yeast-form of Ustilago 
Maydis which had been grown saprophytically for some time. According 
to Ray, the increased growth of the Fungus here shown was due to the 
injury to the plant having caused a change in its metabolism. The change 
assumed is that a certain plastic substance, which serves as food for the 
Fungus, accumulates in consequence of its non-transference by an enzyme 
of the host-plant. 
A series of experiments was carried out with the object of ascertaining 
the infection-powers of the conidia of the first generation produced on leaves 
injured by the action of alcohol, ether, and heat respectively. Leaves 
of barley thus treated were sown with conidia taken from wheat. The 
conidia produced on the treated barley leaves were sown simultaneously on 
1 Until our knowledge of the physiology of the cell has progressed further, it is necessary to use 
the general term vitality to express the sum of the individual physiological processes at work in the 
cell. External factors which affect the normal balance in the working of the individual physiological 
processes increase or decrease the vitality of the plant. 
