326 
well known property of many toxic bodies, that in dilute solu- 
tion they act as growth stimulants, and proliferation of plant 
cells as a result of exposure to various poisonous substances has 
recently been abundantly demonstrated by Smith.* When the 
tissues of the present host were invaded the active principle of 
the fungus would diffuse rapidly through the cells, and at the 
extreme limit of its diffusion range it would appear probable 
that the highly dilute toxic body stimulated the living wood- 
parenchyma cells to the formation of intrusive vesicles. The 
period of time during which the tyloses must have been formed 
could only be very brief, for as has been noted the active prin- 
ciple of Botrytis diffuses rapidly, and the toxic substance would 
very shortly be present in a lethal ‘concentration. Immediately 
this acted upon the cells of the wood-parenchyma, the tyloses 
would collapse and shrivel together choking the lumina of the 
tracheae with dead matter; the rapidity of this process being 
manifested in the brief wilting period of the foliage of the tree. 
These plugs of dead tyloses were sufficiently abundant for it to 
be not improbable that they were primarily responsible for the 
complete interruption of the water stream in the stem of the 
Aesculus Pavia under consideration. 
Longitudinal Extension of Fungus in Host.—a<As the roots of 
the tree were in a normal state of activity the blocking of the 
growth was much more gradual, correspondin 
paratively broad, diffuse, discoloured zone marking the boundary 
the diseased region (Pl. vii. Fig. 3, aca); and the inability 
of the fungus to develop further was probably due to the increas- 
ing desiccation of the wood. 
* Smith, E. F. Mechanism of T Growth in (¢ 1 Jour. Apr. 
Res. vol. viii. 1917. ee OF paeeens Preees ii Crowngall, dour. 2% 
