484 Brooks. — Observations on the Biology of Botrytis cinerea. 
be interesting to know what is the exact physiological condition of these 
yellowing leaves. That the power of carbon assimilation is dwindling 
rapidly seems certain, for under the microscope the colour of the chloro- 
phyll granules is seen to be fading. One naturally inquires what is the 
cause of the direct infection of the yellowing leaves and the non-infection 
of the normal green leaves. 
It may be that some chemotropic substance present in the cells of 
the yellowing leaf attracts the germ-tubes, whereas no such chemotropic 
influence is exerted by the normal leaf. Recently, however, Fulton has 
thrown considerable doubt upon the positive chemotropism of fungal 
hyphae. 
Or is it possible that in the case of non-infection of a normal leaf, 
some substance from the epidermal cells diffuses through the cuticle in 
sufficient quantity to neutralize the effect of the small amount of poisonous 
substance secreted by the germ-tubes, whereas no such substance diffuses 
from the yellowing leaf? This would mean that something in the nature 
of an antitoxin accounts for the immunity of the normal leaf from infection 
by the spores. A third explanation is open to suggestion — it may be that 
the change in the vital activities of the leaf consequent upon yellowing 
induces some alteration in the composition of the external cell-walls, which 
enables the germ-tubes to penetrate them. 
Subsequent research will show whether either of these explanations 
is valid. 
Similar experiments to those outlined above were tried with plants 
which had been placed in darkness. At the time of placing the plants in 
the dark-room, all leaves which showed any trace of yellowing were cut off. 
After the plants had been kept in darkness five days, spores were placed 
upon the leaves, which had by that time begun to show signs of yellowing. 
The areas upon which the spores had been placed were cut out and fixed as 
described before. Upon microtoming and staining it was found that the 
germ-tubes had penetrated the tissues a day after inoculation. Three days 
after placing the spores upon the leaves large disease areas could be seen 
with the naked eye. In the same way, green leaves cut from healthy plants 
and placed under damp conditions in darkness, after being inoculated with 
spores, showed the characteristic disease areas immediately after they had 
begun to turn yellow. F. F. Blackman has shown in his experiments upon 
the respiration of starved Cherry Laurel leaves, i. e. leaves cut off the plant 
and placed in darkness, that soon after the period of maximum C0 2 pro- 
duction prior to death, the common mould fungus Penicillium begins 
to flourish upon the leaves. At this juncture the juices of the leaf-cells are 
escaping, and provide the necessary saprophytic nourishment for the spores 
of the moulds which are lurking on the surface. It seems from the experi- 
ments described above, that the time when Botrytis conidia cause infection 
