K.—BOTANY. 241 
mechanical puncture and resistance to attack by this fungus are definitely 
correlated. These authors also determined by plasmolysis the osmotic 
pressure of the fungal hypha and the pressure required to perforate the 
tissues, and they found that in all cases but one the osmotic pressure 
was sufficient to allow of puncture of the wall of the potato cell by the hypha. 
The question of the nutritive conditions to which the germ-tube is 
exposed when developing on the host tissue is evidently of importance in 
infection. A strong well-developed germ-tube is more likely to succeed 
in penetrating the host tissues than a weakly one. This is in agreement 
with the experience that with forms like Botrytis and Colletotrichum it is 
easier to get infection from drops of weak culture medium than from water. 
In nature, however, the ‘infection drop’ must usually consist of rain or 
dew. That substances which are able to stimulate the growth of the 
germ-tubes can diffuse from the underlying host tissue into the infection 
drop has been shown by W. Brown (loc. cit. 1916). How considerable 
may be the amount of substances diffusing into water on the surface of a 
plant is shown by the analysis of dew from cotton plants given by Smith. 
No less a quantity than 1,300c.c. was collected, and it was found to have 
a content of total solids of 1,023 parts per million, most of the solids 
consisting of calcium and magnesium carbonates. 
The observations of R. J. Noble on Flag Smut of wheat (Urocystis 
tritict) provide another example of the stimulating action of minute 
amounts of tissue extracts. The addition of a few thin slices of wheat 
_ tissue to water in which well-soaked spores of this fungus are lying increases 
_ very markedly the amount of germination over that in ordinary culture 
media. The action is not specific, for tissues of rye, barley, flax, etc., 
_ will produce the same effect, though to a less degree. The distillate from 
watery extracts of wheat seedlings was also found to act, so the stimulating 
substance is volatile, and possibly similar to the substances observed by 
Brown, to which reference has already been made.’ 
It is clear from such observations as these that the conidium may find 
in the infection drop on the leaf a supply of nutritive or stimulating 
substances. Of the chemotropic power of these substances there may be 
some doubt, but of their importance in the production of vigorous germ- 
tubes well equipped for the work of cell-wall penetration there can be 
little question. In the study of the mechanism of entry by various fungi 
_ into the epidermal tissues of their host undertaken by the writers already 
_ mentioned, not only was there no evidence of solution of the cuticle, but 
until the cuticle had been ruptured there was no sign of enzymatic action 
on the cell-wall layers beneath. This suggests that cell-wall dissolving 
_ enzymes are unable to diffuse through the cuticle. It should be pointed 
_ out, however, that Smith,’ in his study of the haustoria of the Erysiphacez, 
- describes a change in the staining reaction of the cell wall below a hypha 
a before the cuticle had been ruptured. Miss Allen also describes a marked 
144C, M. Smith: ‘ Excretion from Leaves asa Factorin Arsenical Injury.’ J. Agric. 
Res., XXVI., 191-4, 1923. The analysis in full, in parts per million, was S,Oy, 13; oxides 
4 Iron and Aluminium, 17; SOs, 26; Cl, 19; CaO, 529; MgO, 100: CO (by titration), 
618. 
18R. J. Noble; ‘Studies on Urocystis tritici Koern, the Organism causing Flag 
Smut of Wheat.’ Phytopathology, 13, 127, 1923. 
16 G. Smith ; ‘A Study of the Haustoria of the Erysiphacex.’ Bot, Gaz., 16,1905, 
—  - 1924 R 
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