106 BOTANICAL GAZETTE [FEBRUARY 
that the entrance of a hypha into a stoma is merely a preliminary 
act, distinct from infection proper, and controlled. by general con- 
ditions, while the fate of the hypha after its entrance is determined 
by complex reactions between parasite and host, which are largely 
specific in their nature. In the light, then, of known facts, no simple 
explanation, such as the theory of chemotropism due to the presence 
of specific chemical compounds, is adequate. Chemotropism may 
possibly be one factor in the complex phenomenon, but it is certainly 
not the predominant factor. 
CONCLUSIONS. 
Various tests upon a number of fungi failed to indicate the exist- 
ence of any definite chemotropic sensibility to nutrient substances 
or other chemical compounds in solution. If positive chemotropism 
exists, it is less prominent than other tropic phenomena involved, 
and was obscured by them. 
Those substances which furnished nutriment to the fungi caused 
a decided growth, often with thickening of the hyphae and an increased 
branching; but they did not cause a more marked turning of the 
hyphae toward the diffusion centers than did non-nutrient and 
toxic substances. 
All of the fungi tested show a tendency to turn from a region in 
which hyphae of the same kind are growing toward one destitute 
of hyphae, or in which the hyphae are less abundant. The turning 
toward a medium in which mycelium has grown, but from which 
the mycelium has been removed, is less marked than that toward 
a medium in which no mycelium has grown. This may be regarded 
as a negative reaction to stimuli from chemical substances, which 
owe their origin in some way to the growing fungus. 
Various fungi show a positive hydrotropism; but an over- 
abundance of moisture may cause a negative reaction in certain fungi. 
The changing of the direction of growth of fungous hyphae is 
a complex phenomenon in which at least two factors, cytotropism 
and hydrotropism, are concerned. Since the complete elimination 
of neither of these factors is possible, all tests must be relative, and 
to that extent unsatisfactory. 
It would seem that the reactions of mycelium to various stimuli 
