84 BOTANIGAL GAZETTE [FEBRUARY 
the substratum if the fungi were grown in a nutrient medium, although 
the mycelial growth was more vigorous. 
SWINGLE (31), in explanation of the effects of Bordeaux mixture, 
advanced the hypothesis, which he based on the studies of REIN- 
HARDT, BiscGEN, and Mryosut, that copper hydroxid may prevent 
the germ-tubes of parasitic fungi from entering the host plant through 
negative chemotropic action. 
NORDHAUSEN (24) accepted MryosuHt’s conclusions, and investi- 
gated the biological bearing of chemotropism upon the penetration 
of plant tissues by certain fungi, without bringing forward any 
additional evidence in favor of chemotropism. 
No further investigation seems to have been made with relation 
to fungi, until CLARK (4), in his investigation of SWINGLE’s hypoth- 
esis, had occasion to test the chemotropic reactions of certain species, 
especially Mucor stolonifer, to toxic substances. For the most part 
he followed Mrvyosut’s methods closely. In every case it was found 
that the hyphae would turn from a nutrient medium and grow into 
media containing such toxic substances as salts of copper, cobalt, 
nickel, zinc, etc., until a concentration sufficient to cause death was 
reached. The hyphae turned toward non-nutrient media and dis- 
tilled water as readily as toward nutrient media. His conclusion 
is that Mucor stolonijer is negatively chemotropic to some secretion 
of its own mycelium, and that this negative chemotropism is much 
greater than any positive chemotropism it may have for food sub- 
stances or oxygen. 
MASSEE (19) found that fungi are attracted to their hosts by 
specific stimuli from substances in the cell sap. In the case of 
saprophytes and facultative parasites, the attractive substance is 
saccharose; the facultative parasites, however, may be repelled by 
more potent negatively chemotropic substances in the cell sap. In 
the case of obligate parasites, the cell sap of the host plant is the 
strongest positive chemotropic agent; malic acid is the specific attrac- 
tive substance for the germ tubes of Monilia fructigena, and the enzyme 
pectase for Cercospora cucumis. Immune plants owe their immunity 
to the absence of the chemotropic substance. 
Other factors affecting the direction of growth of fungous hyphae 
have received but little attention, while the causes of the bending — 
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