1916] GRAVES—CHEMOTROPISM IN RHIZOPUS 343 
glass slide, and since only the low power was necessary, no cover 
glass was used, on account of the danger of shifting 
In working out the results, it was found essential to get an idea 
of the amount of growth which had taken place. This was done 
by estimating the average length of the hyphae in the preparation 
in question. To obtain this average, only those hyphae near the 
holes were considered, since in the regions midway between these 
holes the conditions of nourishment differed from those in the 
immediate proximity of the holes; especially was this true in the 
case of germ tubes in films made of plain agar, with a nourishing 
substance on the other side of the mica plate. A definite area was 
selected near each hole, and the lengths of all the hyphae in this 
area noted. In this way 3 or more holes in different parts of the 
preparation were considered, or until a total of 15 hyphae had been 
counted and averaged. 
Since, as will be shown later, the number of viable spores,‘ 
as well as the number of hyphae present, was found to have a 
marked influence on the results, it was desirable for comparative 
results to have the spore numbers approximately equal. In order 
to get a suspension of spores which would produce the desired 
number of spores to each volume of film covered by a square milli- 
meter of surface, the following method was used. Spore-bearing 
mycelium was allowed to stand in water for a few moments and 
was then strained through a piece of muslin, which had previously 
been freed of its starch by thorough boiling and washing. The 
spore suspension was then tested as to its concentration by the use 
of a micrometer eyepiece containing accurately ruled squares, and 
by diluting, or by centrifuging and pouring off some of the water, 
was brought to the desired strength. 
METHOD OF ESTIMATING THE INTENSITY OF THE CHEMOTROPIC 
REACTION.—In work of this kind, the method of estimating the 
reaction is all-important, since on it depends the accuracy of the 
4It was found that the number of viable spores present bears a definite relation 
to the percentage of germination. When the spores were excessively numerous no 
germination at all resulted. The cause of this non-germination was not worked 
out experimentally, but it is probably knit to the same agencies discussed later 
in this paper; that is, the excretion of toxic products by the spores, in this case in the 
pre-germination stages. 
