1916] GRAVES—CHEMOTROPISM IN RHIZOPUS 363 
katabolic products by the hyphae, or whether they are the result 
of chemical changes which these excreted products induce in the 
medium itself, it is not possible to state with certainty. In any 
case, vital processes of the fungus are primarily responsible. 
Of these two possibilities, however, it is much more probable 
that the fungal excretions are themselves directly the cause of the 
negative chemotropic action, chiefly on account of the fact that ail \ 
the media worked with produce a negative chemotropic reaction | 
after the fungus has grown in them for a time. It is unlikely that 
with these various kinds of media the same repellent substances 
would invariably be formed by chemical action of the fungal excre—/ 
tions. In support of this view, it is also noteworthy that wi 
plain agar the same repellent effect was evidenced as with other \ 
media. — 
That the staling substances are either of a volatile or of a ther- 
molabile nature has already been conclusively demonstrated. 
BALLs (rz) has also shown this, and has ascertained that they exert 
an inhibitory effect on growth, 
RELATIVE VALUES OF POSITIVE AND NEGATIVE CHEMOTROPIC 
FORCES.—The great difference of various germ tubes in their 
capacity for reaction was very noticeable in the examination of the 
preparations. Thus, with two germ tubes at an early stage of 
growth, and at an equal distance from the hole, one would react 
markedly, while the other remained indifferent. At a later stage, 
however, when we may assume that the stimulus was greater, all of 
the germ tubes within the prescribed region about the hole might 
react. 
Our knowledge of the relation between individual variation in 
sensitiveness of the germ tubes and the intensity of the acting 
stimulus is of course very vague,’ but it seems safe to assume that 
a larger percentage of turning means a stronger stimulus. Also 
the rate of diffusion of the stimulating substances would, within 
limits, affect the number of germ tubes reacting in a given volume 
*It is probable that the variation curve of sensitiveness of the germ tubes i is of 
the well known “normal” type, so that the p 
y propor- 
tional to the strength of the stimulus. TRONDLE (x2) has shown that for geotropic 
sensitiveness the curve is of this type. 
