486 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
surface. The fructification comes to maturity in the dark as well 
as in the light, and when the light is thus excluded, the colony is 
invariably elevated at right angles to the substratum. 
The force which causes the fructification to be directed away 
from the substratum and at right angles to it, is in all probability 
negative hydrotaxis. It is true that a colony may creep up the side 
of the test tube in which it is cultivated, finally turning away from 
a glass surface which is apparently not moist, thus leading an 
observer to think that the stimulus may be caused simply by con¬ 
tact with a solid body. Close examination, however, reveals the 
fact that the water lost by excretion covers the creeping pseudo- 
plasmodium and that, consequently, the substratum is wet by the 
excreted water. Even more conclusive evidence that movement 
away from water causes the elevation of the pseudoplasmodium 
may be deduced from the fact that when a colony is moving along 
the glass side of a culture dish, and a moist body, such as a piece 
of dung or wet filter paper, be placed very near the creeping 
pseudoplasmodium, no fructification will be formed for some time 
and the creeping may continue until nearly the whole of the colony 
is utilized in the production of stalk. Evidently the presence of 
the moist body so close to the substratum on which the mass is 
crawling prevents the pseudoplasmodium from becoming sufficiently 
dry to induce it to turn away from the glass. 
The fact that the branches in the case of Polysphondylium are 
almost invariably inclined slightly upward may be also regarded 
as further supporting this belief. It appears at least reasonable 
that negatively hydrotropic forces should cause the small branching 
colonies to be directed upward, away from the moist substratum, 
as well as away from the main stalk from which they arise. 
Gravity does not seem to affect the young colonies in the least, 
for the pseudoplasmodia may be directed upward or downward or 
horizontally, according to the orientation of the substratum. 
The pseudoplasmodia of most of the other species of Dictyoste- 
lium and Polysphondylium further present a curious phenomenon, 
the cause of which is as yet undetermined. The colonies of certain 
species especially tend to move more or less spirally, like twiners. 
Dr. Thaxter (’92) has called attention to an analogous phenomenon 
in the JSIyxohacteriaceae^ in which the aggregated rods which form 
the fructifying colonies tend to move in “ whirlpools ” and the 
