270 MR. F. DARWIN ON A METHOD OF INVESTIGATING THE 
in which the tip of the root rests. Glass tubes were at first 
used, but afterwards straws were employed closed at one end 
by wooden plugs or by cotton-wool; quills were also found 
serviceable. 
Tubes were also successfully made from the nectaries of 
Trop@olum and from the corolla-tubes of various flowers and 
the hollow scape of Taraxacum; and these have the advantage 
of not being hard like glass, quills, or straw, and thus not likely 
to injure the root-tip. But the greatest difficulty was to prevent 
the tip of the root slipping out of the tube. This frequently 
occurred, and spoilt a large majority of the experiments made 
with beans. 
In some experiments I placed a piece of fine gauze in sucha 
way that it hung over the closed end of the tube, and also wrapped 
round about a centimetre of the root. The gauze was kept wet 
by the dripping water, and, without interfering with the growth 
of the root, it tended by its close contact with the root to keep 
the tip in place. At the close of my work I hit upon another 
plan which is perfectly successful in keeping the root in place. 
A very fine brass wire is coiled into a spiral spring and slipped 
over the tube, which is thus continued by a tubular spring. The 
root is placed in the tube, and the spring, very slightly extended, 
is fixed by a thread and a pin to the cotyledon. The root cannot 
slip out of the tube; and since the force needed to produce a 
lateral bend is very slight, there is no obvious reason why 
curvature should not occur. As the root grows in length, the 
spring is of course elongated and must be occasionally re- 
adjusted. I have not made many experiments with this method, 
but I have seen enough to convince me that it is the most 
hopeful for future work. 
Results with Beans (Vicia Faba). 
Forty-four experiments were made ‘in the manner here 
described, and out of these only fourteen showed any result 
worth consideration. Of these, six showed striking curvature of 
the root past the vertical. In the other eight the curvature was 
continued beyond the vertical, but not to any considerable 
extent. 
Among the failures are a few cases in which the root hardly 
bent geotropically at all, whether from too low a temperature oF 
