440 New combe. — The Sensory Zone of Roots . 
positive curves, one negative, and four neutral roots ; the 
fourth set had twelve individuals, and gave six positive curves, 
three negative, and three neutral roots. 
In a plant like Helianthus annuus , not growing well in 
water, it is not at all surprising that occasionally a set of 
seedlings will vary somewhat, as did the first set noted above, 
from the general behaviour. The result of the four experi- 
ments seems to me to teach unmistakably that the root of 
this plant is sensitive to a water-stream at the distance 
of 1 5 mm. from the apex. The negative curves appearing in 
these experiments would nearly all be turned into positive 
curves were the roots not covered by the tubes. This is 
shown in my paper on ‘ Rheotropism,’ and is to be explained 
probably by supposing that the rheotropism of the roots 
overcomes in most cases the initial tendency to contorted 
growth. When the roots are partially shielded by tubes, the 
rheotropic stimulus affects the plant to a much less degree, 
and the tendency to contorted growth manifests itself much 
more strongly. 
If we assume a sensitiveness of the root of Helianthus 
at a distance of 15 mm. from the apex, the recorded results 
will hardly warrant such an assumption for the part of the 
root 20 mm. distant from the apex. For if we take the four 
negative curves as due to contortion and assume an equal 
number of contortions among the positive roots, we shall have 
left but seven positive roots in a total of twenty-nine, a 
proportion almost too small to serve as a basis for any 
conclusion. We cannot assume that the root of Helianthus 
is insensitive 30 mm. distant from the apex, but rather that 
the method employed gives no sufficient evidence for a judge- 
ment either way. 
Five roots of Helianthus annuus were marked and observed 
with the horizontal microscope for the extent of the elongating 
zone, the roots being kept in water, and light excluded. Of 
these, four roots showed growth in the first 7 mm. only, while 
the fifth root showed growth in the first 5 mm * only. It 
must be therefore that the root of this plant is sensitive to 
