Chap. III.] DOWNWARD GROWTH OF THE ROOT. 
103 
from east to west : in other words, they make 
the half-circle which is farthest from the ob¬ 
server from right to left; and the half-circle 
which is nearest to the observer from left to 
right. 
Many plants do the same; and many others 
(as the hop) exactly the reverse. Nothing can 
alter these determinations of growth in these 
plants, which are, possibly, in all cases, owing 
to the action of the same external agent, light, 
on different internal cellular organisation. 
Mechanically , I have forced the heads of plants 
to grow downward, by placing the seeds and 
roots of beans in sponges, and confining their 
heads in glass tubes. I have mechanically 
forced the first or tap-roots of plants to grow 
upward by placing horse-chestnuts in earth, or 
half-covered with water, and confining their tap¬ 
roots in glass tubes. Both the stems and the 
roots will, however, make every attempt to 
double back : and in doing so, I have known 
the root grow spirally up the narrow tube like a 
corkscrew; and having, at last (from becoming 
thin), turned, grow straight down through the 
screw. The tube being then quite filled up, the 
side-fibres of the upper end of the root grew 
straight up in a bunch. In plants which have 
H 4 
