50 Snow. — The Conduction of Geo tropic Excitation in Roots. 
Czapek’s well-known ‘glass-boot’ experiments ( 1895 , p. 255 seq.) there 
is now Haberlandt’s research ( 1908 ) by Piccard’s rotation method. In this, 
various seedlings, including Vida Faba , were fixed to a turn-table that could 
be rotated rapidly about a vertical axis. They were placed so that the 
roots pointed obliquely downwards at 45 0 to the vertical, and so that 
the imaginary prolongation of the axis of rotation intersected the roots at a 
short distance behind the tip. In this way, the tip of the root on one side 
of the axis and the responding region on the other were exposed to trans¬ 
verse centrifugal forces in opposite directions. It was found that the 
responding region possessed a certain sensitivity, which was, however, 
marked by the greater effect transmitted from the tip, when the point 
of intersection was 1*5 mm. or more behind the tip. 
But though it is thus probable that geo-excitation in the root can 
be conducted through gelatine, it is not easy to get such definite evi¬ 
dence of this as can be obtained when the stimulus is that of light or 
of wounding. For in these cases it is easy, after the operation, to stimulate 
the tip of the organ without stimulating the responding region ; but with 
gravity this cannot easily be done, and the attempts to do so described 
earlier were scarcely successful. There remains, therefore, another possible 
explanation. The responding region of the root, as has been shown by 
Haberlandt, is itself slightly sensitive to gravity. It might be, then , that the 
effect of decapitation was to make perception impossible, not only by remov¬ 
ing the highly sensitive tip, but also by destroying the sensitivity of the 
responding region by some effect of wound shock or correlative disturbance. 
When, therefore, the tip was replaced, it might bring about response, not by 
transmitting back geo-excitation through the gelatine, but by transmitting 
some influence that restored the sensitivity of the responding region. 
A process similar to this seems indeed to occur in the Arena seedling. 
For in this and in other grass seedlings, so long as they are intact, the lower 
zones of the cotyledon, as shown by Rothert ( 1896 ), are to some extent 
sensitive to light. But after decapitation they become quite insensitive. 
It has now been shown by Brauner ( 19 . 22 , p. 540) that in Arena , if the 
cotyledon tip is cut off and kept in the dark, and the lower zones exposed 
to stimulus of light, and if the seedling is then put in the dark again and 
the tip replaced, a positive curvature follows. The replacement of the tip 
must, then, have restored the sensitivity of the lower zones. 
It is remarkable, however, that in the grass seedlings it is only com¬ 
plete removal of the tip that makes the lower zones insensitive ; deep 
wounds made in i&do not have this effect. If, then, similar relations held for 
the root, we should expect it to bend when laid horizontal even after the 
infliction of a wound on the tip. But, on the contrary, it is well known that 
any deep cut made in the tip makes the root insensitive to gravity. It does 
not seem, then, that there is anything in the root similar to this sensitivity of 
