400 VEGETABLE PHYSIOLOGY 
which may not inaptly be termed sense-organs, as we can 
localise in them the power of perception of stimulating 
influences. Darwin found that the seedlings of Phalaris 
were not sensitive to the faint light employed in his experi- 
ments, except at a small region extending about ;4 inch 
from the apex. If this part were covered by an opaque 
screen in the shape of a little blackened cap of not sufficient 
weight to cause any flexion of the stem, the seedlings no 
longer bent towards the light. The tip of the root is the 
only part which is sensitive to contact in such a way as to 
cause the growing part to curve so as to carry the tip 
away from the obstacle. The sensitiveness of any par- 
ticular cell is transitory, passing away as other cells are 
formed in front of it. The same region possesses the power 
of appreciating the stimulus of gravitation. This has been 
shown by Czapek in a very ingenious manner. He caused 
the roots of various seedlings, especially using Vicia faba, 
to grow into small and light glass tubes, closed at one end, 
and bent at a right angle about ;, inch from that end. The 
cultivation was carried on on a klinostat for about twelve 
hours, when the root had penetrated to the end of the tube, 
and had consequently become sharply bent at a right angle 
about =; inch from the apex. Roots so prepared were then 
allowed to continue their growth after being placed in 
various positions. When the terminal portion was vertical, 
and the long part of the root consequently horizontal, the 
root continued to grow without any curvature ; when these 
conditions were reversed a geotropic curvature resulted, which 
continued as long as the tip of the root was mechanically 
prevented from becoming vertical. Other observers have 
proved the same thing in different ways. Cisielski ampu- 
tated the tips of certain rootlets, and laid them horizontally 
ona support. They did not then show any sensitiveness 
to gravitation, until they had recovered from the wound 
and a new root-tip was developed upon each. As soon as 
the new tip was formed, the rootlets showed a power of 
reacting to the stimulus of gravitation, and the curvature 
