THE NERVOUS MECHANISM OF PLANTS 409 
as a muscle or a gland-cell, and in the other in a nerve-cell 
belonging to the co-ordinating apparatus. 
We can easily recognise in plants certain structures 
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 7) inch 
from the apex. If this part were covered by an opaque 
sereen 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 particular 
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 Vicia faba, to 
grow into small and light glass tubes, closed at one end, 
and bent at a right angle about ;'5 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 7g inch from the apex. Roots so prepared were then 
fixed in various positions, so that the tip was rigid, while 
the growing zone just behind the tip was free to curve. 
When the tip was vertical, and the long part of the root 
horizontal, the root continued to grow without any curva- 
ture; 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 amputated the tips of certain rootlets, and 
laid them horizontally ona support. They did not then show 
any sensitiveness to gravitation until they had recovered 
