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 ^ 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 -fa 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 

 aboui fa inch from the apex. Boots 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 

 on a 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 



