THE NEEVOUS 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 -^ 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 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 T V inch from that end. Tho 

 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 T V inch from the apex. Eoots 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 on a support. They did not then show 

 any sensitiveness to gravitation until they had recovered 



