n6 



LECTURE XL. 



vapour is continually being evaporated from the damp saw-dust, and the air in its 

 immediate neighbourhood is saturated with it ; a little distance from it, however, the 

 air is relatively drier, and the root apices thus become curved, as the experiment shows, 

 so that they are concave on the damper side. 



In the experiment described the hydrotropism of the root has to overcome its 

 geotropism ; but if the seeds are allowed to germinate on the surfaces of a damp 

 block of peat [T, Fig. 405) which is fastened to the axis ^ of a klinostat and slowly 



Fig. 405- -Cubical block of pea 



A of the klinostat 



rotated, geotropism, as we already know, is rendered ineffectual. In this case 

 then the roots also grow closely apphed along the damp surfaces, it is true, but 

 if it accidentally happens that the apex penetrates into the turf itself it then goes on 

 growing as in /, 0, q, continually deeper into the turf, and, according to circumstances, 

 probably even again comes out from it, as at /, ;/. 



In this, however, there is yet another cause acting on the roots ; for I have 

 found that growing root-apices, when they are pressed on the one side by a solid 

 body, behave like tendrils (only much more tardily) and become curved so as to be 

 concave on the touched side. 



This is perhaps also the proper place to mention that the power which roots 



