240 TEANSACTIONS LIVEBPOOL BIOLOGICAL SOCIETY. 



infinitesinal degree from the percentage contained in the 

 layer of water immediately beneath. Hence the tip of the 

 root, which is the only sensitive part, is equally affected on 

 all sides by the fluid with which it is surrounded. There 

 is therefore nothing to counteract the normal geotropic 

 irritability of the radicles and these grow downwards, 

 though but slowly. 



On the other hand those radicles whose tips are near 

 the surface of the water are affected by the layers of water 

 immediately above being richer in oxygen and bend 

 upwards, either growing along the surface of the water or 

 in some cases emerging above it and growing for some 

 distance upwards. 



If the stagnant oxygenless water over which Peas were 

 germinating was well aerated and kept in an aerated 

 condition the radicles soon bent downwards into it, 

 although it must still have contained large quantities of 

 carbonic acid and an abundance of bacteria. The repellant 

 influence which the water had previously exerted could 

 not therefore have been due to the presence of an excess 

 of dissolved carbonic acid, nor could the curvatures have 

 been produced by an irritant action of the bacteria con- 

 tained in the stagnant water. The curvature is produced 

 by the absence of oxygen in the medium which the radicle 

 is about to penetrate and is the outward and visible sign 

 of what may be termed the Oxytropic, or oxygen seeking, 

 irritability, of the young radicle. 



It is worthy of remark that the oxytropic irritability of 

 the root may under appropriate conditions exercise a 

 greater influence upon the direction of growth of the root 

 than its geotropic irritability. In certain cases a curvature 

 occurred before the tip of the radicle had come into actual 

 contact with the surface of the water and it is a little 

 difficult at first to see the reasons for this apparent 



