VITALITY AND GERMINATION OF SEEDS. 241 



" physiological action at a distance." It might be caused 

 by the under side of a slightly curved root being in a 

 somewhat damper layer of air than the upper, or by its 

 being more exposed to the water vapour given off from 

 the surface beneath. This would tend to keep the cellular 

 tissue of the lower surface in a slightly more turgid con- 

 dition than that of the upper and would hence tend to 

 cause an upward or lateral curvature. It is hardly possible 

 however that the difference in turgidity, if any, so produced 

 could be large enough to markedly affect the normal 

 tendency of the root to grow downwards. It is more 

 probable that the curvature is produced in the same way as 

 it is in the upper layers of the water, the radicle bending 

 to those regions where oxygen is most abundant. In a 

 still atmosphere the diffusion especially of a heavy gas 

 takes place but slowly, hence the layer of air which lies 

 immediately over the surface of water contains much less 

 oxygen and much more carbonic acid gas than the layers 

 of air above. The radicle therefore when it enters this 

 layer may curve upwards in virtue of the Oxytropic 

 Irritability which it possesses. 



Experiments were next made to find out whether the 

 presence of combined oxygen, in the form for example, of 

 a solution of potassium nitrate, would suffice to prevent 

 any oxytropic curvature. This latter is a stable salt and 

 only yields up its oxygen at high temperatures. Hence, 

 since, as might be expected, on germinating Peas over a 

 weak solution of potassium nitrate in boiled water it was 

 found that the radicles behaved just as in the case where 

 oxygen was totally absent, it is therefore the lack of free 

 oxygen in the medium which the radicle is about to 

 penetrate which inhibits or rather antagonizes and over- 

 powers its geotropic irritability. 



Similar experiments were performed with a weak solu- 



