VITALITY AND GERMINATION OF SEEDS. 239 



overcoming the natural geotropic irritability of the 

 radicles. 



If similar Peas are germinated over water which is kept 

 in a well aerated condition, the radicles whether to 

 commence with they are pointing upwards or downwards 

 grow downwards into water, which does not now appear 

 to exercise any repellant influence upon them. The 

 difference between the water in the two experiments is 

 that in the first case it is charged with C0 2 , devoid of 

 oxygen and loaded with bacteria, etc., whilst in the second 

 it contains but little C0 2 , relatively few bacteria and a 

 plentiful supply of oxygen. 



If the aeration of the water be stopped when the 

 radicles have penetrated a little distance beneath the 

 surface, the supply of oxygen which the water contains is 

 soon used up. The still surface of the stagnant water 

 absorbs oxygen from the supernatant air only with great 

 slowness, and this is all used up before it can diffuse to 

 the depths beneath. 



Hence the growth of those roots whose tips are more 

 than an inch below the surface of the water either ceases or 

 goes on but slowly. The fact of any growth in length of 

 the root being possible under such conditions is probably 

 due to the fact that those parts which are above the 

 surface of the water absorb oxygen in abundance and the 

 excess diffuses longitudinally and is finally transferred, in 

 part at all events, to those regions which are especially in 

 want of it. Any growth which does take place in such 

 roots is all in the downward direction being never deflected 

 more than 45° from the perpendicular. This is because 

 at a short distance below the surface of the water the 

 percentage of free oxygen is extremely small and the 

 percentage of oxygen in the layer of water immediately 

 above the tip of the growing root will only differ in an 



