The Erectness of Plants. 



95 



in the opposite direction to that which it takes in the root. The 

 current flows from less negative to more negative points, and 

 therefore flows upwards. It then passes along the upper side of 

 the stem and returns along the under side (fig. 2), causing similar 

 diflferences in permeability, turgor and rate of growth, but it is 

 the upper side which is affected more. The under side, therefore, 

 grows more rapidly than the upper side, and the stem curves 

 upwards. 



09 



MOTOR REG I on 



PERCEPTIVE REGIOri 



Fig. 2 — Diagram of stem apex to illustrate the Hydrion Differentiation Theory of 

 geotropism. The "ci'eaming" is indicated in the apical meristem 

 (perceptive region). The arrows indicate the path of the "action current." 



Using this theory^ as a working hypothesis, bean seedlings 

 were grown under bell-jars in an atmosphere rendered slightly 

 alkaline with ammonia vapour, with the result that the tips of 

 the main roots turned slightly upwards, while the secondary roots 

 or rootlets all grew upwards. Maize seedlings were treated in 

 the same way and the young rootlets in that case also turned 

 upwards. 



The response of the stem was then tested by growing 

 seedlings of Maize under bell-jars in an atmosphere rendered 

 slightly acidic with the vapour of acetic acid, with the result that 

 in almost every case the young shot)t (enclosed in the epicotyl) 

 turned downwards. In one experiment, a shoot which had 

 curved downwards was taken out and grown in fresh air ; then it 

 recovered and slowly turned upwards. The experiment of reversal 



1 Published at some length in the New Phytologist, Vol. XIX., p. 49, 1920, and given 

 briefly in my Textbook of Botany. 



