380 



Soil Factors on Disease Resistance 



normal plants exhibited marked aerotropism and were found to have 

 turned upward towards the air. This continued until a break in the 

 rains and a fall of the ground water restored soil-aeration when normal 

 growth ensued. 



Confirmatory evidence of the view that wilt during the rains is due 

 to the destruction of the fine roots and nodules caused by poor soil- 

 aeration has been obtained in several directions. The actual soil conditions 

 under which wilt naturally occurs can be reproduced in a lysimeter by 

 closing the drainage openings. The slow rise of the water-table leads to 

 the destruction of the fine roots and nodules from below upwards and 

 to the production of wilt. Recovery from monsoon wilt takes place in 



Fig. 3. The root-system of Hibiscus Sabdariffa (left) and H. cannabinus (right). 



the lysimeters and also in the field after the aeration of the soil improves 

 and when the temperature permits of the regeneration of the roots. 

 Indigo grown on porous soil in other parts of India under a high rainfall, 

 such as Dehra Dun and the Chattisgarh Division of the Central Pro- 

 vinces, escapes wilt altogether. In Bihar, wilt is always most severe in 

 years of heavy rainfall when the subsoil water remains at a high level 

 for long periods. It is negligible in years of short rainfall like 1919 when 

 the rise of the subsoil water is very slight. 



Wilt in Bihar during the monsoon is by no means confined to Java 

 indigo. It is common on many deep-rooted varieties of "patwa" 

 (Hibiscus cannabinus L.) and "sann" (Crotalaria juncea L.) while 

 shallow-rooted types of these two species are little affected. Further, 



