184 Deficient Soil Oxygen [382 



the nitrogen it is impossible to be sure that the replacement 

 is ever absolutely complete at the beginning of the experi- 

 ment. 



The cessation of water intake, as shown by the stoppage 

 ^of absorption from the auto-irrigator, is always the first 

 sign of injury. With Coleus and Heliotr opium it is followed 

 in from one to six days by progressively lessened turgor of 

 the shoot and leaves and finally by wilting and death. With 

 Coleus, the admission of oxygen to the soil before death has 

 actually occurred is followed by the slow recovery of the 

 plant. Heliotropium does not so recover, even if oxygen is 

 re-supplied before the wilting has become extensive or 

 severe. With Nerium oleander, which does not wilt, the 

 symptom of injury which follows next after the cessation 

 of water intake by the roots is the yellowing and loss of 

 leaves. 



On removal and examination of the injured plants the 

 root systems are found to be dead and the roots partly dis- 

 integrated. When the injury has been slight or recent, in- 

 dividual roots are determinately dead only in parts of their 

 length, regions of brown discoloration alternating with re- 

 gions of apparently healthy root. When Coleus is first in- 

 jured and then revived by re-admission of oxygen it forms 

 a new root system, the new healthy roots being clearly dis- 

 tinguishable from the older dead ones. These new roots start 

 always from the base of the stem, as in a rooted cutting. 

 They have never been observed to start from any portion 

 of the older root system. If one begins with a Coleus plant 

 which has a small root system, or with an unrooted cutting, 

 or with a previously injured plant which will form new 

 roots, it is possible to grow such a plant with a soil atmos- 

 phere somewhat below normal in oxygen content. In this 

 case the shoot does not attain so large a size as the shoot 

 of a normal control plant and is more susceptible to injury 

 by drouth, as, for instance, by increase in the evaporating 

 power of the air. The root system of such a plant, grown 

 with deficient oxygen, is less developed than that of a normal 



