38 THE COTTON PLANT IN EGYPT CHAP. 



Lastly we have to consider an abnormal limiting factor 

 of root-growth, namely deficiency of soil oxygen, usually 

 due to water-logging of the minute interstices which the 

 soil contains. Owing to the fact that its anatomical 

 structure does not include an elaborate system of inter- 

 cellular air-spaces, such as aquatic plants possess, the 

 cotton root is locally asphyxiated in water-logged soil, and 



FIG. 37. WATER JET EXCAVATION OF ROOTS. 



See Fig. 35 on which the area of this photograph is marked. 



Lateral from neighbouring plant on right is marked with an X. 



Length, 170 cm. +. 



in a few weeks even the stout, woody roots are not merely 

 dead, but decomposed. Here also we require precise 

 information as to the degree of soil saturation which 

 produces these effects under field conditions, but the main 

 facts are perfectly clear, and we shall meet with the serious 

 economic effects of this root-asphyxiation at frequent 

 intervals. 



The stomata. Certain plant physiologists have 



