SYMPTOMS OF DISEASE. i8i 



cured by adding traces of a ferrous salt. The 

 distinction between Icterus, where the organs are 

 only yellow, and Chlorosis proper, where they are 

 nearly white cannot always be maintained. In the 

 typical case only those organs whose cells are still 

 young can become green on adding iron. 



Yellowing or False Chloi'osis may be experimen- 

 tally induced by too much carbon-dioxide in the 

 atmosphere. It also often ensues when the roots 

 of plants in the open are waterlogged, owing to 

 the stagnant water not only driving air from the 

 root-hairs but accumulating dissolved substances 

 which poison the plant. Trees frequently thus 

 suffer from " wet feet " when their roots have pene- 

 trated down to a sodden impervious subsoil. 



Yellowing accompanied by Wilting is a pre- 

 dominant symptom in most cases where transpira- 

 tion is more active than root-absorption beyond a 

 certain limit, as is well known in cases of prolonged 

 drought. It may also be caused in evergreens by 

 the foliage transpiring actively in bright January 

 weather, for instance, while the ground is frozen 

 and the chilled root-hairs cannot absorb. 



In other cases similar appearances are traceable 

 to insects devouring the roots, eg. wireworms, and 

 the malady is sometimes enhanced by their accumu- 

 lations so fouling the wet soil that the roots die 

 off, owing to want of oxygen and to the excess of 

 carbon-dioxide and poisonous matters. 



Yellowing may also result from the presence of 

 poisonous or acid gases in the atmosphere or soil, 

 such as chlorine, hydrochloric acid, sulphurous 



