46 DISEASE IN PLANTS. 



of the mineral, and Nobbe, in 1876, showed that 

 the roots of seedlings reduce potassium perman- 

 ganate, a fact which Molisch confirined in 1887. 

 The latter observer also proved that living root- 

 hairs secrete substances which colour a solution of 

 guaiacum blue, oxidise pyrogallic acid and other 

 organic substances, and rendered it probable that 

 they excrete some substance which inverts cane- 

 sugar, and in some cases even small quantities 

 of a diastatic enzyme. 



Molisch also confirmed an old observation, that 

 roots excrete carbon-dioxide ; and he and Czapek 

 showed that the root-hairs excrete acids more 

 permanent in their nature than carbonic acid, and 

 published a method for showing this by means 

 of a dilute solution, slightly alkaline, of phenol- 

 phthalein. 



Molisch declared that the substances secreted 

 by root-hairs may even be observed, dissolved in 

 drops which ooze from the surfaces of the root- 

 hairs. 



That these root-excretions, and particularly the 

 acids, may be of service in dissolving and render- 

 ing more available various constituents of the soil 

 is an obvious suggestion, and it is borne out by 

 Sachs' discovery of the corrosion of marble, and 

 by Molisch's observation that living roots slowly 

 corrode ivory if continuously kept in contact 

 with it. 



But a deeper insight into the physiology of 

 these organs was only possible when the meaning 

 of the phenomena of osmosis had been rendered 



