114 Otis F. Curtis 



Iron and aluminium 



Bertrand (1911) has stated that compounds of iron, aluminium, and 

 boron may act in a catalytic manner somewhat similar to that of man- 

 ganese. Micheels (1905) earlier suggested such a possibility. The first 

 two compounds, may serve as oxygen carriers. Kastle (1910) cites a 

 number of instances in which investigators have found iron to be closely 

 associated with oxidation processes. 



The iron as used in the present experiments was very evidently in the 

 colloidal hydrated condition. When the solution was first made it was 

 colorless, but within a few minutes the orainge-yellow color of the sus- 

 pended hydrate appeared, which was then easily precipitated by the 

 addition of magnesium sulfate. The solution of 0.0001 molecular strength 

 showed a very marked bluing of guaiacum tincture, as did also that 

 of aluminium chloride of the same strength. The lack of stimulation in 

 the limited treatments may possibly be explained as being due to the fact 

 that no precipitate is formed in close proximity to the living tissues, as 

 is the case in treatment with potassium permanganate, and since the action 

 must be more or less continuous the short treatments are ineffectual. 

 Micheels and De Heen (1905) found similar results with colloidal tin. 

 When the seedlings were entirely removed from the solution no stimulation 

 resulted. 



Sulfur 



The stimulation obtained in the single treatment with ferric sulfate 

 may have been due to the iron, as in the experiment with ferric chloride, 

 or possibly the sulfate ion may have had some influence. It has been 

 reported in several instances that sulfates increase root growth of seed- 

 lings, tho no explanation of this has as yet been offered. 



Rusche (1912) found that as a rule sulfates are the most efficient salts 

 in stimulating root growth in a number of plants. Chlorides, on the 

 other hand, are the most harmful to root growth, while nitrates also have 

 a retarding effect. 



The tables given by Shedd (1914) indicate that the addition of certain 

 sulfates increases root growth to a greater extent than it does top growth. 



Hart and Tottingham (1915) found that certain plants, especially rape 

 and red clover, show greatly increased root growth on the addition of sul- 

 fates. Calcium sulfate was the most efficient in this respect of those used. 



