128 MASS. EXPERIMENT STATION BULLETIN 161. 



As in the previous experiments the roots showed marked differences 

 at the end of the fourth da}^, while the tops showed no differences until a 

 week had elapsed. The roots of the aluminum and iron treated bottles were 

 very much stunted, and either consisted of only one main taproot with- 

 out laterals or root hairs, or else quite a number of short thick roots grow- 

 ing from the base of the stem. The laterals only grew about a sixteenth 

 of an inch and then stopped. All of the stunted roots were thicker than 

 the unaffected ones, and despite their much smaller number and shorter 

 length weighed as much as the healthj^ roots. At the end of six weeks the 

 experiment was discontinued and photographed (Plates I. and II.). The 

 seedlings in the 2.2 parts per miUion iron solution (No. 16) were almost 

 normal, and where calcium carbonate had been added (No. 17) showed 

 practicall}'' no differences from the check. The seedlings in the 4.4 parts 

 per milhon iron solution (No. 13) made little growth after the first week, 

 but did not die, and where calcium carbonate was added the toxic action 

 was in part overcome. In the 11 parts per million iron solution (No. 

 10) the plants died at the end of the fourth week. Calcium carbonate 

 in this case seeminglj^ had no effect. As was already noted in the second 

 experiment calcium sulfate had no effect in counteracting the toxic action 

 of the salts. The results with the aluminum salt were exactly similar 

 to those of the first and second experiment. 



Summarizing the results of the three experiments we find as follows : — ■ 



1. That aluminum sulfate, when present in culture solutions in con- 

 centrations greater than 40 parts per million of aluminum, has a very 

 toxic action on clover seedlings. 



2. That ferrous sulfate when present in culture solutions in concen- 

 trations above 4 parts per million of iron exerts a toxic effect on clover 

 seedlings. 



3. That this toxic effect of iron and aluminum can, in a large measure, 

 be overcome by the use of calcium carbonate up to a certain point, 

 beyond which it has no effect. Calcium sulfate does not have this beneficial 

 effect. Th's would seem to indicate that it was not the presence of calcium 

 alone to which the antitoxic action was due, but rather to the combination 

 in which it is present. Calcium in the form of the carbonate precipitates 

 the iron and aluminum in the form of hydroxides, and thus removes 

 them from solution and counteracts their harmful action. The toxic 

 action of the higher concentrations of iron and aluminum, despite the 

 excess of calcium carbonate present, is due, I think, to the solubiUty of 

 the iron hydroxide. The aluminum hydroxide being less soluble, the 

 toxic effect, even in the most concentrated solutions, is almost entirely 

 counteracted by the calcium carbonate. 



4. The idea that the toxicity of iron and aluminum salts is due to the 

 penetration of the salts into the seedlings does not seem to be borne 

 out. That the toxic action seems to be entirely in the first layer or two 

 of cells in the growing portion of the roots is borne out by the following: 

 a microscopical examination ^ shows that the stunting of the roots is due 



' Made by Mr. G. H. Chapman. 



