Action of Cheviicals on Potato-Tuhers. 283 



alone does not give this colour reaction may easily be con- 

 trolled by steeping similar tubers in an aqueous solution of 

 the indicator containing approximately the same concentration 

 of methyl orange as the acid solution. 



These facts therefore justify the conclusion that the entry 

 by diffusion of the dissolved substance so far tried is exclusively 

 via the buds or " eyes " of intact mature tubers. 



The point demonstrated is, as we shall shortly see, of im- 

 portance when we come to deal with the behaviour of blight- 

 infected tubers on exposure to the action of aqueous sulphuric 

 acid. 



Plate LX. shows the appearances presented by such a tuber 

 as that described above, after 20 hours' steeping in 10 per cent, 

 .aqueous sulphuric acid plus methyl orange. 



Blight-fpee Intact Tubers. Boracic Acid Steeps. 



The steeping of intact tubers in solutions of common salt 

 and sulphuric acid, as we have seen, is attended by very 

 .restricted diffusion of these reagents through the skins of the 

 rsteeped tubers. It was of interest to investigate in this par- 

 ticular respect the action of boracic acid. 



This acid has a much lower solubility than either of the 

 reagents mentioned, functions as a comparatively mild anti- 

 spetic, and in aqueous solution possesses certain physico- 

 chemical properties which differentiate it from both NaCl and 



The following experiment may be taken as typical of the 

 results furnished by steeping intact tubers in aqueous solutions 

 •containing 1.61 per cent, and 0.63 of the acid. 



ExjDei-iment: — ^Two lots (5 each) thick-skinned Victorian 

 tubers were separately steeped in (1) 1.61 per cent, and (2) 

 0.63 per cent, boracic acid solutions for 10 and 11 days respec- 

 tively, and the titres of the solution determined at the com- 

 mencement and termination of the steep experiments. 



Grams boracic acid per 100 cc. 

 of solution. 



(1) (2)^ 



At commencement ... ... 1.61 - 0.63 



At termination 1.60 - 0.65 



7a 



