Action of ClianiicaU on Potato-Tubers. 289 



Critical storage and cooking tests show that the tissue of 

 these tubers undergoes only slight and tardy deterioration provided 

 that these are only slightly infested with blight. Where 

 extensive areas of the skin are involved, their storage quality is 

 plainly defective and unsatisfactory. This stateaient equally 

 applies even in the case of untreated tubers and particularly so 

 under unfavourable conditions of storage. 



It remained to subject blighted tubers to the test with 10 

 per cent, sulphuric plus methyl orange. Badly blighted speci- 

 mens were steeped for varying periods — 5, 10 and 20 hours, and 

 sections of these tubers were prepared similarly to those earlier 

 described. These when submitted to both macroscopical and 

 microscopical examination, showed that wherever the skin was 

 overlying tissue which had been invaded by the blight fungus, 

 there the acid and indicator had freely entered and diffused 

 into the subjacent tissue. Entry of the acid, therefore, is not 

 exclusively confined (as in blight-free tubers) to the buds. In 

 other words, while the undamaged healthy skin of the disease- 

 free tuber functions as a semi-permeable or selective covering 

 to the storage tissue beneath it, the damaged skin of the blight- 

 infected tuber no longer possesses this property. The entry of 

 the acid through the skin and buds is well shown in Plate LX. ; 

 in the original objects which these plates represent, the lighter 

 diffusion zones or areas are very clearly differentiated from 

 those portions of the tissue into which the acid has not ditiused, 

 by a pink coloration (l). 



Certain observations by Wiitrich2 on the toxicity of H2SO4 in 

 dilute concentrations on the conidia and zoospores of Phyto- 

 phfhora are of interest. 



In each instance the period of observation covered 15 hours, 

 and the following results are recorded : — 



strength of acid Phutopldhora iiifestaiif:, L)e Bary. 



0.0049 per cent - - - Within 15 hours conidia 



neither form swarm 

 spores nor directly germ 

 inate 



1 Sections should be examined as early as possible after cutting, since after a few 

 hours, desiccation at the surface leads to the diffusion of alkali. 



2 Zeit. f. Pfl. 1892, 10-31, 81-94. 



