55 



from the starch enough food for its normal growth, and 1 am surprised 

 that this explanation did not occur to me at once. The organism grows 

 fairly well until the small amount of grape sugar present in the potato 

 is exhausted, and thereafter, when thrown wdiolh' upon its own 

 resources, makes only an extreme!}- feeble growth, corresponding to 

 its very feeble diastasic })()wers. This conclusion i(^sts upon the fol- 

 lowing experiments: 



Iodine Starch Reactio.v. 



^ly iminoculated potato cylinders when tested with iodine potas- 

 sium iodide diluted with water, or with iodine crystals dissolved in 

 absolute alcohol to saturation and then diluted with water as required 

 for use, always 3'ielded an inmiediate ))right blue reaction. The starch 

 reaction was also strong after the Ps. Injucuitlti had been grown on 

 them for several weeks, although there was alwa3^s evidence of slight 

 diastasic action to be found in the purplish color assumed by some of 

 the grains. The following are trans^-ripts from my notes. 



(1) Souie fragments of potato scraped from immediately under the 

 yellow slime on a culture 30 days old were put into an old solution of 

 iodine-glycerine. They became black at once, and when crushed out 

 and examined under the microscope were brownish purple — i. e., more 

 brown purple than the starch from a check tube. Tested with alcohol- 

 iodine diluted with fifteen or twenty times its bulk of water, the starch 

 of tlu^ potato in the check tubes became pure blvie. In the culture, 

 inunediately under the A^ellow slime, most of the starch-bearing cells 

 became purple, but occasionally one was nearl}^ pure blue. Cells deep 

 in the cylinder reacted blue. 



(2) On the thirty- first day another tube of the same lot was tested 

 with alcohol-iodine, diluted wdth thirty or fort}" times its bulk of 

 water. When this fluid was put on scrapings from a check tube, the 

 reaction was pure })lue; when it was put on scrapings from imme- 

 diatidy under the yellow slime, the starch reaction was purple and blue 

 pur})le. 



(3) A year previous scrapings were made close under the })acterial 

 layer of a culture 86 days old and tested with iodine potassium iodide. 

 There was a strong blue-black reaction. FndiM* the micioscopv\ how- 

 ever, some of the cells were paler than others, indicating that some 

 of the starch grains had been acted upon slightly. 



(4) On the twenty-ninth day a })otat() cylinder, l)earing a typical 

 growth of the yellow slime and unifoi'iuly grayed, was broken across 

 the middle and tested with iodine alcohol in watei'. The middle ])art 

 of the cylinder reactcnl blue. The outer ])art. close undei" the bacte- 

 rial layer, gave eithei- a reddish or purplish blue reaction. 



No potato cultures of this oiganism were ever tested an liich did not 

 give a very d(M'ided reaction with io(lin(\ The importance of this fact 



