Nov. is, 1920 Vascular Discoloration of Irish Potato Tubers 283 



visible evidence of the presence of other organisms at the time of identifi- 

 cation. In the second column is recorded the number of times the genus 

 in question was found in a tube associated with some other organism. 

 Each tube containing a mixed culture is reported twice, once for each 

 organism. In no case were more than two organisms identified from a 

 single tube. The total number of identifications reported is therefore 

 the sum of all the columns marked ."pure" plus the sum of all the col- 

 umns marked "mixed," while the total number of plantings reported is 

 the sum of all the columns marked pure plus one-half the sum of all the 

 columns marked "mixed." 



One very significant thing shown in Table II is the fact that out of 

 3,203 plantings, all but 161 of which were made from discolored tissue, 

 1,352 gave no growth. There is good reason to believe that in the great 

 majority of these cases the tubes yielded no growth because the tissue 

 transplanted was sterile, or at least free from filamentous fungi. These 

 results are in entire accord with those obtained by the writer in numerous 

 other cases where cultural tests of discolored vascular tissue of potatoes 

 have been carried out. In some instances the discoloration may be a 

 response to parasitic attack on some other portion of the plant, though 

 the tissues of the tuber are not actually attacked. In such cases it may 

 be regarded as a parasitic phenomenon of a secondary character. From 

 the physiological point of view, however, it matters little whether a 

 lethal dose of toxin diffuses from some point in the stem back oi the 

 stolon or from a point within the tuber itself. Likewise, the result is 

 the same whether the tissue is killed by the action of fungi, primary or 

 secondary, or through the operation, directly or indirectly, of malign 

 environment of whatever nature. Conclusions based on field experi- 

 ments with many factors uncontrolled must not be accepted without 

 reserve, but the writer has secured deep vascular discoloration which he 

 believes to be the direct result of too rapid respiration induced in the 

 soil at high temperatures such as prevail during the summer months in 

 the vicinity of Washington and which are occasionally experienced at 

 more northern and western points. This was the case with stock grown 

 at Arlington Farm during the summer of 1917, in which vascular dis- 

 coloration was universal and pronounced, extending throughout the 

 tuber in most cases. While certain lots of this material yielded 

 Fusarium or other fungi from a certain portion of the plantings, other 

 lots yielded only an occasional saprophytic growth out of hundreds of 

 plantings. The results were confirmed by repeated trials, which gave 

 uniformly identical results. 



There seems, therefore, to be good reason to regard some of the stem- 

 end browning of vascular tissue as physiological, even in the cases in 

 which it extends well into the tubers. 



