174 Scientific Proceedings, Royal Dublin Society . 



A healthy plant of the variety Up-to-Date (which is quite susceptible to 

 leaf-roll) was grown in a flower-pot and placed in a position where it was as 

 little as possible exposed to leaf-roll infection through insect agency. As a 

 matter of fact, neither it nor its neighbours ever became infected. On August 8, 

 when the potted plant was well developed, normal in appearance, and just 

 showing its flower-buds, it (together with its pot) was placed during the night 

 (from 5 p.m. to 9 a.m.) in a small ice-chamber, the temperature of which varied 

 between 5-5° C. and 72° C. In the evening, when the plant was first brought 

 in, its leaves contained a normal amount of starch, but after a night in the 

 refrigerator they were found to contain none, and the plant was apparently in 

 no way injured or altered by the treatment. It was then exposed to full day- 

 light in its original position, and again placed at night in the ice-chamber, and 

 this treatment was repeated for eleven consecutive days. The range of tem- 

 perature at night was the same throughout. Periodical tests of the starch- 

 content of the leaves were made during and at the end of the experiment, but 

 an accumulation of starch was never found in the morning, and no rolling of the 

 leaves or any other abnormal symptom appeared. 



In seeking for an explanation of the different results secured by Neger under 

 apparently similar conditions, it is to be observed that this worker used cut 

 shoots and not the whole plant in his experiments. When a normal potato 

 shoot or leaf is cut off and placed with its end standing in water in the dark, it 

 loses its starch in a comparatively short time ; but the process is one of hydrolysis 

 of the starch and eventual consumption of the resulting sugar in respiration, and 

 not of translocation in the generally accepted sense of the word ; for there are 

 no normally functioning organs of storage or growth by which the carbohj^drate 

 can be absorbed, and no soluble carbohydrate passes out of the stem into the 

 water. Even the apparent transference of starch from the leaf-blades to the 

 petiole is largely illusory, although some translocation may perhaps take place, 

 because there is a gradual but less rapid diminution in the total amount of 

 starch present in the petiole also. In the case of larger shoots there would, of 

 course, be a correspondingly more extended field in which such limited trans- 

 location might take place, but the conditions are so unnatural as to render 

 doubtful the applicability of results secured under them to the normal growing 

 plant. An even more rapid disappearance of starch is to be observed (as Neger 

 also noted) in the case of cut leaves and shoots which are not put standing in 

 water, and which wilt in consequence. The obvious conclusion, however, that 

 translocation could not account for the disappearance in this case was not drawn 

 by this worker. 



A similar process of hydrolysis and respiration goes on, but probably more 

 slowly, when diseased shoots and leaves are allowed to stand in the dark with 

 their cut ends in water ; but it is not possible to measure by means of the iodine 

 test the relative rates of carbohydrate translocation in healthy and diseased 

 leaves or shoots kept in this way, iDceause the starch-content of the leaves is very 

 different to begin with, and because the diminution in starch-content is not 

 solely or in the main due to translocation. While practically all the starch 

 contained in a healthy leaf cut ofl' in the evening may be dissolved during the 

 night and retained as sugar, with the exception of that used up in respiration, 

 it is obvious that a diseased leaf cannot under the same conditions be in a position 

 to become free from starch in the same time, even if the starch in it were equally 

 hydrolysable, because of the greater amount of it originally present. A further 

 complication would also ensue, since the resulting sugar, for which there is no 

 outlet but that of respiration, would be present in greater quantity, and would 

 be likely to retard or prevent the final disappearance of the starch. It there- 



