MuRPHV — On the Cams of Rolling m Potato Foliage. 173 



at the same time showed a normal distribution of starch, there being less in the 

 palisade layer than was observed in diseased leaves, and more present elsewhere. 

 Similarly in healthy . stems, while there was sometimes little difference in the 

 starch-content of diseased and healthy plants near the tip, as a general rule a 

 great deal more starch was present throughout than in corresponding tissues of 

 diseased plants. This was particularly so in the underground portions of the 

 stem, the stolons, and the roots. Furthermore, the lower portions of the stems 

 of diseased plants above ground, when cut off in the afternoon and tested at once 

 with Benedict's solution, gave no reaction for sugar, while similar portions of 

 healthy stems gave a strong reaction. 



The view is put forward, based on these results, that in the case of plants 

 seriously affected with leaf -roll the exhaustion of starch in the regions of the leaf 

 nearer the larger veins and the almost complete absence of starch (and sugar) 

 along the vascular bundles in the stem in the track of movement, are probably 

 to be explained by the fact that all the carbohydrate which can be moved has been 

 drawn on for the most part and translocated. The remaining starch, principally 

 in the palisade cells, is apparently not readily mobile, the reason for which is 

 at present unknown. As a result of this, no more carbohydrate (or very little' 

 can be produced in the leaves, owing to previous starch accumulation and the 

 disturbance it set up. The consequence is that at least in the extreme cases 

 now being considerecl the plant as a whole gradually dies from starvation. This 

 view of the case, which supposes the seat of the disturbance of starch transloca- 

 tion in the leaf (whatever and wherever its cause may be) to lie in the leaf -blades 

 rather than in disorganized distant tissue in the stem, is believed to be correct, 

 because the disturbance begins . and ends in the leaves ; but the problem is 

 admittedly a comi)licated one. 



In other stages of the disease and in other varieties, when the plants are not 

 so severely attacked and are still growing fairly vigorously, there is a very large 

 and abnormal accumulation of starch in the petioles of the lower rolled leaves 

 and in the stem near their points of insertion. This in itself might jjerhaps 

 be accounted for by assuming it to be due to an obstruction in the phloem a little 

 further down. But such a supposition would not account for the fact that the 

 accumulation is known in these cases also to start in the blades of the leaves. 

 On the whole, it seems more reasonable to consider the matter as a gradual 

 extension of the disordered condition from the leaf-blades, where it originated, 

 although it is possible that phloem, necrosis, once it has set in, may add to the 

 difficulty of translocation. 



Influence of low temperature on carbohydrate translocation. — A different 

 theory to account for the interruption in the translocation of carbohydrate is 

 put forward by Neger (7), and is based on experiments from which it appeared 

 that the starch in diseased leaves — and in general also in leaves of healthy plants 

 belonging to (assumed) susceptible varieties, or of healthy plants of (assumed) 

 individual susceptibility — is translocated with difficultj'' at low temperatures 

 (10° C), and under conditions of poor aeration of the leaves. The further 

 conclusion was reached by this investigator that high temperature and good 

 aeration promoted the translocation of starch from rolled diseased leaves. 

 Prom these findings the general inference was drawn that the leaf-roll disease 

 was the result of cold nights or of cold wet weather, and was merely the 

 expression on the part of certain plants or varieties of their inability to trans- 

 locate starch normallj^ under such conditions. The experiments of Neger, 

 which were carried out entirely with shoots cut from the plants, will be further 

 discussed below. Before doing so, an experiment to test the effect of low 

 temperature on starch translocation in a growing plant will be described. 



