MuKPHY — On the Cause of Rolling in Potato Foliage. 167 



nevertheless it is sufficiently clear that the lower leaves of diseased plants, which 

 empt}^ themselves of their starch normally at first, gradually accumulate it in 

 the mesophyll at a later stage ; and very soon afterwards begin to roll upwards. 

 This conclusion is confirmed by the results of two further experiments, which 

 will now be described. 



Artificial retardation of starch accumulaiion and leaf-rolling. — At 10 a.m. 

 on June 1, 1922, before any signs of rolling in their leaves were visible, two 

 plants of the variety British Queen growing in the field from "sets" derived 

 from diseased tubers, were darkened by covering them with comparatively light- 

 tight boxes. At that time the lower leaves were found to be practically free 

 from starch; and they were entirely free from it after a further period of 

 twenty-four hours, with the exception of the merest trace in the "starch-sheath" 

 and guard-cells. Carbohydrate translocation may, therefore, be assumed to 

 have been normal at that time. The plants remained covered except for 

 occasional short periods, when the boxes were removed for the purpose of 

 allowing a small amount of photosjaithesis to take place. These periods generally 

 extended from 5 p.m. to 9 a.m., but sometimes the plants were uncovered for 

 shorter intervals in the daytime. Under those abnormal conditions the plants 

 assumed, of course, a somewhat sickly and chlorotic appearance ; but the leaves 

 remained free from roll, and starch accumulation did not occur until a period 

 of from thirty to thirty-six days had elapsed after all the similar, neighbouring, 

 non-covered, diseased plants had shown both rolling of their leaves and starch 

 accumulation. One of the covered plants was first observed to show decided 

 rolling of its leaves on July 11, this occurring after an unduly prolonged 

 exposure to light. Som.e of the leaves on the other covered plant began to 

 roll very slightly on July 12, also following a period of exposure. The two 

 plants were again covered on the dates mentioned, and they were not again 

 exposed to light until seventeen days had elapsed in one case and twenty-one 

 days in the other. In both cases periodical tests were made, and they showed 

 that a large proportion of starch was retained by those more or less rolled leaves 

 which remained green. The leaves which became yellow eventuallj^ showed no 

 starch. While the badly rolled leaves of the first plant, after it had remained 

 covered for seventeen days, turned entirely blacl;: with iodine, the bases of the 

 veins of the slightly rolled leaves of the other covered plant began to clear at that 

 time, and after twenty-one days the basal halves of the leaflets of this plant were 

 quite clear, while their upper halves were full of starch (fig. 9, PL VI). When 

 it was finally uncovered, the second plant developed still more pronounced 

 symptoms of leaf-roll within twenty-four hours. 



Rolling of leaves and starch accunndation artificially produced. — In the 

 second experiment each of four healthy tubers of the variety Up-to-Date was cut 

 in two, and the resulting pairs of four "sets" were planted in two parallel rows 

 as far removed as possible from all likely sources of leaf-roll infection. The 

 four ' ' sets ' ' in one row were planted in mounds of soil about one foot above the 

 general ground-level. Farmyard manure was previously placed at the bottom 

 of each mound, and stakes were provided to which the resulting plants were 

 afterwards tied. The corresponding four "sets" were planted in the other 

 row in the usual manner, i.e. in a drill in which the manure had been placed. 

 All the plants grew well, and appeared normal. On July 7 the soil was care- 

 fully removed from the bases of the four plants growing on the mounds, and the 

 stolons (which were just developing), together with most of the axillary buds and 

 shoots, were removed from them.' The immediate effect of this treatment was 



^ This operation was repeated once during the course of the experiment. 



