MiJKPHv — On the Cause of Rolling in Potato Foliage. 171 



ou the average, about one-quarter of each rolled leaflet had become clear in 

 irregular patches, the clearing being most pronounced in the terminal leaflet 

 as a whole, and generally in the tops of the others (fig. 10, PL VI). 



The same experiment was repeated more than once with plants of the variety 

 Up-to-Date, which were free from leaf-roll, but which proved on examination to 

 be affected with black-stalk rot {Bacillus atrosepticus). The rolled leaves from 

 the top of such plants, while they retained their green colour, contained more 

 starch than similar leaves from healthy plants; and when cut and placed 

 standing in water in the dark became clear at about the same rate and in the 

 same way, from tip to base, as in the case of rolled leaves from, broken stalks of 

 healthy plants. 



Again, excess of starch was demonstrated in the upper rolled leaves of plants 

 of the varieties Up-to-Date and Ally, the rolling being due to obscure and 

 apparently temporary causes, for the plants afterwards recovered. 



It will be observed that in all these eases an upward rolling of the leaves is 

 an accompanying feature of starch accumulation ; and, in the case of the rolling 

 due to the breaking of the stalk at least (the only one in which the relation was 

 determined), an increase in the depth of the spongy parenchjnma was found to 

 be another accompaniment. In the rolled leaves of this plant the spongy 

 parenchjTua and lower epidermis made up 52 per cent, of the thickness of the 

 leaf, while the corresponding figure for healthy leaves was 47' per cent. 



IV. — The Cause of Starch Accumulation in Rolled Leaves of Diseased Plants. 



Since disorganization or necrosis of the phloem has been shown by 

 Quanjer et al. (10) to be an accompanying feature of leaf -roll, and since it is 

 believed by most plant physiologists that the carbohydrates of the plant are 

 translocated through this tissue, the assumption was soon made that such 

 necrosis was the cause of the accumulation of starch in the leaves ; and that, in 

 fact, the association of these two phenomena furnished a settlement of the old 

 question as to the channel through which carbohydrates are principally distri- 

 buted in the plant. 



Investigation was made as to the extent to which the phloem is disorganized 

 in plants affected with 'leaf-roll, and it was found to be ver,y variable. It 

 depends apparently on the susceptibility of the variety to the disease, the 

 severity of the attack in the particular plant under examination, the period of 

 the season at which the examination is made, and probably on many other 

 factors. Thus varieties which commonly show an aggravated form of leaf-roll, 

 such as President (which appears to be synonymous with the Dutch variety 

 Paul Kruger) and Black Skerry, are also generally characterized by severe 

 phloem necrosis, at least in the later stages of the disease. The same may be 

 said for British Queen, and probably for other varieties, when the attack is 

 unusually severe. On the other hand, when the disease occurs in its normal 

 intensity on British Queen, Up-to-Date, and most of the varieties which are in 

 common cultivation in Ireland, the amount of disorganization which can be seen 

 in the phloem during the height of summer does not appear to be sufficient to 

 account either for the external symptoms of disease or for the failure to trans- 

 locate carbohydrates. This seems to be beyond question for the earliest 

 recognizable stage of the disease. Examination made at the end of May and 

 early in June, 1922, of plants in which starch accumulation had just begun in 

 the lower leaves, which were showing the first signs of rolling, failed to reveal 

 any trace of alteration in the phloem, either in the petioles of the affected leaves 

 or in the stem below their insertion. This examination was made repeatedly 



