164 Scientific Proceedings, Royal Dublin Society. 



these investigators had in view the difficulty in upward transfer of food from 

 the planted tubers to the developing shoot ; and some evidence in support of the 

 existence of this difficulty, based on chemical analysis, was brought forward by 

 Spieckermann (14) and Doby (2). The same workers also found that at a later 

 stage of growth there was an excess of food in the upper portion of the plant ; 

 whilst Quanjer (10) clearly expressed the opinion that the abnormal appearance 

 shown by the foliage must be looked upon as being due in some way to a similar 

 disturbance which interfered with the downward transfer of food. 



According to Quanjer (11), Jordi (in 1913) was the first to record the presence 

 of abnormal quantities of starch in the stems and petioles of plants affected 

 with leaf -roll. It was not, however, until the publication of Neger's (6) first 

 paper in 1918 that it was realised that the rolled leaves of diseased plants 

 retained their starch to a ver}^ large extent instead of getting rid of it. In 

 the following year PJsmarch (3), Hiltner (4), and Quanjer (9) published the 

 results of investigations in which each arrived independently at the same general 

 conclusion. According to Esmarch (I.e., p. 17), the leaves of healthy potato 

 plants when darkened become free from starch within a period of from 19 to 

 68 hours, depending on the temperature, weather conditions, age of the leaves, 

 and individuality of the plants. The leaves of diseased plants, on the other 

 hand, become depleted of their starch under the same conditions either not at 

 all or only incompletely. In Esmarch 's experiments the older leaves of diseased 

 plants, after being darkened for from six to eight days, or in many experiments 

 even for twelve days, still retained practically all their starch, in many cases 

 the veins alone being more or less starch-free. In the younger leaves of diseased 

 plants, which were still free from rolling, starch sometimes began to disappear 

 at about the same time as in leaves of the same age from healthy plants; but 

 generally it did so only after a period of from three and a half to eight days. 

 Complete disappearance of the starch from young rolled leaves was seen only 

 rarely. The older the leaf and the more pronounced the rolling the greater 

 was the interference with starch translocation. 



The experiments carried out by Neger gave very similar results. This 

 worker employed cut shoots which, after removal from the plants, were placed in 

 the dark with their ends standing in water. While this method is a very 

 convenient one for demonstrating the difference between the amount of starch 

 in the leaves of diseased and healthy -plants (and was largely used for that 

 purpose in the investigations now to be described), it is entirely unsuitable for 

 experiments on the translocation or movement of carbohydrates in the plants. 

 Consequently Neger's conclusions on carbohydrate translocation, based, as they 

 were, on this method, are open to serious question. 



II. — Starch Accunmlation in the Leaves Invariably Associated with Leaf -Roll. 



In taking up the further study of this subject it seemed necessary in the first 

 place to determine whether excess of starch in the lower leaves of diseased plants 

 was a universal concomitant of leaf-roll under Irish conditions. That this was 

 so was established as a strong probability as a result of experiments carried out 

 here with twenty-one varieties of potato in 1921. Two plants of each variety 

 were selected, one diseased and the other healthy, and from a lower leaf of each, 

 situated on the westerly side of the plant, corresponding leaflets were removed 

 in the morning (10 a.m. to 11 a.m.), and Avere subjected to the well-known iodine 

 test. In the case of all varieties but two there was a very marked difference in 

 the amount of starch present, the affected leaflets containing an abundance of 

 starch, while the healthy ones showed little or none. 



