172 Scientific Proceedings, Royal Dublin Society. 



in comparison with similar tissues of healthy plants, and always with the same 

 result, thus corroborating the work of Oortwijn Botjes (8), and ruling out phloem 

 necrosis as the original or principal cause of the abnormal accumulation of starch 

 in the leaves of diseased plants. 



Further, it is to be observed that there appears to be no spatial relationship 

 between the situation in which starch accumulation first occurs (in the mesophyll 

 of the lower leaves) and the place at which disorganization of the phloem is 

 supposed to be first noticeable, namely, the lower part of the stem. If the 

 necrosis were actually the cause which precluded carbohydrate translocation, it 

 would be reasonable to expect to find excess of starch just above the point of 

 obstraction. This was not found to be the case. I^or, not only was the accu- 

 mulation of starch at that stage confined to the -blades of the lowest leaves, but, 

 as has been stated (p. 166), less starch was found at 10 a.m. in the stems of 

 diseased plants showing the first signs of rolling than in similar healthy stems. 



It is also to be noticed that the disappearance of starch from the very 

 vigorously assimilating upper leaves of affected plants is not interfered with at 

 this time, although the destination it reaches is presumably the same as would 

 have been reached by the starch in the lower leaves were this capable of being 

 moved. 



Again, when rolled leaves are kept darkened for a considerable time, whether 

 remaining on the plant or standing with their cut ends in water, the starch 

 vanishes first from the neighbourhood of the lowest part of the midrib and the 

 base of the leaflet (fig. 9, PL VI). This circumstance shows that the hindrance 

 to translocation, whether mechanical, i^hysieo-ehemical, or otherwise, is less likely 

 to be found a considerable distance away in the stem than in the leaf itself. 



A similar conclusion maj' be drawn from the fact that in the last stages of a 

 very severe attack of leaf-roll, such as is common in the varieties President and 

 Black Skerry, practically the only starch to be found in any part of the plant 

 (with the exception of the tubers) was in the leaves. A systematic examination 

 of plants of the varieties mentioned was made on July 25, 28, 31, and 

 August 2, 1922, the material being taken at 10 a.m. to 1 p.m., and examined at 

 once; and practically the same results were secured throughout from the two 

 varieties. The plants selected were very badly diseased, being dwarfed and 

 chlorotic, with upstanding leaves, which were practically all rolled, and showed 

 considerable pinkish coloration and tip-injury. The greenest of the more or less 

 well-developed leaves were used in the examinations. 



There was a great accumulation of starch in the leaves, and this was largely 

 or entirely confined to the palisade layer. No starch was found, as a rule, in 

 the veins ; but in those portions of the spongy parenchyma furthest away from 

 them starch was sometimes present. The midribs and petioles contained traces 

 of starch in the form of small grains in the "starch-sheath," and sometimes as 

 isolated grains in other parenchymatous cells. The total amount of starch 

 present in the stem was small, and was confined to the "starch-sheath," with the 

 exception of an area some little distance below the growing point, where grains 

 occurred in the parenchyma of the principal vascular bundles and in the pith. 

 In proceeding from the top of the plant downwards the initially small amount 

 of starch present in the "starch-sheath" became gradually less, until at about 

 ground-level it finally disappeared. In the underground organs, with the 

 exception of the tubers, but including the stem, stolons, and roots, starch was 

 either entirely absent or a trace occurred in the cortex of the stolons. It should 

 be noted that in these cases the phloem in the lower half of the stem was consi- 

 derably disorganized. 



In "contrast with this state of affairs, corresponding healthy leaves examined 



