MuKPHY — On the Cause of Rollinrj in Potato Foliage. 175 



fore appears that Neger's conclusions do ]iot follow from his experiments. The 

 latter refer in the main to the hydrolysis of starch in the leaves, and only in a 

 minor degree, if at all, to the translocation of the resulting carbohydrate. The 

 only point proved by them is that rolled leaves of diseased plants contain much 

 more starch than similar healthy leaves." 



V. — Histological and other Symptoms of Leaf -Roll. 



During the course of a systematic examination of the starch-content of 

 various portions of diseased and healthy plants certain histological differences 

 were discovered or re-investigated. As was first pointed out by Schander and 

 V. Tiesenhausen (12), necrosis of the phloem, which Quanjer regards as being 

 the underlying cause and principal symptom of leaf-roll, is not confined to 

 plants affected with the leaf-roll disease. These two workers showed that 

 similar injury to the phloem may be found in plants affected with a number of 

 other diseases, including curly dwarf, black-stalk rot, and blight ; and that it 

 may even occur in maturing healthy plants. It has also been pointed out else- 

 where bj^ the present author (5) that a very thorough disorganization of the 

 phloem (as well as of other tissues) occurs in streak disease. Recent work has 

 again confirmed the fact that this type of injury is not confined to leaf -roll. It 

 was found during the past year to occur in the lower portions of the stalks of 

 potato plants attacked by a disease possibly due to eel-worms, and it has also 

 been seen in the lateral veins, midribs, and petioles of potato leaves attacked by 

 Pliytoplithora infestans. In neither case were the plants affected with leaf -roll. 

 The disorganization in these diseases was not, of course, confined to the phloem, 

 but the effect on that tissue was exactly of the same kind as in leaf-roll, only 

 rather more pronounced. The attack began in the oldest cells, the walls of 

 which became brown in colour, and afterwards collapsed. The walls of such 

 cells no longer gave a cellulose reaction, but the nature of the change in com- 

 position was not further inquired into. In the blighted plants the death of the 

 phloem was apparently due to a toxic substance, which operated some distance 

 in advance of the parasite itself. The first cells to be killed in the petiole were 

 those in the epidermal and sub-epidermal layers (giving rise to the brown 

 stripes characteristic of the disease on stems and leaf-stalks), and from them the 

 parenchymatous ground tissue and the phloem groups nearest the surface of the 

 stalk were attacked in turn. 



A difference manifests itself in the behaviour of diseased and healthy leaves 

 in the matter of starch evacuation which does not seem to have been sufficiently 

 emphasized. It seems to be a general rule that the disappearance of starch begins 

 at the bases of diseased leaflets^that is, if any noticeable clearing whatever takes 

 place — and proceeds for a less or greater distance towards the apex. This 

 feature is most clearly seen when diseased leaflets in the earliest stage of rolling 

 are darkened for prolonged periods (fig. 9, PL VI). In such cases a sharp line 

 divides the tissue which is free from starch from that which retains it in 

 apparently undiminished quantity. So far as our experience goes, it also seems 

 to be invariably, true that the starch begins to disappear from healthy leaflets in 



'In a subsequent paper, a note of whicli has just been seen (Centralbl. f. Bakt. Abt. II, 

 54 Band, 1921, p. 512), Neger records an experiment in which living plants diseased with leaf- 

 roll were kept at night under different conditions. From this it appears that those kept 

 during the night at 20o C. became entirely healthy, and showed normal ("good") translo- 

 cation, as measured by the application of the iodine test to the leaves. However this may 

 be (and it should be noted that the respiration factor seems to have been left out of account), 

 it does not prove the author's contention that leaf -roll, or the liability of a plant to it, is an 

 expression of inability to translocate carbohydrate at a low temperature. 



