410 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Potato Leaf-roll, Histological Studies of. By E. L. Artschwager {Jour. Agr. 

 Res. xv. pp. 559-57°. Dec. 1918 ; plates). — An investigation running counter 

 in some ways to Quanjer's observations. Too long to abstract — the original 

 should be consulted. — F. J. C. 



Potato, Mosaic Disease of the. By E. S. Schultz, D. Folsom, F. M. 

 Hildebrandt, and L. A. Hawkins {Jour. Agr. Res. xvii. No. 6, Sept. 1919, 

 pp. 247-273, 8 plates). — Mosaic disease of the potato is now widely distributed 

 over the United States, and has a very detrimental effect upon the yield. It 

 produces characteristic symptoms upon the foliage and these may be modified 

 or obscured by differences in environment or variety. The disease may be 

 transmitted from plant to plant by the juice, winch may be carried by at least 

 two species of aphides. The mosaic disease has a tendency to increase the 

 sugar contents of the leaves at the expense of the starch. Removing the 

 "mosaic" plants before aphids become abundant is helpful in checking the 

 disease. 



A short bibliography is appended. — A. B. 



Potato, Parasitism of Pythium Debaryanum (Hesse) on the. By L. A. 



Hawkins and R. B. Harvey {Jour. Agr. Res. xviii. No. 5, Dec. 1919, 

 pp. 275-298, 3 plates). — The physiology of parasitism and the relations existing 

 between host and parasite have been the subject of numerous investigations. 

 The authors find that Pythium Debaryanum destroys the pentosans, starch, 

 and sugar of the potato tuber ; that the fungus secretes a toxin which kills the 

 cells of the potato and that an enzyme is secreted by the fungus which destroys 

 the middle lamellae of the cells of the host. The cut surface of the cortex, when 

 dried for three hours, was much more resistant to puncture than the freshly 

 cut surface. Thus there is a co-relation between resistance to infection by the 

 fungus, and resistance to mechanical pressure. The osmotic pressure within 

 the fungus filament, as determined by plasmolysis, is sufficient to develop the 

 pressure necessary to puncture the cell walls in the potato tubers in nearly 

 every case where infection occurred. Mechanical pressure exerted by the fungus 

 hyphae seem to be the most important factor in cell-wall penetration by this 

 fungus, and resistance to infection is apparently due to resistance of the cell 

 walls to mechanical puncture. Microscopical examination and observations 

 seem to corroborate this theory. — A. B. 



Potato Seed. By H. A. Edson and W. Stuart (U.S.A. Dep. Agr., June 1918, 

 pp. 1-8 ; 4 figs.). — Seed potatos should not be selected from the whole crop after 

 it has been harvested. It is better to plant a special plot with a good strain, 

 grow the plants well and eliminate weak and diseased plants and sports. Select 

 seed from the most productive plants yielding tubers of uniform size. — 5. E. W. 



Potato Stem Lesions. By H. A. Edson and M. Shapovalov (Jour. Agr. 

 Res. xiv. No. 5, July 1918, pp. 213-219 ; 3 plates). — The authors find that 

 numerous species of fungi are concerned in the formation of lesions on the stems 

 of the potato. Amongst these are several parasitic species of Fusarium, Alternaria, 

 Botrytis, Zygorhynchus, Corethropsis, Phoma, Clonostachys, Acrostalagmus, as 

 well as Rhizoctonia. The lesions produced by these fungi are practically 

 indistinguishable from each other under field conditions, but show certain 

 characteristics when reproduced under control conditions in the greenhouse. 



A. B. 



Pruning Investigations: Second Report. By J. R. Magness, A. F. Edminster, 



and V. R. Gardner (U.S.A. Exp. Stn. Oregon, Bull. 146, Oct. 1917; 19 plates). 

 — Contains accounts of studies in fruit- bud formation, the relation between 

 length, angle, and diameter of shoots, and the development of side shoots and 

 fruit spurs from lateral buds, the influence of bending dormant shoots upon 

 their subsequent behaviour, and the relative effects of winter heading back and 

 thinning out of apple shoots in young trees. — A. P. 



Pyracantha Gibbsii var. yunnanensis. By A. Bruce Jackson (Gard. Chron. 

 May 31, 1919 ; with 2 figs.). — The right name for the plant known as P. crenulata 

 v. yunnanensis and P. crenulata v. obtusata. It differs from the type in more 

 spiny branches, with leaves coarsely crenate, instead of entire or finely serrate 

 in the upper two-thirds only, and fruits maturing and lasting later. — E. A. B. 



Pyrethrum. By W. S. Abbott (U.S.A. Dep. Agr., Bull. 771, pp. 1-7). — 

 Pyrethrum powder deteriorates in the course of 12 months unless it is kept in 

 sealed jars. The whole flower-heads are not damaged by storage. — S. E. W. 



