AuausT 17, 1929] NATURE 267

anxiety, especially among biologists, who are used to pycniospores of one sex are applied to a pycnium of checking the adequacy of their methods by control opposite sex, the pycniospores are stimulated to experiments. The difficulty of obtaining decisive germinate and to produce haploid hyphoo which grow results often flows from heterogeneity of material, down to the hypha! wefts near the lower epidermis and often from causes of bias, often, too, from the diffi• there fuse with cells of opposite sex. The solution of culty of setting up an experiment in such a way as to the p:oblem of tracing the hyphoo from the germinating obtain a valid estimate of error. I have never known pycmospores to the base of the oocium must await difficulty to arise in biological work from imperfect further investigation. W. F. HANNA. normality of the variation, often though I have Dominion Rust Research Laboratory, examined data for this particular cause of difficulty; Winnipeg, June 24. nor is there, I believe, any case to the contrary in the literature. This is not to say that the deviation from The Crystal Structure of Solid Nitrogen. " Student's " t-distribution found by Shewhart and RECENT researches on the luminescence of solidified Winters, for samples from rectangular and triangular gases have shown that systems consisting of mixtures distributions, may not have a real application in some of nitrogen with inert gases give a great variety of technological work, but rather that such deviations oscillatory bands, which are intimately connected have not been found, and are scarcely to be looked for, with the oscillations which the nitrogen atoms are in biological research as ordinarily conducted. able to perform in the crystalline state. As pointed R. A. FISHER. out in previous papers, the determination of the Rothamsted Experimental Station, crystal structures of these systems will be of funda• Harpenden, July 26. mental importance for the interpretation of the oscillatory bands characteristic of the solid state. Nuclear Association in the lEcium of Researches on the structure of solidified gases have Puccinia graminis. already been taken up by W. H. Keesom and his IN a letter to NATURE (July 23, 1927, p. 116) collaborators at the Cryogenic Laboratory of Leyden Craigie announced the discovery of heterothallism in (Oomm., Leyden, 178). For pure argon they find a the rust fungi. In a second letter (Nov. 26, 1927, face-centred lattice. In the case of nitrogen, which p. 765) he showed that the oocial stage of Puccinia is the most important substance for our purpose, graminis-the parasite which causes the black stem they merely state that they have obtained powder rust disease of cereals-can be produced on the leaves diagrams, which, however, are so complicated that of the common barberry at will simply by applying they have not been able to interpret them by means to the pycnia of a monosporidial pustule of one sex of a definite unit cell and a corresponding crystal some of the pycniospore-containing nectar excreted system. from the pycnia of another monosporidial pustule of As the oscillatory bands mostly originate from opposite sex. nitrogen, it is indeed the nitrogen structure which is A series of experiments on Puccinia graminis, of the greatest importance for the interpretation of recently made by me, has yielded some information the luminescence phenomena, and we have therefore as to the manner in which the change from the haploid also attacked the problem at the Physical Institute to the diploid condition in this fungus is brought of Oslo. We obtained good powder diagrams with about. a conveniently constructed apparatus, and after The sporidia are uninucleate. In a pustule of having tried a number of crystal classes, we finally monosporidial origin, the mycelium and the pycnic• succeeded in showing that the spectrum could be spores produced by the mycelium are also uninucleate. interpreted by a unit cell belonging to the cubic In such a haploid pustule there appear, near the lower system. epidermis of the barberry leaf, numerous sterile wefts The side of the unit cube was found to be 5·65 A. of mycelium. These wefts, which appear to be units, and the density measurements of Dewar lead crescent-shaped in transverse sections of the leaf and to the result that the unit cell contains 8 nitrogen are made up of hyphoo with uninucleate cells, are evi• atoms. dently haploid rudiments of oocial cups waiting to be In these investigations I have been most ably stimulated into further developmental activity. assisted by Mr. Alf Maurstad, Mr. S. Stensholt, and vVhen nectar which contains pycniospores of one Mr. E. Tonsberg. L. VEGARD. sex is applied to the pycnia of a monosporidial pustule Physical Institute, University, of opposite sex, the wefts of hyphoo situated along the Oslo, June 28. base of the pustule soon undergo a change from the haploid to the diploid condition. About 48 hours Nature of Disease-Producing Viruses. after the pycniospores have been applied, the nuclei TWICE recently I have noticed in the columns of at the base of each weft become enlarged. Neigh• NATURE criticism of the hypothesis that disease-pro• bouring hyphoo then fuse in pairs in a manner similar ducing viruses are intermediate between molecule and to that described by Christman for Phragmidium living cell because no non-parasitic forms are known. speciosum, and two nuclei become associated in each Until viruses can be known other than by the effects fusion cell. The fusion cells, which initiate the diplo• of their parasitism, it would seem to be quite impossible phase, elongate and cut off chains of binucleate cells to detect corresponding bodies that are not parasitic. which later divide and thus form intercalary cells and I agree with the critics that if such bodies exist they oociospores. Sections through pustules fixed 65 hours may be much more numerous than the viruses, but after the application of pycniospores have shown until some method is devised by which the constituents young oocial cups with as many as four oociospores in of the viruses can be recognised, it would seem to be several of the oociospore chains. useless to look for them. For the hypothetical inter• The part which the pycniospores play in bringing mediate combining molecular structure, metabolism, about oocial development is not yet completely under• and reproduction, I have been using the designation stood. Some of the pycniospores have been observed vitamol. J. J. DAVIS. to germinate, and, in one instance, the germ-tube from Department of Botany, the spore had attained a length of 15 1'-· In view of the University of Wisconsin, fact that binucleate hyphoo are to be found only at Madison, Wisconsin, U.S.A., the base of the oocium, it seems probable that, when July 9. No. 3120, VoL. 124] © 1929 Nature Publishing Group