September i6. iS<)j] 



NATURE 



483 



from external infection, and sowed the grains in sterilised soil 

 in specially constructed greenhouses, through which the air can 

 only pass viA cotton-wool filters. Between the double-glass 

 windows water was allowed to stream, and the plants thus kept 

 cool. Some of these protected plants became rusted. 



Before we draw any conclusions from such difficult experi- 

 ments as the above, let us see the results of microscopic 

 examination. 



Reference has already been made to the mycelium and spores 

 in the tissues of the pericarp of the grain ; no trace could be, or 

 ever has been, detected in the endosperm or embryo. In some 

 cases the seedlings, four to eight weeks old, showed the first 

 uredo-pustules on their leaves, and the mycelium but no spores 

 could be detected in the seed-coats. 



The tissues of the leaf, in the neighbourhood of young uredo- 

 pustules, frequently showed curious clumps of protoplasm in the 

 ceils, either free in the cell-cavity, or attached to the primordial 

 utricle, and looking like haustoria. Eriksson assumes that we 

 have here the key to the puzzle ; he regards these " plasmatic 

 corpuscles " as the protoplasm of the fungus which, after leading 

 a dormant life commingled symbiotically with the living proto- 

 plasm of the cell, is now gaining the upper hand and beginning 

 to form a dominant mycelium. 



We are therefore to suppose that when the spores of rust, even 

 if of the right variety, alight on the tissues of a wheat-plant, it 

 is a matter decided by external and internal conditions whether 

 the perm-tubes forthwith infect the plant and grow out into a 

 dominant, parasitic, sporiferous mycelium, as we know they 

 usually do, or simply manage to infect the cells with enough 

 protoplasm to live a latent symbiotic life for weeks— or even 

 months — as a Alycoplastiia, which may, under favourable circum- 

 stances, gain the upper hand, and grow out in the form of a 

 mycelium. 



This is a startling hypothesis, and brings us to the most 

 advanced point along this line of biological speculation. We 

 must distinguish sharply and clearly between such a view, which 

 is by no means inconsistent with all we know of parasites, so far 

 as the dormant mycelium goes, and all the hazy, mystical sugges- 

 tions as to " infective substance " and so forth, which were so 

 freely flung about at the beginning of this epoch, and which De 

 Bary's strictly scientific methods put down so firmly. 



The idea of symbiosis is now comparatively old, and there are 

 many cases of dormant life now well established. Even the 

 astounding notion of blended protoplasms can no longer be 

 regarded as new. I need only remind you of Cornu's Rozella, 

 which invades the thallus of Saprolegnia and Woronina in 

 Vaucheria, the protoplasm of the two organisms apparently 

 blending and living a common life for some time before the true 

 nature of the parasite manifests itself. Eriksson has avowedly 

 been influenced by these and other cases among the Chrytridi- 

 aieit. That the remarkable intra-cellular fusions of Plasmodi- 

 ophora and the now well-established symbiosis of the organism 

 of the leguminous root-nodules have also had their influence on 

 his work may well be assumed, and I think we may trace also 

 the effects of our knowledge of the latent life of Ustilago during 

 the vegetative period of the attacked cereal. 



But there are other cases which prevent our casting aside as 

 impossible the view that Eriksson has put forward. 



I showed some years ago that the mycelium of the Botrytis 

 of the lily disease can lie dormant for some time in the cell- 

 walls, and I have observations showing that other forms of 

 Botrytis which attack roses and chrysanthemums only gain the 

 upper hand when the cold autumn nights so chill the attacked 

 cells that they succumb ; the mycelium was there long before, 

 but so long as the cells were active no progress could be made, 

 and only when the plasmolysed chilled cells exude their sap can 

 the mycelium advance. 



Many cases of similarly dormant mycelia appear to exist in 

 those cortex and cambium diseases which result in the production 

 f cankers — e.g. Nectria ditissitna and Peziza Willkoinmii, and 

 I ubeufs experiments with Gyintiosporangium are even more 

 uggestive. Tubeuf found that if G. clavariafarme is sown on 

 hawthorn seedlings the fungus forms yellow spots and induces 

 marked hypertrophy, and normal spermogonia and jecidia — 

 Rocstelia lacerata—?^x^ developed ; but if Pyrus Acuparia is used 

 as the host, no yellow spots or hypertrophy result, though a 

 mycelium is formed and will even produce a few starved sjiermo- 

 gonia. On allied species of Pyrus the fungus may even succeed 

 in forming a few poorly developed aecidia. But on the quince 

 the fungus only just succeeds in establishing an infecting { 



NO. 1455, VOL. 56] 



mycelium, and soon dies ; and Wagner describes similar events 

 with fungi on Stellaria. 



These cases point to a struggle between the protoplasm of the 

 cells of the different hosts, and of the fungus respectively : some- 

 times one wins, sometimes the other. The following cases are 

 also suggestive. De Bary found that the germinal hypha; of 

 Perottospora pygmtta, which is parasitic on Anemone, will pene- 

 trate the tissues of Rajtunculus Ficaria, but cannot maintain its 

 hold, and the mycelium soon succumbs and dies. 



Still more remarkable and to the point is the following case. 

 Soppitt and Plowright in England, and Klebahn and others on 

 the continent, have gradually unravelled a curious case of 

 hetercecism and specialised parasitism among certain Puccinias 

 found on Smilax, Convallaria, Paris, and Digraphis. The 

 story is too long to recount in detail, but the Puccinia- spores 

 from Phalaris were found by Klebahn to refuse to infect Poly- 

 gonatum leaves successfully, though they readily infect the allied 

 Convallaria. Close investigation showed, however, that, although 

 the sporidia failed to develop a mycelium in the Polygonatum 

 leaves, they really penetrate the cells, and the delicate germ- 

 tube is killed off by the protoplasm, a red spot marking the 

 place of entrance. 



The perennial mycelia of Witches' Brooms, aecidia in 

 Euphorbia, Taphrina, and many other perennial mycelia are 

 also cases in point. 



It is not my purpose to hold a brief for Eriksson's hypothesis, 

 but I may point out that it is in no way contradictory to the facts 

 already known since De Bary's time. Its most serious aspect is 

 with regard to possible treatment, and it is obviously essential 

 that we should have it tested to the utmost, for it must be re- 

 membered that no method of spraying or dusting has been, or 

 apparently can be, devised for cereals ; hence the questions as to 

 the existence of really resistent forms, and whether dormant 

 mycelia lurking in their tissues have deceived us in these cases 

 also, require sifting to the bottom. Experience, so far, points 

 to the selection of pedigree wheats and careful cultivation as the 

 first necessities ; how far the question of spring versus winter 

 wheat aids us is still matter for further experiment ; early and 

 late ripening are also concerned. Climate we cannot hope to 

 control, but it remains to be seen — when the facts are known — 

 how far it can be "dodged." 



Clearly what is needed, then, is experiments with varieties or 

 wheat under all conditions, and we may congratulate the Aus- 

 tralian, Swedish, and United States experimental stations on 

 their preliminary efforts in this direction. 



I have only been able to give a mere sketch of this rapidly 

 growing subject, but I think you will agree that we are justified 

 in saying that an epidemic of parasitic fungi depends on the 

 interaction of many factors, congenital variations of the host- 

 plant and topical variations of its cell-contents being probably 

 among the most important ; and since we cannot hope to control 

 the variations of the parasite, or the meteorological conditions, 

 it behoves agriculturists to pay more systematic attention to the 

 selection of those varieties of the cereal which are least pre- 

 disposed to rust. 



When we find the annual losses from wheat-rust alone put 

 down at sums varying from i,ocx),ooo/. to 20,000,000/. in each 

 of the great wheat-growing countries of Europe, India, Australia, 

 the United States, and elsewhere, it strikes one as very remark- 

 able that so little should be done to encourage the scientific in- 

 vestigation of these practical questions. I need hardly say that 

 the establishment and maintenance of a fully equipped labora- 

 tory and experimental station does not cost the interest on the 

 smallest of these sums. 



It should be also clear that in the further development of our 

 knowledge of the treatment of parasitic diseases of plants the 

 farmer, gardener, and forester can alone supply the experimental 

 evidence which will enable us to put theory to the test in the 

 field, garden, and forest. The botanist, by means of his pure 

 cultures of the fungus, can now show clearly what stage in the 

 life-history of a parasite is vulnerable. In his " microscopic 

 gardens " he can show what antiseptics may be employed, how 

 strong they should be, and when and how they should be 

 employed. 



But we must not forget that it is one thing to kill a 

 fungus when grown pure, and another to kill it when 

 growing on or in, or even associated with, other plants, without 

 harming the latter. We may compare the first case to the de- 

 struction of weeds on a gravel path, where the antiseptic dressing 

 may be employed lavishly and at any time, because there are no 



