304 



NATURE 



[November 3, 1921 



tigation of the sex-phenomena in the pea-crab, the 

 female of which is known to be sedentary, spending 

 its life imprisoned within bivalves such as mussels, 

 cockles, and occasionally oysters. It was found that 

 the female fulfils its mating functions probably once 

 and for all (see Nature, December 23, 1920, p. 533) 

 when quite a tiny mite, and probably not more than 

 a month or two old. It is unlikely afterwards that 

 the female has any more need of a male. Now the 

 same result— excepting for any advantage there may 

 be in cross-fertilisation — might be obtained by the 

 pea-crab being born as a male and changing into a 

 female, and at the outset of the investigation such a 

 change was suspected. It is, indeed, possible, that 

 evolution in sex-characters in a similar species carried 

 a stage further would end in cutting out the males of 

 that species altogether— as has taken place in some 

 Cirripedes — and leaving a hermaphrodite form which 

 starts life as a male and changes into a female. In 

 the case of the pea-crab, therefore, bisexuality is 

 maintained by a very precocious association of the 

 sexes, while the males still retain their active mode 

 of life. 



A_ similar dodge on the part of Nature has been 

 applied in a large number of other cases, such as in 

 some parasitic Copepods, some Echiuroids, some 

 Cirripedes, some parasitic Isopods, and no doubt in 

 many other cases. But in these cases the male is 

 often minute and permanently attached to the female. 

 It is clear, therefore, that it is necessary to be on 

 the look-out for adaptations which retain in a species 

 an appearance of bisexuality, but which bring about 

 a close resemblance to the hermaphrodite state. The 

 conditions which determine whether both sexual pro- 

 ducts shall ripen together in an individual or only 

 successively are entirely unknown, but it is hoped 

 that the suggestions put forward in this letter may 

 help towards securing some information bearing on 

 the problem. J. H. Orton. 



The Laboratory, The Hoe, Plymouth, 

 October 20. 



The Presence of Perennial Mycelium in Peronospora 

 Schleideni, linger. 



Since Peronospora Schleideni, Unger, the onion 

 mildew fungus, is known to produce its sexual organs 

 in the leaves of the host-plant, the assumption has 

 been made that the parasite is dependent on its 

 oospores for perpetuation from year to year. It has 

 now been proved, however, that the mycelium is 

 capable of a perennial existence in onion bulbs, and 

 that the shoots produced if such bulbs are planted 

 are infected ah initio. 



The presence of non-septate mycelium permeating 

 the bulbs of the potato- and common onion, sent in 

 from several parts of Ireland in the late spring of 

 1920, first directed attention to the subject; All 

 attempts to induce this fungus to fruit on the bulb- 

 scales, to infect other onions, or to grow on artificial 

 media, met with failure. In some cases, however, 

 where green leaves were present, the mycelium was 

 traced from the bulb to the apical portion of a leaf 

 on which conidiophores of P. Schleideni were being 

 produced. All the infected bulbs shrivelled and died 

 during the course of the autumn or winter, as did 

 further infected specimens gathered in the autumn 

 from badly diseased plots. Practically 66 per cent, of 

 the smaller-sized onions from one particular plot 

 which was badly mildewed in 1920 contained non- 

 septate mycelium; Such infected bulbs sprouted prema- 

 turely, but although the mycelium grew up within 

 some of the new shoots, it failed to develop conidio- 



NO. 2714, VOL. 108] 



phores on the surface under winter conditions in the 

 greenhouse. 



Further material became available in the spring 

 which proved the relationship of this fungus to P. 

 Schleideni. Bulbs of the common onion (^Allium Cepa) 

 and of the shallot (A, ascalonicum) which contained 

 the same non-septate mycelium were grown in the 

 early spring under conditions which excluded the 

 possibility of external infection, as proved by the 

 fact that numerous control plants which were initially 

 free from mycelium remained free from mildew, even 

 when kept for a week under conditions favourable to 

 the disease. The infected plants, on the other hand, 

 when placed under a bell-jar for one night were found 

 practically covered with mildew next day. In some 

 cases the original infected leaves, which had not 

 developed any mildew, were cut away. The new 

 shoots which replaced them again came up permeated 

 with mycelium and again became mildewed under 

 favourable conditions, while similarly treated control 

 plants remained healthy. 



There is a time in the early spring, generally in the 

 month of April, when the mildew is found only on 

 onions the bulbs and leaves of which are permeated 

 with mycelium. These plants appear to act as impor- 

 tant centres of infection. For a long period no ap- 

 parent harm results to the host, but the tips of the 

 leaves ultimately turn yellow and droop. Under 

 favourable weather conditions mildew then breaks out, 

 at first just below the withered portion of the older 

 leaves, and then on all the leaves except the 

 voungest. There appears to be a nice balance 

 between host and parasite, seeing that both can 

 go on flourishing so long together, and that it is 

 apparently only at a certain stage of maturity of the 

 leaves that the internal mycelium breaks out into 

 conidiophores. Even when this happens, the host- 

 tissue is not killed for a considerable time. 



Non-septate mycelium, apparently that of P. 

 Schleideni, has been found in the bulbs of the common 

 onion, in those of the potato- or underground-onion, 

 and of the shallot. In the case of the common onion 

 and shallot this mycelium has been definitely con- 

 nected with the mildew fungus. It has also been 

 proved that the mycelium survives when infected bulbs 

 are left in the soil during the winter. This is pos- 

 sibly an important point in the case of Tripoli onions, 

 which are sown in the autumn. The role of perennial 

 mycelium in the shallot, potato-onion, and in onions 

 grown from "sets" is obvious. 



Plainly visible mycelium was observed in infected 

 bulbs every time they were examined in spring, 

 autumn, and winter. In these circumstances, it 

 appears unnecessary to suggest the presence of myco- 

 plasmic infection. It may be added that the mycelium 

 is so obvious, the hyphae being stout and well differen- 

 tiated from the cells of the host-plant, into some of 

 which large convoluted haustoria extend, that no 

 better subject for demonstrating intercellular mycelium 

 and haustoria is known to the present writer. No 

 staining is required. 



The effect of the fungus on the keeping qualities 

 of infected bulbs requires further study, as do some 

 other points in this strangely overlooked phase in 

 the life-history of such a common and destructive 

 parasite. Paul A. Murphy. 



Royal College of Science, Upper Merrion 

 Street, Dublin, October 15. 



The Development of Optical Industries. 



In Nature of October 20, p. 238, Messrs. Zeiss 's 

 publicity manager questions a particular period of 

 British supremacy in the manufacture of optical glass. 



