This is said to prevent the attacks of the onion-fly, for it is believed that the flies select the younger onions for the deposition of their eggs in preference to those of more mature growth.

Where possible all mildewed material should be burnt, for no fact is better known than that mycelium or fungus spawn is often perennial.

Like many other plant ailments, the diseases of onions require investigation. Several forms of disease are known to growers, of which no explanation has at present been forthcoming. In one form the whole crop turns sickly yellow just before ripening, the tops soften, the bulb becomes detached, the roots decay, and the entire growth soon becomes rotten. Another disease causes the onion to become thick and soft-necked, the bulb in proportion being small. Sometimes mildew attacks the full-sized and apparently not thoroughly-ripened bulb after it is harvested, and commences from the outside.

CHAPTER X.

MOULD OF ONIONS.

Mucor subtilissimus, B.

THE fungi known as Mucors or Moulds (mukēs, mould) are extremely common on decaying bulbs, fruits, provisions, etc. They are said not to be the immediate cause of decay, but there can be no doubt that they greatly accelerate putrescence when they grow upon exposed or injured places. Sometimes they grow in the inner substance of plants, like the one under description, which has been named by the Rev. M. J. Berkeley, in reference to its extreme smallness and delicacy, Mucor subtilissimus.

A species of fungus closely allied to the onion Mucor is the common Mucor mucedo, L., so frequent on paste, jam, damaged fruit, etc. The onion Mucor differs from all its allies, in its extremely small size; it is said to be the most microscopic of all fungi found in Great Britain.

In the Mucor disease the whole substance of the neck of the onion near the bulb, and sometimes the bulb itself, is traversed by fine threads of mycelium, and in the midst of this mass of spawn may be seen innumerable black atoms like minute grains of gunpowder. These little grains have been described as fungi under the name of Sclerotium Cepa and Sclerotium cepavorum, B.; the nature of Sclerotia is described under Peziza postuma, B. and Wils. The present Sclerotia not only differ from the potato Sclerotia in their much smaller, almost microscopic size, but also in their less compact and more filamentous structure. The less compact a Sclerotium is, the more readily it will germinate, and in the present instance the

Sclerotium will protrude germ tubes in a few hours if placed in a drop of water. The whole process of growth may be easily watched under the microscope. On germination in water, the filamentous mycelium of which each Sclerotium is formed, protrudes, elongates, and branches in a flexuous manner in all directions; this spawn is sometimes jointed and sometimes free from joints, and whilst in water no farther progress is made in growth beyond the protrusion of these threads. As soon, however, as the mycelium reaches the edge of the water, a change takes place, and the branches become furnished on their tips with minute globose heads, technically termed Sporangia or spore cases. These extremely minute spherical heads at length become filled with elliptic spores or sporidia (so called because they are produced within a spore case), which, on the natural bursting of the Sporangium, or spore case, are set free in the air. The little oval sporidia now soon germinate and reproduce the species. The small mycelial threads protruded from the sporidia are capable of forming little white knots, which at length become, on exposure to the air, the minute black granules termed onion Sclerotia.

Now, although these small dark-coloured granules will germinate very readily under favourable conditions in water, it must not be supposed that every example germinates after a few hours' rest. There can be no doubt that this Sclerotium condition answers precisely the same purpose in the onion as it does in the very large Sclerotium of the potato; that is, it carries on the life of the fungus in a hibernating state through the winter.

Individual fungi vary in their habits of growth precisely in the same way as they vary in their specific characters. No hard and fast line can bind down every individual fungus to specific characters or to habits of growth. For instance, in Puccinia mixta, Fl., already described, although nine hundred and ninety-nine teleutospores out of a thousand may go to rest for several months,

the thousandth example will germinate at once; and, in the allied fungus of the Hollyhock disease, Puccinia malvacearum, Mont., where nearly all the teleutospores germinate as soon as ripe without any rest, it may constantly be observed that a certain number of erratic examples do certainly hibernate. Provisions of this nature are favourable to the existence of fungi, for, if plants were strictly bound by inexorable laws as to habit of growth, they might, by some untoward circumstance, be all suddenly swept out of existence. When unfavourable conditions arise, a species may be kept in existence by individuals of erratic habit.

No doubt the attacks of the onion Mucor may be palliated by the destruction by fire of infected plants and refuse. By this means all Sclerotia and perennial spawn will be destroyed.

At present Mucor subtilissimus, B., has only been recorded as growing upon the cultivated onion, but it doubtlessly grows upon other and allied plants.

CHAPTER XI.

ONION SMUT.

Urocystis cepulæ, Far.

IN 1879 a disease of onions appeared in France, known as the American Onion Smut," Urocystis cepulæ, Farlow, a disease which had hitherto been confined to America. Fungi belonging to Urocystis are parasitic on living plants, and familiar allies in this country are the bunt of wheat, Tilletia caries, Tul.; corn smut, Ustilago carbo, Tul.; the common black smuts of violets and colchicum, named Urocystis viola, B. and Br., and U. colchici, Tul.; potato smut, Tubercinia scabies, B. and Br., and many others. It is probable that we already have this disease in Britain, as onion growers have sorely complained of late of their onions falling into a dusty black mass after harvesting. Whether this fungus is really distinct from the common smut of colchicum, U. colchici, Tul., seems somewhat uncertain. The name of the parasite is derived from uro to burn, kystis a bladder, and capa the onion plant.