The resting-spores of this fungus have also been detected in the wallflower, Cheiranthus Cheiri, L.; the Shepherd'spurse, Capsella Bursa-pastoris, D.C.; Camelina sativa, Cranz., and in other plants.

Peronospora parasitica, Pers., not only grows on the wild and all the cultivated varieties of the cabbage and turnip, Brassica oleracea, L., and B. campestris, L.; but it often grows on Whitlow Grass, Draba verna, L., and on the Shepherd's-purse, Capsella Bursa-pastoris, D.C., in company with one of the white-rust fungi named Cystopus candidus, Lev., described farther on. It also grows on Garlic Mustard, Alliaria officinalis, D.C.; Pennycress, Thlaspi arvense, L.; Tower mustard, Arabis perfoliata, L.; Coral root, Dentaria bulbifera, L.; D. heptaphyllos, Clus. ; Neslia paniculata, Des.; Hairy Bittercress, Cardamine hirsuta, L.; Narrow-leaved Bittercress, C. impatiens, L., and other plants.

The many common weeds just mentioned act as nurse plants for the putrefactive mildew of our turnips and cabbages. The fungus lives through the winter in a hibernating state not only in rotten turnip and mangel roots, but in the decaying remains of such extremely common weeds as the Shepherd's-purse and other worthless plants. It is obvious, then, that it is not only desirable to burn all fungus-infected turnip and cabbage material, but as far as possible to keep the fields and hedgerows clear from the cruciferous weeds just mentioned. may be answered that it is impossible to keep down the weeds and burn the decaying cruciferous rubbish. This may be partially true, but the moral to be drawn from the life history of this fungus is, Do not let putrid refuse and worthless and dangerous weeds interfere more than is necessary with the healthy growth of food-plants.

It

CHAPTER XVI.

WHITE-RUST DISEASE OF CABBAGES, ETC.

Cystopus candidus, Lev.

THERE is no more familiar parasite of cruciferous plants than the fungus of White Rust, Cystopus candidus, Lev. The generic name is derived from kystis, a bladder, and pous, a foot; candidus, of course, refers to the white colour of the fungus; the name is intended to indicate the white pustular appearance of the fungus on the attacked plants. White rust is extremely common on cabbages, excessively so on the common Shepherd's-purse, Capsella Bursa-pastoris, D.C., and many other cruciferous weeds and garden flowers. The appearance of the fungus is known to every one who has walked in a kitchen garden. Cabbages and cauliflowers are seen with their leaves and stems swollen, distorted, and spotted with white streaks and blotches, as if sprinkled over with whitewash. If typical examples of the parasite are carefully examined on invaded leaves, it will be noticed that the white splashes are really somewhat elongated swollen pustules, often arranged in a concentric or spiral manner, and measuring half an inch or more across. On the leaf stalks and flower stems the pustules are disposed in a more irregular manner. The parasite invades every part of the host plant above ground, sometimes sweeping off every seedling in the earliest stages of growth, at other times attacking the flowers, and so stopping the production of seeds. The fungus, in whatever form it appears, reduces and damages the produce of the plants attacked. Experienced observers can detect the presence of white rust long before the

white pustules are visible, by the swollen and distorted appearance of the leaves and stems, caused by the presence of the spawn of the parasite within the plant. As in Peronospora, the mycelium of Cystopus traverses the host plant by the intercellular passages. The spawn threads resemble the mycelium of Peronospora parasitica, Pers., in being provided with suckers which become affixed to the constituent cells within the leaves and stems of the host. When the white pustules are examined with a microscope they are found to be not dissimilar in character although different in colour from the pustules belonging to Puccinia mixta, Fl., already described, or of the rust fungus of corn, Uredo linearis, Pers., described further on. Instead, however, of simple red Uredo spores or compound blackish Puccinia spores being found within the pustules, chains of almost colourless round or ovate spores or conidia are seen in the white-rust fungus. Chains of conidia or spores belonging to Cystopus candidus, Lev., are illustrated in different stages of growth, enlarged 400 diameters, at A, Fig. 32. The fungus grows beneath the epidermis of the plant after the manner of Puccinia mixta, Fl., already described; the pustules produced by the white-rust fungus are, however, very much larger than the blisters of the Puccinia. The spores or conidia grow in chains, a fact first pointed out by the Rev. M. J. Berkeley, in the Journal of the Royal Horticultural Society, vol. iii., p. 269, 1848. The spores or conidia are formed in Cystopus in the following manner :-At first simple clublike growths are produced as at B; a constriction forms towards the apex of the club, which speedily takes the form of a joint or septum as at C; in the process of growth another constriction occurs as at D, which in turn speedily becomes a septum or joint. As this process is repeated each club at length supports a short chain of conidia, each conidium being attached to the conidia next in order by joints as at DD. When large numbers of conidia have been produced in this

manner in the disease pustules, the epidermis of the host plant bursts in an irregular manner, and the conidia are set free. Each conidium is filled with finely granulated protoplasm or vital material.

When the conidia approach maturity in damp air or water, the interior substance of each may be seen divided into a definite number of portions, generally from five to eight; each portion presenting a pentagonal or hexagonal form bounded by a white line, precisely in the manner of the conidia of the fungus of the potato disease described

farther on in this work. Each of the contained portions within the conidium is now really a secondary spore, and the body which was at first a simple conidium has now become a sporangium or spore case-technically, in this instance, called a zoosporangium, or case containing zoospores or spores endowed with an animal-like motion.

If we take a perfectly ripe conidium, sporangium, or

zoosporangium, and enlarge it 1000 diameters, we shall see it as at E, Fig. 32; the former point of attachment is seen at F, and the sporangium is shown in the act of discharging its differentiated contents, in the form of zoospores, from its apex. These secondary spores were at first the polyhedric contents of the sporangium; but as they emerge in water or on any damp surface the angles become rounded, and they are at last expelled as minute ovoid bodies as illustrated at G. At first these small secondary spores or zoospores remain immovable at the mouth of the burst sporangium; soon, however, they begin to slightly oscillate, and two excessively attenuated hairlike cilia are developed from beneath as at H. At a special moment the foremost cilium is distended in a straight line as shown, whilst the hindermost cilium at the same time suddenly quivers, and the zoospore sails away over any moist surface, as if endowed with animal life. Each zoospore exhibits within one or more lustrous, perhaps contractile, vacuoles.

The phenomena just described can only be seen when a zoosporangium of the white-rust fungus, has been placed in water upon a glass slide, and viewed under a coverglass with a high power of the microscope. In dry air no differentiation of the contents of the conidium takes place. It is certain that rain, dew, or moisture of some sort is essential for the bursting of the sporangia and the expulsion of the zoospores. The bursting, as seen in water, under the microscope takes place in an hour or two after immersion; the conidia retain the power of producing zoospores for about a month. The zoospores are

able to swim about for several hours; their cilia then vanish, the zoospore retakes a globular tailless form, bursts as at J, produces a germ tube, and this germ tube is then a spawn thread of white rust capable of producing a new series of clubs capped with zoosporebearing sporangia.

It is obvious from the above description that the white