themselves with the matted threads of the parasite, and went home as if clothed in bear skins. Clover dodder is generally said to be an annual; but observers are not wanting who have expressed a belief that it is often perennial, and lives on from year to year irrespective of the introduction of new seed, which, indeed, it very seldom produces in Britain. The dodders are said to be acrid and purgative, and mischievous to flocks and herds. It is singular that the parasite should be capable of elaborating acrid principles from the juices of a sweet non-acrid host.

Dodders are still largely imported to Britain in unclean foreign seed. Prof. Lindley has stated that both clover dodder and flax dodder were first imported to this country from Afghanistan so lately as 1843. Dodder is so common now that Prof. Buckman, to whom we are indebted for a most interesting and instructive essay on dodders, records an instance of seventy bushels of flax dodder seeds being sifted out of a single field of flax seed, whilst a year or two afterwards almost as much was separated from a crop of flax grown at the Royal Agricultural College.

On rare occasions clover dodder produces seeds in Britain; and as there is evidence that the threadlike stems are sometimes perennial, dodder refuse should never be left on the ground to rot. Every patch of dodder should be carefully raked together and burnt, and by this process and careful sifting its appearance in the fields can generally be prevented. Some agriculturists, on first seeing the yellow patches in the clover fields, remove all the clover from the outer edges of the invaded patch for a width of about eighteen inches; this leaves nothing for the dodder to prey upon, as the threadlike stems cannot stretch across the eighteen inches of vacant ground. The clover is removed because it is extremely difficult to entirely remove dodder.

CHAPTER XX.

GRASS MILDEW.

Erysiphe graminis, D.C..

THERE are few fungi more common or injurious in warm dry weather than the fungus of grass mildew, grass blight, or wheat rust,- —or white rust, as it is termed in America, named Erysiphe graminis, D.C. It is possible that the familiar straw blight of agriculturists, already described, may be caused by the mycelium of the Erysiphe, and this is an additional reason for directing special attention to grass mildew or blight. The generic name Erysiphe was the term given to mildew by the Greeks; the specific name graminis needs no explanation. The fine, creeping, jointed mycelium of Erysiphe graminis, D.C., forms a white superficial mildew on the living stems and leaves of cereals and other grasses in the summer and autumn. When the white mildew patches are examined in the autumn with a very strong lens, they will be seen sprinkled with minute black dots, as illustrated twice the natural size on the wheat stem in Fig. 55. The spawn of this mildew is generally supposed to be incapable of penetrating the tissues of plants; but suckers have been described as belonging to the mycelia of some allied species of mildew. With these minute suckers the Erysiphe adheres to its host, if the suckers do not indeed pierce the leaf cells and derive nourishment therefrom, after the manner of Peronospora. In the summer the mycelium gives rise to vast numbers of vertical moniliform, or necklace-like groups or chains of conidia or spores, as illustrated at Fig. 56, enlarged 400 diameters. This peculiar

fruit of the mycelium was at one time considered a perfect fungus, and was described under the name of Oidium monilioides, Lk. The meaning of Oidium has been already explained, and monilioides means necklace-like, and refers to the growth of the fungus, which resembles a string of beads.

A

X.1000

X.2

FIG. 55.-GRASS MILDEW.

Wheat Stem invaded by Erysiphe graminis, D.C. Twice the natural size.

X.400

FIG. 56.

Oidium monilioides, Lk. The early condition of Erysiphe graminis, D.C., enlarged 400 diameters. Germinating conidium enlarged 1000 diameters.

The Oidium is extremely common on the Gramineæ in the summer, and it may always be found on grass, and especially rankly growing grass in damp positions. The necklacelike growth of the beadlike conidia is so delicate that the slightest touch or breath destroys their chainlike arrange

ment. If, however, infected grass is kept in damp air, a fragment of a leaf may generally be successfully cut, examined without water with a low power of the microscope, and the Oidium seen in a growing condition. When thus examined the profuse chainlike growth may be easily observed. If placed under a cover glass for examination under the higher powers of the microscope, the beadlike spores or conidia instantly break away from each other, so inconceivably slight is their attachment. The moniliform habit can, therefore, only be seen with a low power applied to the dry living fungus whilst in situ. In water or damp air the conidia, and especially the topmost conidium, quickly germinate and produce thin threads, as illustrated at Fig. 56, A, enlarged 1000 diameters. This constant spore production and germination is incessantly continued through the summer months, till at last a thick grayish-white coat of mildew more or less covers all infected plants. The conidia are so small that it would take about a million to cover a square inch. The Oidium state of grass blight may be compared with the Oidium of the turnip, illustrated to the same scale at Figs. 27 and 28. The grass Oidium is somewhat taller, but the spores of the turnip Oidium are more than twice the length of those of our present plant. Oidium monilioides, Lk., is a typical plant. The Oidium growth of the fungus, however, and the production of the profuse mycelium is only a preparatory stage of growth for the perfect Erysiphe which generally follows; it has been observed that when the Oidium does not appear till late in the summer, the Erysiphe or perfect condition is never produced, and the whole growth of the fungus is confined to the Oidium stage. Under favourable conditions of growth, the Oidium threads of the summer produce in the autumn little brown globose bodies termed conceptacles. This condition of the mildew is shown on the wheat stem in Fig. 55. When examined even with a powerful lens, these little blackish dots, termed conceptacles, appear less in size than fine grains of dust;

but when magnified with a moderately high power of the microscope, they appear as spherical bodies, furnished with a large number of slender, curved, radiating, tentacle-like arms or branches, the whole growth being partly buried in the spawn, as illustrated in Fig. 57, enlarged 100 diameters. The limit of the page will not admit of a higher magnification than 100 diameters; it must there

fore be borne in mind that this Erysiphe is a comparatively large fungus when placed side by side with others which are magnified 200 and 400 diameters. The difference in size between this Erysiphe and Peronospora exigua, W.Sm., illustrated, enlarged 400 diameters, in Fig. 2, is very striking.

K