confirm, extend, modify, and check the observations of other men. Confirmations are not always to be trusted, for it often happens that a beginner is over anxious to confirm the statements of a master, as by that means the pupil hopes to secure some of the credit belonging to the original teacher. It is not only necessary to know what is confirmed, but who confirms it. Some older views of our own, in which we have now no belief, have been repeatedly “confirmed.”

All phenomena which on the face of them are unusual should be carefully examined and re-examined, and constantly tested and retested. Reasonable statements may be more readily accepted than unreasonable, but it often happens that the more wonderful and unreasonable a phenomenon is-according to the descriptions-the more avidiously it is accepted, especially by beginners.

Opinions often vary as to the meaning of the phenomena connected with disease, as in the appearances presented by the fungi of corn mildew, of the potato murrain, and some other diseases. In these instances we shall not disrespectfully advance our own views to the disadvantage of other observers, but shall clearly and impartially state both sides of any disputed question. We shall, however, consider it our duty to say how our mind has been impressed by the evidence. Although certain facts are themselves often undisputed, yet the deductions made from them are hotly contested.

In concluding these brief introductory remarks, we strongly advise such of our readers as have the opportunity, to carefully examine the phenomena hereafter mentioned for themselves, and to accept nothing on mere faith. Any new observers who will sift and resift any statements of fact or deduction which appear to be unreasonable will be doing a real service to science.

CHAPTER II.

CLOVER SICKNESS-CLOVER MILDEW.

Peronospora trifoliorum, D.By.

THE well-known weakly growth of clover, termed clover
sickness, is said-perhaps on insufficient grounds-to be
due to a deficiency of potash in the soil, especially the
soluble salts of potassium in the subsoil. When clover is
grown too frequently in the same fields, and without alter-
nation of crops, the ground becomes "clover sick." Two
nematoid or thread worms (nema, a thread) of minute
size, and allied to the so-called " worms " of stale vinegar
and paste, and to the Nematode which causes ear-cockle
in wheat, oats, and rye, have been described as attacking
clover. These thread worms have been described under
the names of Tylenchus devastatrix and T. Havensteinii;
but the impoverished condition of clover when due to
these parasites is said to be distinguishable from clover-
sickness proper.
Some observers have said that the ail-
ment is due to the presence of a fungus known as Sphæria
herbarum, Pers., sometimes described as Pleospora, and
more frequently as Cladosporium. This fungus is over-
burdened with synonyms and varietal names, and is so
common on all herbaceous plants that it is hardly likely
to be the sole cause of clover sickness. A second fungus,
named Peziza ciborioides, Fr., it is also credited with being
the cause of this ailment, but probably on insufficient
evidence, although, from what we have recently learned
of the attacks of the spawn or mycelium of a Peziza upon
living potato plants, the presence of P. ciborioides, Fr., on
clover deserves attention. A third fungus, named Pha-

cidium medicaginis, Desm., has also with insufficient reason been referred to as the cause of the disease. It is almost impossible to say what may be the chief cause of clover sickness, and there may be several forms of the disease. The spawn of fungi is sometimes confined to the interior of plants, where it causes serious disturbance, and this spawn or mycelium, if without fruit, even when seen under the microscope, is commonly so indefinite in character that no one can say for certain what fungus it is destined to produce.

A frequent fungus on dying clover leaves is Ascobolus trifolii, Riv., which is the same with Phacidium trifolii, Boud.; and another is Polythrincium trifolii, Kze., which is said by some authors to be a second condition of Dothidea trifolii, Fr. A rust fungus named Uromyces appendiculata, Lev., is also at times very prevalent on the peaflower tribe. The fungus parasites of the Leguminosa, to the pea-flower tribe of which our clovers belong, are but few in number, although their individual power for destruction is great. The best known are the mildews of our garden and field peas named Erysiphe Martii, Link., and E. communis, Schl., the latter of which also occurs upon the Ranunculacea and the vine. A close ally named E. graminis, D.C., is parasitic on grasses.

A very frequent parasite of clovers in Britain, and one to which we are inclined to refer a great deal of clover sickness, is Peronospora trifoliorum, D.By., a pest which appears to have attracted little or no attention in this country till late years.

It, like all other species of Peronospora, attacks living plants; it is common on purple clover, Trifolium medium, L.; T. alpestre, L.; crimson clover, T. incarnatum, L.; on Lucern, Medicago sativa, L., and other plants. There is some diversity of opinion as to the meaning of the name Peronospora, as Corda, the botanist who first used the name, gave no explanation of its derivation. Corda probably had in view the word peronao (Teрováw-Homer and

Theocritus) to pierce, pin, or transfix, or perone, the pin or tongue of a buckle, a pointed or piercing object, and spora (σорá), a seed or spore. He probably used the

word in reference to the power of the fungus to pierce the tissues of the plant it attacks as distinguished from other fungi which have no such power. The specific name trifoliorum explains itself. Peronospora trifoliorum, D.By., is closely allied to the potato fungus, and it grows within and upon the under surface of the leaves of the plants invaded. By this habit of growth, and its putrefactive power, it not only chokes up the organs of transpiration of the host plant, but causes decomposition of the tissues by contact. It is remarkable for the profuse production of its minute, oval, transparent spores or conidia. A spore in fungi is a reproductive body, answering to the seed of flowering plants, but with no embryo or rudimentary plant within. Certain spores in Peronospora and in many other fungi are often called conidia from konis, dust, to distinguish them as secondary spores, or spores of an inferior class, the fungus itself being capable, under favourable circumstances, of producing other spores of a much higher order and more complex structure. The conidia in Peronospora, as the name indicates, are like fine, generally transparent dust. These conidia are filled with colourless protoplasm, or vital material, and they do not readily germinate except in water. When a conidium of Peronospora trifoliorum, D.By., falls upon any damp surface it bursts at the side, and the protoplasm exudes somewhat in the form of an amœba, one of the simplest animal organisms. From this irregular amoeba-like form, other fertile stems of Peronospora trifoliorum, D.By., speedily arise.

The disease spots on the leaves, as caused by the Peronospora, are at first white, and speedily become pallid or brownish. At length the corroded fragments drop from the leaf to the ground. This species of Peronospora produces oospores, egg-like spores or resting-spores; these fall to the ground in the autumn, and rest in a hibernat

ing state till the following summer, when they germinate, and produce threads carrying secondary spores or conidia; these conidia drop off from the parent plant, sail through the air, and are carried in different directions by currents of wind. Such spores as light upon clover plants cause the production of the mildew; such as fall on unsuitable places perish. A description of how resting-spores are produced, and their nature, is given farther on in this work.

The illustration of this fungus (Fig. 1, A) shows the parasite enlarged 400 diameters, growing from the undersurface of the foliage of Medicago sativa L., whilst one of the very pale gray spores or conidia is enlarged to 1000 diameters at B. In the illustration the fungus is really inverted so that its characters may be more easily understood. All species of Peronospora usually grow from the under surface of leaves, where they may be seen by the unaided eye as small white cottony masses. They commonly burst through the organs of transpiration—stomata ; sometimes, however, the fungi push the leaf cells aside and so get access to the air from the interior of the leaf. Two stomata are seen in section at C, D. The fertile threads which carry the spores are termed by botanists conidiophores or conidia-bearers. The spawn of the fungus causes putrefaction of the tissues by mere contact. spores, on falling on the foliage and bursting, also cause putrefaction of the leaf. The destruction of invaded clover is further aided by the conidiophores or stems of the fungi obliterating the organs of transpiration.

The

Imperfect drainage and thick planting favours the growth of all the Peronosporea, whereas a free circulation of dry air is often fatal to them. When once they make their attack, it must be remembered that they establish themselves within the tissues of the invaded plants, in a position where it is impossible to reach them with any curative material. Attention, therefore, to the mode of cultivation may tend to stop the spread, if not to prevent