traversed in every part by the spawn of the fungus. During warm, humid conditions of the weather the black decomposed spots are sometimes present for several days on the leaves before the fungus is seen. These blotches indicate that the putrefactive spawn of the fungus is within the leaves, awaiting favourable conditions for its complete development as a white bloom outside. The phenomena just mentioned are accompanied by a peculiar and very offensive odour well known to every person who has walked through a field of potatoes suffering from disease. The odour is caused by the putrescence set up in the tissues of the host plant by the contact of the mycelium of the potato fungus. Although the attack of disease in potato plants is apparently sudden, and made on apparently sound plants, yet all known facts point to the probability of the existence of the fungus in a nascent state during at least several weeks prior to its general recognition. The belief in the extreme suddenness of fungoid growths is, in many instances, a mere popular delusion. The common field mushroom is supposed by rustics to grow in a single night; but it is well known to careful observers that the infant mushroom exists just beneath the surface of the soil in a growing state for several weeks before it suddenly bursts through the earth and expands its cap or pileus. We have ourselves seen fields of potatoes which were apparently undiseased one day, prostrate on the ground the next, and the haulms blown away by the wind on the third day. This apparent suddenness of the attack in the early autumn appears to be well known in America; for Professor W. G. Farlow of Harvard University writes in reference to the "Potato Rot." (Bulletin of the Bussey Institution, part iv. p. 319): "At times its advent is so sudden that, within a few hours, the potato fields change from green to brown and black, and the plants which, in the morning, gave promise of an abundant crop, before night present a mass of decaying vegetation, in which are involved not only the 66 leaves and stems, but also the tubers." The following interesting and instructive sentence occurs in an excellent essay written by Dr. W. Peard, LL.B., on Certain Enemies of our Roots," and published in the Journal of the Bath and West of England Society and Southern Counties Association, vol. iv., third series, p. 14 :- "At that time (Aug. 1845) we were spending some weeks at Ballyshannon, and close to our cottage was a magnificent field of potatoes, about twenty acres in extent, through which we passed regularly every morning and evening. One day, during the last week in August, as we brushed through the dark-green foliage, earthy disagreeable odours, before unknown to us, rose from the plants. On the following morning the entire crop looked as if it had been exposed during the night to the action of steam. Stems and leaves were soft, pulpy, and blackened; in six-and-thirty hours a few sickly stems and discoloured leaves were all that remained. The crop had ceased to exist." Mr. J. G. Baker, F.R.S., of Kew, is of opinion that the potato plant, Solanum tuberosum, L., in its present "tuberbearing state is in a disorganised, unhealthy condition, a fitting subject for the attacks of fungi and aphides ;" and he quotes Mr. T. A. Knight to the effect that the formation of the tubers more or less deprives the potato plant of its requisite amount of nutriment. He considers that the potato is grown in a necessarily unnatural way in masses in our fields, instead of in isolated examples as in Nature; and that the fact of the almost total absence of flowers and fruit in many cultivated varieties shows that the plant is in a disorganised state. Mr. Baker, from an examination of a large number of examples, has come to the conclusion that all the garden varieties have originated from S. tuberosum, L. Out of 700 or 900 species of Solanum it appears that only six produce tubers or potatoes at all; the rest "maintain their hold on the world as most plants do, by their flowers, fruits, and seeds." Other observers hold an opinion at variance with the one advanced by Mr. Baker, and say it is impossible to over-cultivate any plant; that seedling potatoes are as badly affected as those grown from cut tubers; and that animals and plants placed in an artificial position by man only need an extra amount of care corresponding with their new position. Race horses are said to live as long as cart horses, domestic as long as wild animals, and the delicate children of towns as long as the more roughly nurtured children of country villages. It is acknowledged that extra care is required, but it is maintained that the constitution is not impaired. In the potato disease, as in every other disease, both of animals and plants, it is necessary that the ailment should be completely understood before any serious attempt can be made towards the prevention or palliation of the attacks. We will, therefore, closely examine the structure and mode of growth of the potato fungus. For an exact examination of Peronospora infestans, Mont., a very minute and extremely thin and transparent slice must be cut from a diseased leaf at a spot where the white bloom caused by the presence of the fungus is visible underneath. A good plan is to cut a diseased leaf in two through a disease spot, and then with a sharp lancet cut an extremely thin slice off from one of the exposed cut surfaces. If the slice last cut is somewhat longitudinally wedge-shaped, it will often best show the structure of the leaf and the contained fungus at the thinner end of the section. Such slicing requires great care and experience, and the art is only acquired after many failures. To those, therefore, who are unequal to the task we advise the purchase of slides ready prepared by the Rev. J. E. Vize. The atom to be examined should be placed on a glass slide in a drop of glycerine (this is preferable to water, as the latter often dries too quickly), and then covered with a clean thin cover-glass. The magnification given by an ordinary lens is useless for the observation of the minute fungus now before us, so we must at once place it under the higher powers of the microscope. If the slicing through a disease spot is successful, we shall probably see the atom when magnified 100 diameters, as at Fig. 127. The thickness of the lamina of the leaf is shown at A, B ; the under side of the leaf is represented at A, from which surface the fungus almost invariably springs. The fungus, therefore, really grows downwards. The true upper surface is shown at B. This reversal of the leaf in the illustration is merely, as in other instances in this book, to show more clearly the treelike branching growth of the fungus. If we confine our attention for the present to the section of the leaf, we shall note that it is made up of minute bladder-like cells, loosely packed together; and that the cells at top and bottom, representing the lower and upper cuticle of the leaf, are devoid of the shading, which is meant to indicate the green colouring matter or chlorophyll within. An opening into the interior of the leaf will be seen at C; this is one of the stomata or organs of transpiration, sometimes referred to as "breathing pores." The stomata are like the gates to a camp or to an entrenched position; they are the weak points through which an enemy may enter, and when once these gates are passed, the whole interior of the plant is at the mercy of the invader. D may be seen a hair built up of four transparent cells, the two lower being traversed by a mycelial thread of the potato fungus. On the upper part of this hair, attached to the outside at E, may be seen one of the small branches of the fungus; this branch has burst and thrown out a mycelial thread from its side. Every fragment of the potato fungus is capable of growth, and of ultimately reproducing the parent fungus. The cells immediately under the true upper cuticle of the leaf at F are termed pallisade cells; and their disposition in the manner illustrated serves to give the necessary firmness to the exposed upper surface of the leaf. At If we now look within the fragment of the leaf we |