Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/619

Rh POTATO 597 and when the disease takes a bad form the dying leaves give out a highly offensive odour. The fungus, which is chiefly within the leaves and stems, seldom emerges through the firm upper surface of the leaf ; it commonly appears as a white bloom or mildew on the circumference of the disease -patches on the under surface. It grows within the tissues from central spots towards an ever- extending circumference, carrying putrescence in its course. As the patches extend in size by the growth of the fungus they at length become confluent, and so the leaves are destroyed and an end is put to one of the chief vital functions of the host plant. On the destruction of the leaves the fungus either descends the stem by the interior or the spores are washed by the rain to the tubers in the ground. In either case the tubers are reached by the fungus or its spores, and so become diseased. The fungus which undoubtedly causes the mischief is very small in size, and under the microscope appears slightly whitish or colourless. The highest powers are required to see all parts of the parasite. The accompanying illustration, drawn from nature, shows the habit and structure of the fungus, Peronospora infcstans, Mont. The letters A B show a vertical section through a frament of a Q Peronospora infestans, Mont. Fungus of Potato Disease. potato leaf, enlarged 100 diameters ; A is the upper surface line, and B the lower ; the lower surface of the leaf is shown at the top, the better to exhibit the nature of the fungus growths. Between A and B the loose cellular tissue of which the leaf is partly built up is seen in section, and at C the vertical pallisade cells which give firmness to the upper surface of the leaf. Amongst the minute spherical cells within the substance of the leaf numerous transparent threads are shown ; these are the mycelial threads or spawn of the tungus ; wherever they touch the leaf-cells they pierce or break down the tissue, and so set up decomposition, as indicated by the darker shading. The lower surface of the potato leaf is furnished with numerous organs of transpiration or stomata, which are narrow orifices opening into the leaf and from which moisture is transpired in the form of fine vapour. Out of these small openings the fungus threads emerge, as shown at D, D, D. When the threads reach the air they branch in a tree-like manner, and each branch carries one or more ovate reproductive bodies termed spores&quot; or &quot; conidia,&quot; bodies roughly comparable with seeds, as shown at E, E, E. Sometimes other reproductive bodies roughly comparable with the anthers and pistils of flowering plants are borne inside the leaf, stem, or tuber, as at F ; the larger body of these is female, and is termed an &quot;oogonium,&quot; and the smaller, winch at length pierces the oogonium, is male, and is termed an antheridium.&quot; When the spores or conidia are magnified 400 diameters they are seen as at F, and the contained protoplasm often breaks up into a definite number of parts, as at G. When a spore like F germinates it protrudes an amceba-like mass of protoplasm, as shown at H, which is capable of reproducing the potato fungus at once ; and when a differentiated conidium as at G germinates it expels about eight minute mobile bodies called &quot;zoospores,&quot; each zoospore being furnished with two extremely attenuated vibrating hairs termed &quot;cilia,&quot; as shown at J. These zoospores swim about in any film of moisture, and on going to rest take a spherical form, germinate, and produce threads of mycelium as at K ; the mycelium from the germinating conidia or zoospores soon finds its way into the tissues of the potato leaf by the organs of transpiration, and the process of growth already described is re peated over and over again till the entire potato leaf, or indeed the whole plant, is reduced to putridity. The oogonium and antheridium as seen at F are further enlarged to 400 diameters at L ; it will here be seen that the smaller male organ or antheridium has projected a fine beak through the walls of the oogonium or female organism ; through this boak some of the protoplasm from the antheridium passes into and mingles with the protoplasm of the oogonium ; this is the act of fertilization, and an oospore or resting spore (M, N), a body roughly comparable with a seed, is the result. After fertilization the oospores quickly drop from their supporting threads and become free like most ripe fruits. As the potato fungus causes the potato to become putrid the mature oospores or resting spores are necessarily confined to the portions of the potato plant which have been destroyed by the fungus, i.e., either to the decayed leaves or stems or to the diseased tubers ; they are brown in colour and generally more or less spinulose or warted. They will not germinate till after a rest of nine, ten, or twelve months, or in some instances even two years. They germinate by protruding threads, which speedily bear spores or conidia as at E, or more rarely zoospores as at 3. The resting spores were seen by Dr Kayer and Dr Montague in 1845, and named (in ignorance of their true nature) Artotrogus liijdnospoms. The Rev. M. J. Berkeley shortly afterwards identified them as the resting spores of the potato fungus ; but they were not seen by any one between the years 1845 and 1875, when in the latter year they were discovered in great abundance and artificially produced from the potato fungus by the writer of this article. At first believed to be rare, they are now known to be amongst the commonest of vegetable productions. The potato fungus is easily made to produce resting spores, and their germination after a } r ear s rest is an observation of no special difficulty. At one time these resting spores were confused by some botanists with a little fugitive transparent fungus, bearing oogonia not half the size of the oogonia of Peronospora infcstans, and named Pythium vexans, D. By. ; the latter plant perfects itself in twenty-four hours or at most a day or two, instead of taking a year or more as do the resting spores of the fungus of the potato disease. Pythium vexans has no connexion whatever with the fungus or the potato disease. The germinating conidia of the potato fungus, as at E, are not only able to pierce the leaves and stems of the potato plant, and so gain an entry to its interior through the epidermis, but they are also able to pierce the bark of the tuber, especially in young examples. It is therefore obvious that, if the tubers are exposed to the air where they are liable to become slightly cracked by the sun, wind, hail, and rain, and injured by small animals and insects, the spores from the leaves will drop on to the tubers, quickly germinate upon the slightly-injured places, r.nd cause the potatoes to become dis eased. Earthing up therefore prevents these injuries, but where practised to an immoderate extent it materially reduces the pro duce of tubers. The labour entailed in repeated earthing up is also considered a serious objection to its general adoption. All diseased potato material should be gathered together and either deeply buried or burnt, as the hibernating germs of the disease (oospores) rest in the decaying potato refuse, and the my celium itself sometimes hibernates. See Berkeley s essay, &quot; On the Potato Disease,&quot; in Journal nfthe Royal Horti cultural Society, vol. i., 1S46 ; Professor A. de Bary, &quot;On the Nature of the Potato Fungus,&quot; in Journal of Eoyal Agricultural Society, vol. xii., 1S7G ; Earl Cathcart, &quot;The Cultivated Potato,&quot; in Journal of Roy. Agr. Soc., 1884; J. G. Baker, &quot;The Tuber-bearing Species of Solan-urn,&quot; in Journal of the Linnean Society, vol. xx., 1884 ; and Worthington G. Smith, Diseases of Field and Garden Crops (1884). In the latter vork a full bibliography is given. (W. G. SM.) POTATO, SWEET. This plant (the Convolvulus batatas, or Ijiomcea batatas of some authors) is generally culti vated in the West Indies and most tropical countries for the sake of its tuberous root, which is an article of diet greatly in request. It is a climbing perennial with cordate, entire, or palmately-lobed leaves borne on slender twining stems. The flowers are borne on long stalks in loose clusters or cymes, and have a white or rosy funnel- shaped corolla like that of the common bindweed of English hedges. The edible portion is the root, which dilates into