Page:The American Cyclopædia (1879) Volume XIII.djvu/796

 772 POTATO ROT POTEMKIN or tuber, where it grows at the expense of the contents of the cells ; it may exist to some ex- tent within the tissues of the potato without seriously injuring the -plant, and be quite un- suspected, as it can only be detected by care- ful microscopic observation. When from any c.iuse the mycelium of the fungus increases so rapidly that its demand upon the potato plant for nourishment is greater than it can supply, then the plant assumes a sickly aspect, black spots appear on the leaves, and death soon fol- lows. When the weather is cool and dry the fungus vegetates very slowly within the tissues of the potato, but warm weather accompanied by dampness favors its rapid and destructive development. The mycelium can only spread through the tissues of the particular plant it in- habits ; but it produces something correspond- ing to seeds, by which it may be propagated from plant to plant. When fructification, as it may be called, takes place, the fungus seeks the open air ; some of the threads make their way through the stomata or breathing pores of the leaf, and appear like a slight frost on its under surface ; the threads often branch, and their tips and those of the branches swell up to form the egg-shaped bodies called spores, which when perfected fall away and are ready to reproduce the fungus. They germinate either by pushing out a germi- nal tube, or sprouting at once, or their con- tents undergo a change and form several small bodies (zoospores), which to the number of 6 to 15 escape, and have the power of mov- ing over a moist surface by means of two cilia or vibrating hairs ; when these zoospores come to rest they produce at one end a germinal tube, the same as formed by the spore ; these spores or zoospores may fall upon other plants, or may be carried there by the wind or other means, and when they germinate are able to push their germinal tube directly into their tis- sues and there grow to form a mycelium. These spores are asexual, being produced by the sub- division of an individual without any coope- ration of two distinct plants, or two distinct organs of the same plant. They retain their germinating power for several weeks, but their vitality is destroyed by a winter's exposure. This is a brief outline of the life of this fungus so far as known, but its history is not complete ; other similar fungi at some time in their career produce sexual spores or oogonia; and this may and does take place upon quite another plant. Another peronospora, which is very destructive to lettuce, produces on that plant only asexual spores as described for the po- tato fungus; but these asexual spores when they germinate on the groundsel produce a mycelium which has organs corresponding in f auction to stamen and pistil, and these by their union form an oogonium or sexual spore, which differs from the others in being much tougher, is not killed by freezing, and may even pass through the alimentary canal of animals and retain its vitality, and through the medium of manure be brought around to the lettuce again, when it can go on with its destructive work. This alternation of generations is very common among fungi, and botanists are now seeking for the sexual spore, or resting spore, of the potato fungus. Statements that it has already been discovered are without foundation in fact. There is some evidence pointing to clover as the plant upon which the sexual spore is pro- duced. Prof. W. G. Fax-low of the Bussey institution, Harvard university, has proposed a series of questions to cultivators to call out such evidence as may be afforded by the effect of certain rotations of crops upon the rot in potatoes. There is no cure for the potato rot ; as it does not appear until August, early varie- ties which ripen before that time escape it; while some more vigorous kinds seem to be better able to resist the disease than others, none are quite free from it. One of the most obvious methods of avoiding the disease is to plant only tubers that are apparently sound, although, as stated, the fungus may be within the tissues in so small quantities as to produce no rot or other manifestation of its presence. Should the suspicion that the oospore is formed in clover be confirmed, then the avoidance of animal manures will be one of the principal means of preventing the spread of the fungus. POTATO WORM. See HAWK MOTH. POTEMKffl, Grigori Alexandrovitch, prince, a Russian soldier, born in the government of Smolensk in 1736 or 1739, died in October, 1791. He was an ensign in the army when in 1762, soon after the accession of Catharine II., he attracted her attention by his fine ap- pearance. He gradually superseded Orloff as her lover, and by his influence over her be- came practically the arbitrary ruler of the em- pire. It was mainly under his influence that Turkey was crippled, and that Kherson was founded in 1778 as the chief emporium of southern Russia. He conquered the Crimea and the Kuban territory, restored the ancient names of Taurida and Caucasus, and became governor of the new possessions with the sur- name of Tavridtcheskoi (Tauridan), and the empress presented him with the Taurida pal- ace at St. Petersburg, built especially for him. He dazzled Catharine with the vision of a new Byzantine empire, and when she visited him in Taurida (1787) he resorted to many strata- gems to impress her with the splendor of her new dominions. (See CATHARINE II.) In the new war with Turkey he was commander-in- chief, and conquered Otchakov, while Suva- roff won victory after victory. Potemkin now thought the time had come for taking Con- stantinople, but the Russian exchequer was exhausted, and while he went to St. Peters- burg to urge the continuation of war, Catha- rine concluded an armistice with the Turks. The empress had loaded him with honors and riches, and he left an immense fortune. See Vie de Potemkin, by Mme. de Cerenville (Paris, 1807-' 8) ; Zhizn Potemkina, by Lev-