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Pyrenomycetes (Which see The Perithecia vary)

(which see). The Perithecia vary a good deal in form, and in mode of grouping together; but they are always small, and are usually flask-shaped. They may be almost free from the mycelium, or may be imbedded in it, and, in the latter case, the wall is hardly distinguishable from the mass in which it lies. When free, the wall is often rather brittle, but moderately strong, and is formed of squarish or rounded cells, usually dark brown or black. The Perithecia may be scattered about singly, or crowded on specialised bodies. When mature, each usually contains several long, transparent cells of uniform width, or windened upwards (asci), and in each ascus there are generally eight spores, less often from two to six, or an indefinite number, for reproduction. These escape through a narrow neck, in which is a circular pore, or the aperture may be a long slit; or there may not be an aperture, the Perithecia bursting ultimately. Intermixed with the asci in many are numerous slender filaments, as long as the asci. They are the paraphyses, and their presence or absence is of consequence in distinguishing the genera in the group.

PERITOMA (from peritome, a cutting round about; referring to the base of the calyx). SYN. Atalanta (of Nuttall). ORD. Capparideae. A small genus of hardy, North American, annual herbs, now included, by the authors of the "Genera Plantarum," under Cleome. Calyx four-fid, cut round at the base; stamens six. Capsules stalked. Leaves compound, generally three to five-foliolate. For culture of P. aurea, see Cleome.

P. aurea (golden). fl. yellow; petals oblong-elliptic, sub-sessile. July and August. l. three to five-foliolate; leaflets oblong-lanceolate, acute at both ends, entire. h. 1ft. 1840. The proper name of this plant is Cleome lutea. (B. R. xxvii. 67.) PERNETTYA (named after A. J. Pernetty, 1716-1801, who accompanied Bougainville in his voyage, and was author of "A Voyage to the Falkland Islands"). ORD. Ericaceae. A genus comprising about fifteen species (the number is inordinately multiplied by some authors) of hardy, or nearly hardy, rigid, very glabrous, hispid, or ciliated shrubs, some small; one is a native of the mountains of Tasmania and New Zealand, another is Brazilian, and the rest inhabit the mountains of Western America, from Tierra del Fuego to Mexico. Flowers white or rose-colour, small, nodding, solitary in the axils, or disposed in axillary or terminal racemes; calyx five-parted, not changing in the fruit, persistent; corolla urceolate or globose, shortly five-lobed, the lobes recurved; pedicels curved, bracteate and bibracteolate. Fruit a globular berry, varying considerably in colour and size, generally about the size of a pea. Leaves usually small, alternate, short-stalked, serrated, penninerved, coriaceous, hard, persistent. The species best known in cultivation are those described below. For culture, see Gaultheria.

P. ciliaris (ciliated). fl. white; peduncles glandular. June. l. ovate-lanceolate, acute, having the margins denticulated and ciliated with bristles. Branchlets setose. h. 2ft. Mexico, 1849. Half-hardy. (G. C. n. s., x. 39.) P. furens (maddening).* fl. white, in almost sessile, solitary, many-flowered, axillary racemes; lobes of corolla spreading, rather obtuse. March. l. alternate, 1 1/2in. long, ovate-lanceolate, rarely sub-obovate, with serrated margins; petioles red. Chili. A low, moderately-branched shrub. (B. M. 4920.) P. mucronata (mucronate).* Prickly Heath. fl. white; pedicels axillary, bracteate, about equal in length to the leaves. May to July. l. ovate, cuspidate, denticulate, serrulate, stiff, shining on both surfaces. h. 6ft. Magellan, 1828. (B. R. 1675.) SYN. Arbutus mucronata (B. M. 3093; L. B. C. 1848). A considerable number of handsome seedling varieties have been produced from this species, varying in the size and colour of the berries, these ranging from almost black to nearly white. Perhaps the finest forms have been raised by Mr. L. J. Davis, Hillsborough, co. Down.

P. m. angustifolia (narrow-leaved). A form with narrower leaves than the type. (B. M. 3889; B. R. 1840, 63, under name of P. angustifolia.) P. Pentlandi (Pentland's). fl. white, 1/3in. long, ovate-globose, solitary, axillary, drooping; pedicels red. June. fr. a berry, about the size of a large pea, dark blue-purple. l. crowded or sparse, 2/3in. long and under, ovate or ovate-lanceolate, acute, serrulate, very coriaceous, shining. Branches and branchlets angled, setulose or glabrous, leafy. Andes, 1875. A rigid, branched, small shrub. (B. M. 6204.) P. pilosa (pilose).* fl. white; corolla ovate, with blunt, revolute teeth. May. l. ovate-elliptic, ciliately serrulated, coriaceous, without mucro, and callous at the point. Stem pilose, procumbent. h. 6in. Mexico, 1839. SYN. Arbutus pilosa. (B. M. 3177.) PERONOSPORA. A genus of minute Fungi, belonging to the "White Moulds," one of the groups included among the mildews. There are many species in the genus, and all of them are parasitic on plants, i.e., they grow in and upon living plants. They prefer the leaves and young stems--at least they are most conspicuous upon these--but they run through all parts, from roots to seeds, though showing themselves usually only on the younger green organs. They are confined to herbs as their hosts, except in one or two cases, e.g., P. sparsa, on leaves of Roses. Some of them grow on cultivated herbs in gardens, and are frequently most injurious to these plants. Several of the more destructive species are briefly described below; but a short account of the general structure and modes of reproduction characteristic of the genus must first be given. We select as a type Peronospora nivea, which is very common upon the leaves, stems, and roots of wild Umbelliferae, and also upon the garden Carrot, Parsley, and Parsnip. A portion of a leaf attacked by this Fungus may be recognised by its yellowish-green colour above, passing into brown. The lower surface of the patch bears a white coating, which, under the microscope, is seen to consist of erect, branched stalks of very small size. These stalks emerge through the stomata of the leaf. Each is forked once or twice, and bears, towards the tip, from one to four horizontal branches, each forked once, twice, or thrice, and ending in straight, short branchlets. Each of the latter bears on its tip a small cell, called a conidium On tracing the erect stalks downwards into the leaf, they are found to arise as branches from an abundant mass of filaments (mycelium) of a Fungus. This mycelium is spread through all parts of the plant, lying in the intercellular spaces, or boring through the cell-walls, and drawing nourishment from the contents of the cells by means of numerous minute suckers (haustoria). The host-plants are soon destroyed by the Fungus, and the diseased parts either dry up, or, if fleshy, e.g., roots and tubers, they become soft, rotten, and pulpy. Reproduction is effected by means of the conidia and of oospores. The conidia do not at once reproduce the Fungus, but, when placed in water, their contents divide, into five or six small, egg-shaped masses, or zoospores, each furnished with two hairs (cilia), which they can move, and thus they can swim in water, e.g., in a dewdrop or raindrop. After a time, when on a suitable part of the plant, they push out tubes, and these pass through the stomata into the interior of the plant, and there reproduce the Fungus. This is the usual process in summer. The conidia and the zoospores, however, are unfit to resist intense cold; hence, another process of reproduction occurs in autumn, resulting in the formation of the oospores in the tissues of the host-plants. This latter process is as follows. On two neighbouring branches of mycelium-threads, inside the tissues of the host, two projections begin to form. One of these (oosphere) is globular; the other (antheridium) is oval, or clubbed, and much smaller, and a cell is cut off from the rest, at the end, by a partition. The antheridium comes to lie against the oosphere, and sends into it a kind of beak, which pierces its wall, and fertilises the contents. These contract, become covered with a cell wall, and form the oospore. The outer wall of the oospore often bears ridges, or outgrowths, characteristic of the species. The oospores remain unchanged during winter, and in spring they reproduce the Fungus.

Remedy. Owing to the fact that the Fungi live inside the host-plants, and only produce their conidia outside them, it is impossible to destroy the parasite without destroying the host. Efforts should be directed against the spread of the disease, since a cure of the diseased plants is practically hopeless. All tissues attacked by Peronosporae are liable to communicate the disease to healthy plants; hence, the former should, if possible, be burned, that being the only certain means of preventing the spread of the disease. Damp places, and confined air favour the Fungi; hence, drainage is of value, and the plants liable to injury should be separated, so far as can be done, to permit free circulation of air. Any means to strengthen the plants will probably render them better able to resist the evil influence of the Fungi. Warm, close, showery weather is peculiarly favourable to the spread of the latter, since it promotes the production of conidia, and aids the zoospores in diffusing themselves over neighbouring healthy plants by means of the drops of rain or dew.

In the case of the Potato-disease Fungus (P. infestans), numerous remedies have been proposed, e.g., cutting off stems and leaves as soon as disease appears, Jensen's method of earthing-up the drills, &c. See Phytophthora. The species of Peronospora are distinguished by differences in the erect branches that bear the conidia, and also in the markings of the outer coat of the oospores; but the latter bodies have, as yet, not been detected in some of these Fungi. The genus has been divided into four sections by Professor de Bary, of Strasbourg, who has employed, as the basis of his arrangement, the modes of development of the zoospores, or mycelium, from the conidia, in forming new plants. This arrangement, however, is not of much assistance in making out the species of such Fungi as may be injuring any cultivated plants; and for ease of reference, therefore, in the following descriptions, the nature of the branching of the erect, fertile stems, and the markings of the outer coat of the resting spores, are chiefly employed. 1. P. parasitica, abundant on, and destructive to, many cruciferous plants, including Turnips and Cabbages. On the larger kinds of Cruciferae, the Fungus is often confined to spots on the leaves. It has large, branched haustoria, or suckers, which often nearly fill the cells of the host-plants, from which they draw nourishment. The fertile stems are rather stout below, and five to eight times bifurcated; the branches are rather crowded, and end in slender, short branches, each of which bears on its tip an elliptical, colourless conidium. The conidium shows no papilla, and, in germination, emits a mycelium tube from any point of its surface. The oospore is globular, and has a thin, nearly smooth, outer coat. 2. P. Viciae grows on Peas and Vetches, often doing considerable damage. The fertile stems are rather slender, six to eight times bifurcated, with the branches rather stiff and spreading, and the terminal branchlets rather longer than in most of the other species, and very slender. The conidia are grey, with a pale violet tint, but otherwise much as in P. parasitica. The oospore is globular, pale brown, with a network of ridges on the outer coat. 3. P. effusa attacks Spinach, and other Chenopodiaceae, causing the leaves to become thickened, and to speedily decay. The fertile stems are rather short and thick, and bifurcate from two to seven times, ending in short, straight or hooked branches. They are grey, with a tinge of violet, as are also the elliptical conidia. The latter germinate as in P. parasitica. The oospore is pale brown, with irregular, brown ridges which render it, though globular, decidedly angular in outline. This species forms very dense tufts on the lower surface of the leaves. 4. P. Schleideniana grows on Onions and their allies. The fertile threads are rather stout, not, or scarcely, jointed, giving off branches along the sides alternately; branches usually three to five times bifurcated, and ending in short, hooked or spreading branchlets; conidia large, obovoid or nearly pear-shaped, all parts reddishgrey or dull violet. The conidia germinate from any part of their sides. The oospores are globular, brown, smooth, except for an open network of ridges. 5. P. arborescens lives on Poppies only, and hence is of no very great practical importance in Britain. It often causes distortions in the plants affected by it. The fertile stems are rather longer and more slender than usual, and are seven to ten times bifurcated; and the branches end in spreading, or hooked and very slender tips. The conidia are nearly round, small, colourless, and germinate from the side. The oospores are globular, brown, marked with a network of ridges. 6. P. sparsa is one of the few Peronosporae on woody plants. It grows, as a delicate grey mould, on the under side of the leaves of Roses, in conservatories. The leaves become spotted with brown, and soon shrivel up. The fertile stems of the Fungus bifurcate several times, and end in curved tips, bearing round conidia, which germinate from the side. The oospores of this species have not been observed. 7. P. nivea has been already described. The spreading branches, bifurcated from one to three times; the ovoid, rather small conidia, with a papilla at the tip, and producing, not a germinal mycelium tube, but four or five zoospores; and the smooth, unridged coat of the oospore, distinguish this parasite of the Umbelliferae. 8. P. ganglioniformis is parasitic on many Compositae, and among these is the Lettuce, to which it, at times, does much harm. The fertile stems are somewhat flattened and broad, bifurcated three to eight times, and bear at their tips a flattened enlargement, like a nerve ganglion (whence the specific name), on which, chiefly round the margin, stand from two to eight short, slender branchlets. Each of these bears a pale or colourless, nearly round conidium, provided with a minute wart at one end, from which the mycelium tube is emitted in germination. The oospore is yellowish-brown, its outer coat being slightly wrinkled. 9. P. pygmaea lives on Anemone nemorosa and on Hepatica triloba, as well as, occasionally, on cultivated Anemones. It is easily known by the fertile stems being short and stout, and thickened upwards to a kind of head, on which grow from two to six short, slender branchlets. Each of these bears an ovoid or elliptical conidium, which, in germination, gives out all the contents, through a small wart at one end. The oospore is pale brown, and has a smooth, wrinkled coat.

The three following species may possibly occur in Britain, though not yet recorded from this country.

P. viticola is parasitic in North America, on the native American Vines, and on the introduced European Vine. It has not yet crossed the Atlantic, but may easily be brought on the Vines so often imported from America to Europe, since it is abundant in the Eastern United States. It grows on all green parts, and destroys them, rendering them brown. The fertile stems are trifurcated far more often than bifurcated, and end in crowded, short branchlets, bearing elliptical conidia, which form zoospores. The oospore has a smooth, yellow, thick outer coat.

P. Cactorum causes decay and rottenness of the fleshy tissues of cultivated species of Cactus. The fertile stems bear a few branchlets, ranged along one side. The conidia are colourless, ovoid or elliptical, with a considerable wart on the tip, and through this the mycelial filament emerges in germination of the conidium. The oospore has a smooth, thick, pale brown outer coat. This disease has been the cause of a good deal of injury in botanic gardens in Germany.

P. Schachtii has frequently proved very destructive to Beets on the Continent. It forms a bluish-grey coat on the lower surface of the leaves, which become yellowish-green, wrinkled, and fleshy, and remain small. Frequently, the young shoots and the heart-leaves are destroyed. The fertile stems bear several short branches; the terminal branchlets are short, straight, blunt, spreading, and bear ovate, dull violet conidia. The oospores are unknown. The mycelium lives through the winter in the diseased plants, and resumes its growth in the young leaves during the following year. The seeds from diseased plants are often affected as shown by the appearance of the Fungus on the seedlings. The Potato-disease Fungus, frequently described under the name of Peronospora infestans, Mont. (e.g., in Cooke's "Handbook of British Fungi"), has, of late years, been placed in a separate genus, under the name Phytophthora infestans. For the characters distinctive of the genus, and the habits of and remedies for P. infestans, and of the only other species, P. Fagi, see Phytophthora. PERSEA (a name applied by Theophrastus to an Egyptian tree). Alligator or Avocado Pear. ORD. Laurineae. An extensive genus (about 100 species have been enumerated) of mostly stove trees or shrubs, all of which (except one species from the Canary Islands) are natives of tropical and sub-tropical Asia and America. Flowers disposed in panicles; perianth tube short; limb segments six, almost equal, or the three outer ones smaller; perfect stamens nine. Fruit often large and fleshy. Leaves alternate or scattered, coriaceous, penniveined, or more or less distinctly tripli-nerved. The undermentioned species is the only one introduced, and it is rarely seen in cultivation. It thrives best in a compost of loam and fibry peat. Propagated, in autumn, by layers of the ripened shoots, or, in May, by cuttings of firm shoots, inserted in sand, under a bell glass, in heat.

P. gratissima (most-grateful). Alligator or Avocado Pear. fl. green, about 1/2in. across. fr. pear-shaped, yellow and brown, often tinged with deep purple, and about 4in. long. l. elliptical, narrow towards the base, about 6in. long. h. 25ft. to 30ft. West Indies, 1739. The flavour of the fruit somewhat resembles that of butter, or marrow, hence it is called "Vegetable Marrow" in the West Indies. It is so rich and mild that most people make use of some spice, or pungent substance, to give it poignancy; and wine, sugar, lime-juice, but mostly pepper and salt, are used. Marks made with the stone of the seed on a white wall will turn as red as blood, and cannot be effected until the wall is white-washed, and even then only with difficulty. (B. M. 4580; B. R. 1258.)