IRRITABILITY. 



JACOB'S-LADDER. 



portion, for the purposes of irrigation, appear at 

 least equally important. 



Liebig informs us that, "in the vicinity of 

 Liegen (a town in Nassau), from three to five 

 perfect crops are obtained from one meadow, 

 and this is effected by covering the fields with 

 river water, which is conducted over the mea- 

 dow, in spring, by numerous small canals. 

 This is found to be of such advantage, that 

 supposing a meadow not so treated to yield 

 1000 Ibs. of hay, then from one thus watered 

 4500 pounds are produced. In respect to the 

 cultivation of meadows, the country around 

 Liegen is considered to be the best in all Ger- 

 many." 



There is no agricultural question, therefore, 

 of more consequence, in a national point of 

 view, than that of the improvement of the soil 

 by the practice of irrigation ; for, in its prose- 

 cution, all the rich, organic, and other matters 

 diffused through the rivers, which would other- 

 wise be carried into the sea, are saved to agri- 

 culture. This is not, therefore, a question like 

 that attending most other modes of fertilizing 

 the soil, merely transposing manure from one 

 field or district to another ; but it is the abso- 

 lute recovery, as it were, from the ocean, of a 

 mass of finely divided, enriching substances, 

 constantly draining from the land. It is the 

 effectual diversion of a stream which is ever 

 steadily impoverishing all cultivated soils, and 

 which unnoticed, and in too many instances 

 deemed* worthless, gliding into the ocean, is 

 almost the only drawback to the steadily in- 

 creasing fertility of our country. 



There are papers on irrigation by Mr. J. 

 Purdy, of Castle Acre, in Norfolk, Com. Board 

 of dgr. vol. vii. p. 112; by Mr. D. Shank, of 

 Wigtonshire, ibid. p. 170 ; by Mr. Beck, of Nor- 

 folk, ibid. p. 108 ; on the irrigation of Lombardy 

 and Piedmont, by Don R. S. Coutinho, ibid. p. 

 189 ; in Aberdeenshire, ibid. vol. iv. p. 263 ; in 

 Denbighshire, ibid. p. 266 ; by Mr. Eyres, of 

 Norfolk, ibid. vol. vi.p. 328; by Professor Ren- 

 nie, Quart. Journ. of Jlsr. vol. v. p. 24 ; on the 

 foul water irrigation of Edinburgh, ibid. vol. x. 

 p. 256. (Quart. Journ. of Jlgr. vol. x. p. 558 ; 

 Stephens' s Practical Irrigator Brown's Rural Af- 

 fairs, p. 263 ; Sinclair's Hortus Gram. p. 382 ; 

 Davis 1 's Wiltshire ; Driver's Hampshire.') 



IRRITABILITY OF PLANTS. See BOTA- 

 NY, TEMPERATURE, and ACCLIMITATION OF 

 PLANTS. 



ISNARDIA (named in memory of Anthony 

 Isnard, member of the Academy of Sciences). 

 These plants are mere weeds, or creeping 

 aquatic herbs, growing in marshy situations. 



The marsh isnardia (/. palustris) is the only 

 indigenous species. 



It is an annual, growing~in ponds and wa- 

 tery places, blowing axillary, solitary, sessile, 

 small, green, and inconspicuous flowers in 

 July. The herb is floating, smooth, with nu- 

 merous long filamentous roots. The stems 

 are several, about a span long, simple, or 

 slightly branched, leafy, bluntly quadrangular. 

 The leaves are opposite, stalked, ovate, acute, 

 entire, scarcely an inch in length, bright green, 

 somewhat succulent, the mid-rib often red or 

 purplish. (Etig. Flor. vol. iv. p. 264.) 



The I. Palustris is common to America as 

 676 



[ well as to Europe, and is found from Canada 

 to the West Indies. In Pennsylvania another 

 species is met with the Alternate-leaved Is- 

 nardia (/. alternifolia), commonly called Bas- 

 tard Loose-strife. The roots of both species 

 are perennial. 



Besides these, there are about a dozen addi- 

 tional species, as the genus is now constituted, 

 in the United States. (Flor. Cestrica.) 



ITALIAN RYE-GRASS. See RYE-GRASS. 



ITCH. In farriery, a cutaneous disease. 

 See MANGE. 



IVY (Hedera helix'). The name appears to 

 be derived from hedra, a Celtic word signifying 

 a cord ; and the English name ivy is derived 

 from ivo, a word in the same language signify- 

 ing green.) A hardy evergreen climber, com- 

 mon everywhere in Europe, which is excel- 

 lent as a screen planted against trellis-work. 

 The common ivy is very often employed for 

 covering naked buildings or trees, which latter 

 it invariably kills. The stem is branched, either 

 trailing on the ground and bearing 5-lobed 

 white-veined leaves, but no flowers ; or climb- 

 ing, flattened and attached by dense tufted fibres, 

 which serve for support, not nourishment; 

 the flowering branches are loosely spread- 

 ing, round, bearing ovate, undivided leaves. 

 Umbels aggregate, green, many-flowered, their 

 stalks covered with starry pubescence, and ac- 

 companied at the base by several small brae- 

 teas. The berry is the size of a currant, 

 smooth, black, internally whitish and mealy, 

 with seldom more than five seeds. The whole 

 plant is somewhat aromatic ; and a very fra- 

 grant resin exudes from the old stems when 

 bruised. 



IVY, AMERICAN, Virginia Creeper (Jim- 

 pelopsis hederacca}. Ivy-like ampelopsis. The 

 stem of this climbing plant sometimes grows 

 30 or 40 feet long, branching diffusely, climb- 

 ing fences, trees, and walls, and clinging to 

 them by adhesive expansions at the points f 

 the tendrils. .The leaves are digitate in clusters 

 of fives, on long stems. The plant is com- 

 mon in the United States in woodlands, fence- 

 rows, thickets, &c. Sir J. E. Smith (in Rees' 

 Cyclop.) contends that this is a true vitis ; but 

 Dr. Darlington thinks, that although nearly 

 allied to, it may be very properly separated 

 from our grape vines. Two or three other 

 species of ampelopsis are enumerated in the 

 United States. 



IVY, GROUND. See ALKHOOF. 



IVY, IRISH (Canatiensis), is a fast-growing 

 climber, with large lobed leaves, which soon 

 covers walls and nouses. It is propagated by 

 layers, or slips taken off and planted where 

 they are to grow. 



J. 



JACOB'S LADDER, BLUE or GHEEK VALB- 

 RIAN (Polcmonium cceruleum). This plant is a 

 common ornament of flower borders in rustic 

 gardens, of no particular qualities, notwith- 

 standing its name of valerian, derived perhaps 

 from the leaves, which resemble those of some 

 of the Valeriance. The root is fibrous, not 

 creeping, herb nearly smooth, perennial, 1J or 



