exog] 



€f)£ CreaSurg at Sootanji, 



484 



determined by counting the number of 

 zones or circles in the woody stem, each 

 circle marking one year's growth, and the 

 last-formed circle being external. All the 

 native trees of Britain are exogenous. The 

 characters of the class are given under 

 the head Dicotyledons. [J. H. B.] 



EXOGENOUS. Growing by addition to 

 the outer parts of the stem. 



EXOGONIUM. A genus of Convolvulacece 

 very closely allied to Convolvulus and 

 Ipomcea, from both which it is distin- 

 guished by its stamens projecting from 

 the tube of the corolla; and from the for- 

 mer by its button-like stigma. E. Purga, 

 a Mexican climbing plant, with salver- 

 shaped purplish flowers, furnishes the 

 true Jalap tubers of commerce. These are 

 roundish, of variable size, the largest 



Exogonium Purga. 



being about as large as an orange, and of 

 a dark colour. They owe their well-known 

 purgative properties to their resinous in- 

 gredients, and hence worm-eaten tubers 

 are more valued than sound ones, as the 

 insects eat the farinaceous and woody por- 

 tions of the tuber and leave the resin. Vari- 

 ous species of Ipomcea are also said to fur- 

 nish a spurious kind of jalap. [M. T. M.] 



EXORHIZ.E. A name given to exoge- 

 nous or iicotyledonous plants, from the 

 mode in which the young root sprouts 

 when the seed is placed in the ground. 

 The term is derived from the Greek exo 

 outwardly, and rhiza a root, meaning root 

 pushing outwardly, in allusion to it push- 

 ing out directly in a tapering manner, and 

 not coming out in the form of numerous 

 rootlets through sheaths as in the Enclo- 

 rhiza, or monocotyledons. [J. H. B.] 



EXORHIZAL. That kind of germina- 

 tion in which the point of the radicle itself 

 becomes the first root. 



EXOSMOSE. That force which causes a 

 viscid fluid lying on the outside of an 



i organic membrane to attract watery fluid 

 through it. 



I EXOSTEMMA. A genus of tropical 



I trees or shrubs of the Cinchona family. 



They have whitish or pink flowers of a 



funnel-like form, the segments of the limb 



j linear and rolled back ; the Ave stamens 



I project to a considerable distance from 



the corolla, hence the name of the genus. 



The ovary is two-celled, with a long style, 



and almost undivided stigma ; capsule 



two-seeded : some of the kinds are in culti- 



! vation. The barks of the "West Indian 



! species possess febrifugal qualities, as in 



the closely allied Cinchona. [M. T. M.] 



I EXOSTOME. The aperture in the outer 

 integument of an ovule. 



EXOSTOSIS. A name given to a dis- 

 eased condition in plants, in which hard 

 masses of wood are produced, projecting 

 like warts or tumours from the main 

 stem or roots. Most cases seem to arise 

 from tissues developed round adventi- 

 tious buds which do not properly break 

 through the bark. These are sometimes 

 completely concealed, as the knaurs in 

 beech, which are often quite free. Some- 

 times there is a continued multiplication 

 of fresh buds, and in proportion as these 

 are more or less developed, we have the 

 besom-like bodies on birch, or the rough 

 tumours on elms. Cypress knees, which 

 sometimes grow to a great size, and when 

 hollowed are used for beehives in the 

 i United States, grow by a similar disease 

 ' on the roots of Taxodium. Fine specimens 

 may be seen at Sion. The tumour at the 

 junction of a graft with its stock seems to 

 arise from some different cause, which is 

 not at present ascertained. [M. J. B.] 



EXSERTED. Projecting beyond the 

 orifice of an organ. 



EXSUCCOUS. Juiceless. 

 EXTINE. The outer coat of a pollen 

 grain. 



EXTRA. On the outside of, or beyond ; 

 as Extra- axillaris, beyond the axil; Extra- 

 foliarius, beyond a leaf; Extra-medianus, 

 beyond the middle. 



EXTRORSE. Turned outwards from 

 the axis of growth of the series of organs 

 to which it belongs. 



EYE. A term in gardening for a leaf- 

 bud : also for the centre or the central 

 markings of a flower. 



EYEBRIGHT. Euphrasia. 



EYSENHARDTIA. A genus of Legumi- 

 nosce nearly related to Amorpha and Da- 

 Ua, but differing from the former— which 

 has only one petal, and that the standard — 

 in iN corolla of five petals, and from the 

 latter, in its little sabre-shaped pod being 

 much longer than the calyx. 



E. amorphoides is a much-branched shrub 

 or small tree, five to twenty feet high, 

 found in Texas and Mexico ; its slender 

 ash-coloured branches are furnished with 



