TRANSPIRATION 



3365 



TRAGIA (from Hieronymus Bock [or Tragus], a 

 German botanist, 1498-1554). Euphorbiaceje. Herbs, 

 sometimes twining and often with stinging hairs, rarely 

 cult.: Ivs. alternate, toothed or lobed: fls. monoecious, 

 racemose, apetalous, inconspicuous; stamens simple, 

 generally 3; styles united above the base, with the 

 apex free: ovules 1 in each of the 3 cells. About 50 

 species of tropical and temperate regions; related to 

 Plukenetia and Dalechampia. T. cannabina, Linn., of 

 Trop. Asia, is used for fiber to some extent. T. volubv- 

 li*. Linn. TWLXIXG COWTTCH, of the W. Indies, has 

 stinging hairs. Probably not cult. Several species of 

 Tragia are native in the'U. S. j. B. S. NORTON. 



TRAGOPOGON (Greek for goat's beard). Comp6sitae. 

 GOAT'S-BEARD. Erect biennial or perennial herbs with 

 narrow grass-like leaves and heads of yellow or purple 

 flowers, belonging to the ligulate section of the com- 

 posite family (tribe Cichorioceas). 



Mostly weedy plants with a tap-root: florets perfect, 

 with slender style-branches and sagittate anthers; 

 pappus composed of bristles in a single series; involucre 

 cylindric or nearly so, 

 with approximately 

 equal-length bracts in 

 a single row. Between 

 30 and 40 species, 

 native to S. Eu., X. 

 Afr.. and Cent, and S. 

 Asia. One of them is 

 cult, for its edible tap- 

 root (salsify) and 

 another is now a fre- 

 quent weed in this 

 country. The fls. of 

 these open only in the 

 morning. 



A. Fls. purple. 

 porriffilius, Linn. 

 SALSIFY. VEGETABLE 

 OYSTER. OYSTER 

 PLANT. Figs. 3532, 

 3832. Tall strict bien- 

 nial, sometimes 4 ft. 

 high when in bloom, 

 glabrous: Ivs. keeled, 

 tapering from a broad 

 often clasping base: 

 fls. showy, closing at 

 noon or before, the 

 outer rays exceeded by the involucral scales: peduncle 

 thickened and hollow beneath the heads. S. Eu. 

 Naturalized in many parts of the country, often becom- 

 ing a persistent weed. See Salsify. 



AA. Fls. yellow. 



pratensis, Linn. GOAT'S-BEARD. More or less 

 branched, 3 ft. or less tall: Ivs. as in the preceding: outer 

 rays exceeding the involucral scales; rays yellow, showy, 

 about 2^2 in- wide: peduncle scarcely swollen. A weecl, 



L. H. B. 



TRAGOPfRUM LANCEOLATUM var. LATTF6LIUM: Atra- 

 pharif Muschkttoirii. 



TRAILING ARBUTUS: Epigxa. 

 TRAINING: Pruning. 



TRANSPIRATION is the term applied to the escape 

 of water from leaves and other parts of the plant in the 

 form of vapor. 



There is no closed "circulation" of sap in plants com- 

 parable to the streaming of blood in animals. Water 

 which generally contains various mineral soil constit- 

 uents in solution enters the roots, and most of it passes 



3832. Flowers of salsify or oyster 

 plant. Tragopogon porrifolius. 

 (XH) 



upward through the stems directly to the leaves, where 

 it is evaporated. In plants of a succulent character, 

 the intake often exceeds the loss, and a large amount of 

 surplus liquid may be accumulated in various organs, 

 from whence it may pass to the leaves in times of 

 drought. 



The movement of water set up by transpiration car- 

 ries water and the contained salts to the centers of 

 food formation in the leaves, and the evaporation 

 process facilitates exchange of gases with the air. The 

 evaporation also tends to equalize temperatures. That 

 an enormous amount of work is performed by the 

 plant in transpiration may be seen when it is known 

 that a single sunflower plant will evaporate a pint of 

 water from its leaves in a single day, and about seventy 

 times this much in the course of its development. A 

 birch tree with 200,000 leaves will transpire from 700 

 to 1,000 pounds of water daily in the summer. A single 

 oak tree will throw 120 or 130 tons of water into the 

 air during the course of a season, and an acre of beech 

 trees containing 400 to 600 specimens will transpire 

 about 2,000,000 pounds in a single summer. It is 

 estimated that 98 per cent of the energy derived from 

 sunlight, by leaves is expended in the work of trans- 

 piration. 



To determine the exact amount of water transpired 

 by a plant, a specimen not more than a yard in height, 

 growing in a pot, may be used. Set the pot on a square 

 of oilcloth, then bring the cloth up around the pot 

 and tie closely to the stem of the plant. This will pre- 

 vent evaporation except from the shoot. Now set 

 the prepared plant on one pan of a scale, together with 

 a small measuring -glass, and balance. Allow the 

 plant to remain in the warm sunshine for eight hours, 

 then note the amount of water which must be poured 

 into the glass to reach the original level and restore 

 the balance: this will represent the amount of trans- 

 piration. 



To demonstrate that water actually does come from 

 the leaf, cut off a small leafy shoot of any convenient 

 plant and thrust the base of the stem through a piece 

 of cardboard into a tumbler of water: seal the opening 

 around the stem with wax or gelatine: then cover the 

 exposed part of the shoot with another tumbler and 

 set in a warm light place. Moisture which could have 

 come only from the leaves will soon gather on the glass. 

 Some transpiration occurs over the entire surface of 

 the plant, although only about one-thirtieth as much is 

 given off by the stem as from the same amount of 

 leaf-surface. 



The structure of the leaves is such as to facilitate 

 transpiration. The interior of the leaf is made up of a 

 great number of loosely arranged cells which evaporate 

 water into the air between them. The air in the leaf 

 communicates with the atmosphere through openings 

 called stomata, which are generally placed on the lower 

 side of the leaf. Consequently the watery vapor 

 diffuses out through the stomatal opening. Near the 

 outer end of the stomatal chimneys are guard-cells 

 which undergo- alterations in form that change the size 

 of the opening of the guard-cells and modify the rate 

 of water loss. The position and shape of the guard- 

 cells are affected by the age of the leaf, turgidity of the 

 tissues, wind, and sunlight. The rate of transpiration 

 may also be affected by cutinization or other altera- 

 tions in the cell-walls through which evaporation into 

 the stomatal chambers takes place, or by the presence 

 of colloids retentive of water in the cells. 



Species characteristic of arid regions generally have 

 limited waterproofed surfaces with a comparatively 

 small number of stomata. This modification is exem- 

 plified by the cacti, which transpire not more than one 

 three-hundredth as much water as a broad-leaved plant 

 of the same volume; and such succulents may lose as 

 tittle as one forty-thousandth of their total weight by 

 transpiration in a day. D T . 



