LEA 



I 



circumstances, except when light is 

 present. Now, the only source from which 

 they can obtain hydrogen is by decom- 

 posing water; and how light assists in 

 the decomposition, may, perhaps, be ex- 

 plained by the dis-oxygenizingpowerwith 

 which it is gifted. The violet rays of the 

 spectrum, and those just beyond them, 

 have this power in the greatest degree; 

 and Sennebier has ascertained by experi- 

 ment, that those rays have the greatest 

 influence in producing the green colour 

 of plants. 



"When leaves are of any other hue than 

 green they are said to be coloured. This 

 variegation is often considered to be a 

 symptom either of tenderness or debility ; 

 and it is certain, when the leaves of a 

 plant become generally white, that that 

 individual is seldom long-lived. Mr. 

 Knight, however, has demonstrated that 

 variegation is not a certain indication of 

 a deficiency of hardihood. 



The functions of the leaves appear to 

 be a combination of those of the lungs 

 and stomach of animals ; they not only 

 modify the food brought to them i'rom the 

 roots, so as to fit it for increasing the size 

 of the parent plant, but they also ahsorb 

 nourishment from the atmosphere. The 

 sap, after elaboration in these organs, 

 differs in every plant, though, as far as 

 experiments have been tried, it appears 

 to be nearly the same in all vegetables 

 when it first reaches them. The power of 

 a leaf to generate sap is in proportion to 

 its area of surface, exposure to the light, 

 and congenial situation. 



The transpiration of plants decreases 

 with that of the temperature to which 

 they are exposed, as well as with the pe- 

 riod of their growth. This explains why 

 the gardener finds that his plants do not 

 require so much water in cold weather, 

 nor during the time that elapses between 

 the fall of their blossoms and the ripen- 

 ing of their seed. During this period 

 they do not transpire more than one-half 

 so much as during the period preceding 

 and attending upon their blooming. 



The transpiration takes place from the 

 upper surface of the leaves. Hence arises 

 the benefit which plants derive in rooms, 

 greenhouses, and other confined inclo- 

 sures, by keeping these surfaces cleansed 

 with the sponge and syringe. Some 

 plants are particularly sensitive to injury 

 from any check to their transpiration, 

 among which are the tea-scented roses ; 



4 ] LED 



and it thence arises that they cannot now 

 be cultivated in nursery gardens near 

 London, whe,re they once flourished when 

 that metropolis was less extensive. It 

 must be remembered, however, in using 

 the sponge and syringe, that the under 

 side of leaves is also an absorbing sur- 

 face, benefited by being kept clean, and 

 by the application of moisture. 



During the day leaves absorb car- 

 bonic acid gas, which they decompose, 

 retaining its carbon, and emitting the 

 greater part of the oxygen that enters into 

 its composition. In the night this opera- 

 tion is, in a certain measure, reversed, a 

 small quantity of oxygen being absorbed 

 from the atmosphere, and a yet smaller 

 proportion of carbonic acid emitted. 



Carbonic acid gas in small proportions 

 is essential to the existence of leaves ; yet 

 it only benefits them when present in 

 quantities not exceeding one-twelfth of 

 the bulk of the atmosphere in which they 

 are vegetating, though one-twenty fifth 

 is a still more favourable proportion ; and 

 as hotbeds, heated by fermenting matters, 

 have the air within their frames rapidly 

 contaminated to a much greater extent 

 than the proportions above named, thence 

 arises the injury to the plants they con- 

 tain from a too long neglected ventilation. 

 The leaves turn yellow from the excess of 

 acid, which they are unable to digest, and 

 which consequently effects that change 

 of colour which also occurs in autumn. 



LECANO'PTEEIS. (From lefcane, a basin, 

 and pteris, a fern. Nat. ord., Ferns [Poly- 

 podiaceee]. Linn., 24^-Cryplogamia 1-JF'i- 

 lices.) 



A fine stove Fern. See FERNS. 

 L. carno'sa (fleshy-/eaed). Yellow. May. Java. 



LECHEKAU'LTIA. See LESCHEKAU'LTIA. 



LEDEBOU'RIA. (Named after M.Lcde- 

 lour, a botanical writer. Nat. ord., Lily- 

 worts [Liliacea?.]. Linn., Q-Hexandria 1- 

 Monogynia. Allied to Bessera.) 



Greenhouse bulb, not to be confounded with 

 Ledebu'ria, a name given to an umbel-bearirig 

 penus. Offsets; sandy loam, with a little peat; 

 bulbs protected from cold and wet in winter. 

 L. hyaci'nthlna (hyacinth-like). ^.Greenish-white. 

 E. Ind. 1832. 



LEDON GUM. Ci'slus le'don. 



LEDOCA'RPUM. (From Icdon, the Cistus, 

 and karpos, a fruit; fruit like that of the 

 Cistus. Nat. ord., Oxallds [Oxalidacese]. 

 Linn., 10-Decandria 5-Pentagynia.) 



Greenhouse evergreen. Cuttings in sandy loam, 

 under u bell-glass ; sandy loam. Winter temp., 

 40 to 48. 



