LEA 



[ 341 ] 



LED 



sists in the decomposition, may, per- , 

 haps, be explained by the dis-oxygen- ; 

 i/ing power with which it is gifted. The 

 violet rays of the spectrum, and those i 

 just beyond them, have this power in ! 

 the greatest degree ; and Sennebier ; 

 has ascertained by experiment, 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 j 

 debility; and it is certain, when the : 

 leaves of a plant become generally j 

 white, that that individual is seldom i 

 long-lived. Mr. Knight, however, has i 

 demonstrated that variegation is not a i 

 certain indication of a deficiency of 

 hardihood. 



The function 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 from 

 the roots, so as to fit it for increasing 

 the sr/e of the parent plant, but they 

 also absorb nourishment from the at- 

 mosphere. The sap, after elaboration 

 in these organs, differs in every plant, 

 though, as far as experiments have i 

 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 

 period 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 blos- 

 soms and the ripening of their seed. 

 During 1 this period they do not tran- 

 spire more than one half so much as 

 during the period preceding and at- 

 tending 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 con- 

 fined inclosures, by keeping these 

 surfaces cleaiised 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 ; and it thence arises 

 that they cannot now be cultivated in 

 nursery gardens near London, where 

 they once flourished when that metro- 

 polis was less extensive. It must be 

 remembered, however, in using the 

 sponge and syringe, that the under side 

 of leaves is also an absorbing surface, 

 benefited by being kept clean, and by 

 the application; of moisture. 



During the day leaves also absorb 

 carbonic acid gas, which, they decom- 

 pose, retaining its carbon and emitting 

 the greater part of the oxygen that 

 enters into its composition. In the 

 night this operation is in a certain 

 measure reversed, a small quantity of 

 oxygen being absorbed from the atmo- 

 sphere, and a yet smaller proportion of 

 carbonic acid emitted. 



Carbonic acid gas in small propor- 

 tions 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 atmo- 

 sphere in which they are vegetating, 

 though one-twenty-fifth is a still more 

 favourable proportion ; and as hotbeds, 

 heated by fermenting matters, have 

 rapidly the air within their frames con- 

 taminated to a much greater extent 

 than the proportions above named, 

 thence arises the injury to the plants 

 they contain from a too long neglected 

 ventilation. The leaves turn yellow 

 from the excess of acid, which, they 

 are unable to digest, and which con- 

 sequently effects that change of colour 

 which also occurs in autumn. 



LECANO'PTERIS. (From Jckane, a 

 basin, and jrtcris, a fern. Nat. ord., 

 Ferns [Polypodiacea?]. Linn., 2-Cryp~ 

 toyamia l-Filiccs.) 



A fine stove fern. Se Ferns. 

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



LECIIEXA'ULTIA. See Lcschvna'iiltia. 



LEDEBOT;'KIA. (Named after M. 

 ^Lcdcbonr, a botanical writer. Nat. ord., 

 Lilyicorts [Liliacece]. Linn., ti-Hcxan- 

 dri'a 1-Monoyynia. ' Allied to Bessera.) 



Greenhouse bulb, not to be confounded with 

 Ledeburia, a name given to an umbel-bearing 

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

 bulbs protected from cold and wet in winter. 



