LICKS. 



LIGHT. 



comes, indeed, a diet recommended to invalids. 

 Others are used as tonic medicines, as Variola 

 rin fainnc.a and Parmelia parietina. Their prin 



cpal 



is, however, that o( furnishing the 



dyer with brilliant colours; orchall, cudbear 

 and perolle, with many more, are thus employed. 

 (Brunde's Diet, of Science.) See Moss. 



LICKS. A term applied in the United States 

 to places where salt springs escape from the 

 earth and impregnate the soil, and sometimes 

 give rise to an efflorescence of common salt. 

 To such spots the deer, buffalo, and almost all 

 graminivorous animals resort, for the purpose 

 of licking the surface. See SALT. 



LID. In botany, the calyx which falls off 

 from the flower in a single piece. 



LIFE EVERLASTING. See CUDWEED. 



LIGHT, ITS INFLUENCE ON VEGETA- 

 TION. That light has a considerable influence 

 upon the growth of plants, is an observation 

 that must have been very early made by man- 

 kind. The inferior green colour of plants 

 growing in the shade, as in woods, or when 

 covered with earth, or inverted vessels, would 

 clearly indicate to the most careless observer, 

 that light at least influenced the colour of ve- 

 getation : every gardener, in truth, takes advan- 

 tage of this fact, when he is blanching his 

 culinary vegetables. But it was not till after 

 the days of Priestley, that the other chemical 

 effects which light produces upon a growing 

 plant were so much better understood. 



It is probable that this influence commences 

 at a very early period in the life of the plant, 

 with even the germination of the seed. Ingen- 

 houz, says Dr. Thomson, found that seed al- 

 ways germinate faster in the dark than in the 

 light. (Exper.surla Veg. 11.) And these expe- 

 riments were repeated by Sennebier with equal 

 success. (Mem. Physico-Chem. vol. iii. p. 41.) 

 But the Abbe Bertholjn, who distinguished 

 himself so much by his labours to demonstrate 

 the effect of electricity on vegetation, objected 

 to the conclusions of these philosophers, and 

 affirmed that the difference in the germination 

 of seeds in the shade and in the light, was 

 owing, not to the light itself, but to the differ- 

 ence in the moisture in the two situations, the 

 moisture evaporating much faster from the seeds 

 in the light than from those in the shade; and 

 he affirmed that when precautions were taken 

 to keep the seeds equally moist, then those in the 

 sun germinated sooner than those in the shade. 

 (Jour, de Physique, 1789.) But when Sennebier 

 repeated his former experiments, and employed 

 every possible precaution to insure equality 

 of moisture in both situations, he constantly 

 found the seeds in the shade germinated sooner 

 than those in the light. We may conclude, 

 therefore, that light is injurious to germination ; 

 and hence one reason for covering seeds with 

 the soil in which they are grown. But from 

 ;he more recent experiments of Saussure, there 

 is reason to believe that light is only injurious 

 Jo vegetation in consequence of the heat it 

 produces ; for where the direct rays of the sun 

 were intercepted, though light was admitted, 

 ihe germination of the seeds was not sensibly 

 retarded. (Rech. Chem. sur la Veg. p. 23; Thorn- 

 t.-n's Chem. vol. iv. p. 307.) 



And with regard to the after-gi ^wth of plants, 

 710 



light exercises a very considerable influence. 

 It is now clearly ascertained that plants vege- 

 tating in the light, absorb carbonic acid gas 

 from the atmosphere, and emit oxygen gas ; 

 but when vegetating in the dark different ef- 

 fects are produced, for then carbonic acid gas 

 is emitted, and oxygen gas absorbed. (See 

 GASES.) This latter process is thus explained 

 by Liebig : " It is true that the decomposition 

 of carbonic acid is arrested by the absence of 

 light; but then, namely, at night, a true che- 

 mical process commences, in consequence of 

 the action of the oxygen in the air upon the 

 organic substance, composing the leaves, blos- 

 soms, and fruit. This process is not at all 

 connected with the life of the vegetable organ- 

 ism, because it goes on in a dead plant exactly 

 as in a living one. The substances composing 

 the leaves of different plants being known, it 

 is a matter of the greatest ease and certainty 

 to calculate which of them during life should 

 absorb most oxygen by chemical action where 

 the influence of light is withdrawn. The leaves 

 and green parts of all plants containing vola- 

 tile oils, or volatile constituents in general, 

 which change into resin by the absorption of 

 oxygen, should absorb more than other parts 

 which are free from such substances. Those 

 leaves, also, which contain either the consti- 

 tuents of nutgalls, or compounds in which ni- 

 trogen is present, ought to absorb more oxygen 

 than those which do not contain such matters, 

 The correctness of these inferences has been 

 distinctly proved by the observations of De 

 Saussure; for whilst the tasteless leaves of the 

 Jlgave Americana absorb only 0-3 of their vo- 

 lume of oxygen in the dark during 24 hours, the 

 leaves of the Pinus abies which contain volatile 

 and resinous oils absorb 10 times, those of the 

 Quercus robur containing tannic acid 14 times, 

 and the balmy leaves of the Populus alba 21 

 times that quantity. This chemical action -is 

 shown very plainly also in the leaves of the 

 Cotyledon calydnum, the Cacalia faoides, an< 3 

 others; for they are sour like sorrel in the 

 morning, tasteless at noon, and bitter in the 

 evening. The formation of acids is effected 

 during the night by a true process of oxydation ; 

 these are deprived 1 of their acid properties 

 during the day and evening, and are changed, 

 by separation of a part of their oxygen, into 

 compounds containing oxygen and hydrogen, 

 either in the same proportions as in water or even 

 with an excess of hydrogen, which is the com- 

 position of all tasteless and bitter substances. 

 When the green leaves of the poplar, the beech, 

 the oak, or the holly, are dried under the air- 

 pump, with exclusion of light, then moistened 

 with water, a'nd placed under a glass globe filled 

 with oxygen, they are found to absorb that gas 

 in proportion as they change in colour. The 

 chemical nature of this process is thus com- 

 pletely established. The diminution of the gas 

 which occurs can only be owing to the union of 

 a large proportion of oxygen with those sub- 

 stances which are already in the state of oxides, 

 or to the oxydation of the hydrogen in those ve- 

 getable compounds which contain it in excess. 

 The fallen brown or yellow leaves of the oak 

 contain no longer tannin, and those of the poplar 

 no balsamic constituents. (Org. Chem. p. 28.) 



