FUNCTIONS OF THE FLOWER-LEAVES. 95 



the sap of plants, is derived from the soil. The under side of the 

 leaves of plants is also sui)posed by some to be capable of absorbing 

 moisture from the air, either in the form of watery vapour, or when it 

 falls upon the leaves in the state of dew. Like the roots also they may 

 absorb with the dew any substances the latter happens to hold in solu- 

 tion. And thus plants may, in some degree, be nourished by the vola- 

 tile organic substances which ascend from the earth during the heat of 

 the day, and which are again in a great measure precipitated with tlie 

 evening dew. 



Whether the leaves ever absorb nitrogen gas from the air has not as 

 yet been determined with sufficient accuracy. If they do, it must in gene- 

 ral be in very small quantity only, since it has hitherto escaped detec- 

 tion. In like manner it is doubtful how far they regularly absorb any 

 other substances which the air is supposed to contain. Thus it is known 

 that nitric acid exists in the air in very minute quantity. Some chem- 

 ists also believe that ammonia is extensively diffused through the atmos- 

 phere in an exceedingly diluted slate. Do the leaves of plants absorb 

 these substances? Is the absorption of them one of the constant and ne- 

 cessary functions of the leaves ? The reply to these questions must be 

 very uncertain, and any principle which professes- to be based upon such 

 a reply must be regarded only as a matter of opinion. 



8°. The petals of flower-leaves perform a somewhat different function 

 from those of the ordinary leaves of a plant. They absorb oxygen at 

 all times — though more by day than by night — and they constantly emit 

 carbonic acid. The bulk of the latter gas evolved, however, is less than 

 that of the oxygen taken in. The absorption of oxygen gas, and the 

 constant production of carbonic acid, is, in some flowers, so great as to 

 cause a perceptible 'ncrease of temperature — and to this slow combus- 

 tion, so to speak, the proper heat observed in the flowers of many plants 

 has been attributed. 



According to some authors, the flower-leaves also emit pure nitrogen , 

 gas. — [Sprengel, Cheinie, II., p. 347.] This fact has not yet been deter- 

 mined by a sufficient number of accurate experiments; it is in accord- 

 ance, however, with the results of Boussingault, that, when a plant 

 flowers and approaches to maturity, the nitrogen it contains becomes 

 less. If confirmed, this evolution of nitrogen would throw an interest- 

 ing light on the most advantageous employment of green crops, both for 

 the purposes of manure and for the feeding of cattle. 



9°. When the leaves of a plant begin to decay, either naturally as in 

 autumn, or from artificial or accidental causes, they no longer absorb 

 and decompose carbonic acid, even under the influence of the sun's rays. 

 On the contrary, they absorb oxygen, like the petals cf the flower, new 

 compounds are formed within their substance — their green colour disap- 

 pears — they become yellow — they wither, die, and drop from the tree — 

 their final function, as the organs of a living being, is discharged. They 

 then undergo new changes, are subjected to a new series of influences, 

 and are made to serve new purposes in the economy of nature. These 

 we shall hereafter find to be no less interesting and important in refer- 

 ence to a further end, than are the functions of the living leaf to the 

 growth and nourishment of the plant. — [See' subsequent lectu^ *' On the 

 law of the decay of organic substances.^ ^] ^ 



