iS AMMONIA DECOMPOSED IN THE DARK ? 165 



have seen [notes to pages 136 and 138,] that, according to the results 

 of the best experiments, the whole of the oxygen of the carbonic acid 

 absorbed, is not given off by the leaves of all plants even in the sun- 

 shine, — while in the dark this gas is largely and directly imbibed from 

 the air. If in the sap of a plant there be present at the same time a 

 quantity of ammonia, the hydrogen of this ammonia may unite directly 

 with the oxygen of the carbonic acid, forming water and a proportionate 

 quantity of one or other of the several compounds (p. 112), which may 

 be represented by carbon and water. Thus 



3 of Carbonic Acid, = C3 Oq and the hydrogen of 

 2 of Ammonia (NH3) = Hg 



X of Grape 3 of 



Sugar. Water, 



give . . . . C3 He Oe = C3 H3 O3 -f 3HO 



so that where ammonia is present, and circumstances are favourable, 

 sugar or starch may be formed in variable quantity, without the neces- 

 sary evolution of oxygen gas. This change will take place in the inte- 

 rior of the leaf. And, if the direct decomposition of carbonic acid, and 

 the evolution of its oxygen by the agency of the sun, take place at the 

 same time — with a rapidity proportioned to the intensity of the light, — 

 this simultaneous production of sugar, &c., from the presence of ammo- 

 nia, must aid the increase and growth of the plant; and may be one 

 main cause of the fertilizing action of this compound, which has been so 

 long and so generally recognized. 



When the hydrogen of the ammonia is thus worked up, the quantity 

 of ox\gen which escapes from the leaf must be less in proportion ; and 

 hence another cause (p. 138) for those discrepancies which have been 

 observed in regard to the bulk of oxygen given off, compared with that 

 of the carbonic acid taken in, by the leaves of different plants. 



But at the same time the nitrogen is set free. This nitrogen will 

 either be again compounded in the plant with other elements, or, if not 

 required for its healthy growth — that is, if more largely present than is 

 required by the plant — it will be directl}' emitted by the leaves, or sent 

 downwards and permitted to escape by the root. Hence the reason 

 why pure nitrogen is evolved from the leaves of some plants (p. 95), 

 and why ammonia exercises a beneficial action upon vegetation, in 

 cases where all the nitrogen it contains is neither retained nor required 

 by the plant. 



Does this decomposition necessarily require the agency of light? 

 May it not take place in the absence of the sun ? 



I will mention one or two facts which seem to throw light upon this 

 point. 



1°. Plants grow in the dark. Though feeble and blanched, they in- 

 crease largely in bulk ; they must, therefore, have the power of assimi- 

 lating their food to a certain extent, independent of the sun's rays. 



2°. Several species of Poa, Plantago, Trifolium arvense, Cheiran- 

 thus, (fee, become green in the perpetual darkness of mines (Hum- 

 boldt). 



3°. When a little hydrogen is mixed with the air, plants become 

 greenish, even in the dark (Sennebier) ; and when exposed to the sun, ^ 

 the green becomes unusually intense in such a mixture (Ingenhouss). 

 8 



