ASSIMILATION OF CARBON. 



the evening. The formation of acids is 

 effected during the night by a true process 

 of oxidation : these are deprived 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 composi- 

 tion of all tasteless and bitter substances. 



Indeed, the quantity of oxygen absorbed 

 could be estimated pretty nearly by the dif- 

 ferent periods which the green leaves of 

 plants require to undergo alteration in colour, 

 by the influence of the atmosphere. Those 

 which continue longest green will abstract 

 less oxygen from the air in an equal space 

 of time, than those the constituent parts of 

 which suffer a more rapid change. It is 

 found, for example, that the leaves of the 

 Ilex aquifoliiim, distinguished by the dura- 

 bility of their colour, absorb only O86 of 

 their volume of oxygen gas in the same time 

 that the leaves of the poplar absorb 8, and 

 those of the beech 9$ times their volume ; 

 both the beech and poplar being remarkable 

 for the rapidity and ease with which the 

 colour of their leaves changes. 



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, and 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 completely established. 

 The diminution of the gas which occurs can 

 only be owing to the union of a large pro- 

 portion of oxygen with those substances 

 which are already in the state of oxides, or 

 to the oxidation of the hydrogen in those 

 vegetable 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. 



The property which green leaves possess 

 of absorbing oxygen belongs also to fresh 

 wood, \vhether taken from a twig or from 

 the interior of the trunk of a tree. When 

 fine chips of such wood are placed in a 

 moist condition under a jar filled with oxy- 

 cren, the gas is seen to diminish in volume. 

 But wood, dried by exposure to the atmo- 

 sphere and then moistened, converts the 

 oxygen into carbonic acid, without change 

 of volume ; fresh wood, therefore, absorbs 

 most oxygen. 



MM. Peiersen and SchSdler have shown, 

 by the careful elementary analysis of 24 dif- 

 ferent kinds of wood, that they contain car- 

 bon and the elements of water, with the 

 addition of a certain quantity of hydrogen. 

 Oak wood, recently taken from the tree, and 

 dried at 100 C. (212 F.,) contains 49,432 

 carbon, 6.069 hydrogen, and 44.499 oxygen. 



The proportion of hydrogen which is ne- 

 cessary to combine with 44.498 oxygen in 

 order to form water, is of this quantity, 

 namely, 5.56 ; it is evident, therefore, that 



oak wood contains ^ more hydrogen than 

 corresponds to this proportion. In Pinus 

 Larix, P. Mies, and P. picea the excess of 

 hydrogen amounts to \, and in Tilia euro- 

 pcua to . The quantity of hydrogen sta nds 

 m some relation to the specific weight of the 

 wood; the lighter kinds of wood contain 

 more of it than the heavier. In ebony wood 

 (Diospyros Ebenum) the oxygen and'hydro- 

 gen are in exactly the same proportion as in 

 water. 



The difference between the composition 

 of the varieties of wood, and that of simple 

 woody fibre, depends, unquestionably, upon 

 the presence of constituents, in part soluble, 

 and in part insoluble, such as resin and 

 other matters, which contain a large pro- 

 portion of hydrogen: the hydrogen of such 

 substances being in the analysis of the vari- 

 ous woods superadded to that of the true 

 woody fibre. 



It has previously been mentioned that 

 mouldering oak wood contains carbon and 

 the elements of water, without any excess 

 of hydrogen. But the proportions of iu 

 constituents must necessarily have been dif- 

 ferent, if the volume of the air had not 

 changed during its decay, because the pro- 

 portion of hydrogen in those component 

 substances of the wood which contained it 

 in excess is here diminished, and this dimi- 

 nution could only be effected by an absorp- 

 tion of oxygen, and consequent formation 

 of water. 



Most vegetable physiologists have con- 

 nected the emission of carbonic acid during 

 the night with the absorption of oxygen 

 from the atmosphere, and have considered 

 these actions as a true process of respiration 

 in plants, similar to that of animals, and like 

 it, having for its result the separation of 

 carbon from some of their constitutents. 

 This opinion has a very weak and unstable 

 foundation. 



The carbonic acid, which has been ab- 

 sorbed by the leaves and by the roots, to- 

 gether with water, ceases to be decomposed 

 on the departure of daylight ; it is dissolved 

 in the juices which pervade all parts of the 

 plant, and escapes every moment through 

 the leaves in quantity corresponding to that 

 of the water which evaporates. 



A soil in which plants vegetate vigor- 

 ously, contains a certain quantity of mois- 

 ture which is indispensably necessary to 

 their existence. Carbonic acid, likewise, i; 

 always present in such a soil, whether it 

 has been abstracted from the air or has 1 been 

 generated by the decay of vegetable matter. 

 Rain and wellwater, and also that from 

 other sources, invariably contains carbonic 

 acid. Plants during their life constantly 

 possess the power of absorbing by their 

 roots moisture, and, along with it, air and 

 carbonic acid. Is it, therefore, surprising 

 that the carbonic acid should be returned 

 unchanged to the atmosphere, along with 

 water, when light (the cause of the fixation 

 of its carbon) is absent? 



