136 LECTURES ON 



It is, however, only in the sun-light, and with many plants (accord- 

 ing to the recent researches of Corenwinder) only in direct sun-light 

 that carbonic acid or, more properly, carbon is assimilated. We have 

 already alluded to the fact that oxygen is exhaled by the plant. This 

 oxygen comes from the decomposition of carbonic acid (and water) in 

 the interior of the plant. The vegetable cell aided by the sun has 

 the power of separating the elements of this compound with the great- 

 est ease, and it retains the carbon to add to its structure while the 

 oxygen escapes entirely or in part into the general atmosphere. 



As already mentioned, however, oxygen itself, under certain circum- 

 stances, more particularly at certain stages of vegetable development, 

 is absorbed; and as a consequence of this and at just the same time, 

 carbonic acid is evolved. This separation of carbonic acid may be 

 observed in all young plants (still depending upon the disorganization 

 of the parent seed) when situated in the shade ; and some plants exhale 

 it at all periods of their growth when not exposed to direct sun-light. 



All plants exhale carbonic acid during the night or in the entire 

 absence of sun-light; but the amount of this gas that is absorbed and 

 decomposed by day vastly exceeds that evolved by night. In fact, 

 one hour or half hour of direct sunshine enables it to absorb and de- 

 compose more than has escaped from it in a whole night. 



Carbonic acid gas is unquestionably the chief source of the carbon 

 of agricultural plants. Some writers, with Liebig, consider it to be 

 practically the exclusive means of supplying this element. Others, 

 after Saussure and Mulder, regard the slightly soluble compounds re- 

 resulting from the decay of vegetable matter (humus) in the soil, as 

 capable of directly supplying a portion of carbon to a new generation 

 of plants. While there is perhaps no satisfactory evidence that humus 

 is entirely excluded from immediately nourishing vegetation, it is plain 

 from considerations founded in the growth of forests and prairie grasses 

 that the atmosphere, and indeed carbonic acid is now entitled to rank 

 as the great storehouse of carbon for this purpose, as once, before 

 humus existed, it must have betrn the exclusive source of this element. 



From what has been already remarked with regard to the compo- 

 sition of the vegetable carbo-hydrates, it is seen that a certain general 

 theoretical view of their formation in the plant may be at once gath- 

 ered from the facts now set forth. In order to form the members of 

 the cellulose group, it is only needful that the carbon retained by the 

 cells from the carbonic acid which they decompose so readily, should 

 enter into union with a due amount of the water that perpetually streams 

 upward through them. By the elimination of a portion of oxygen 

 from the water itself, we have remaining the elements that form the 

 fats and fixed oils. To yield the vegetable acids and the pectose 

 group, it suffices that a portion of oxygen be retained or be reabsorbed. 

 These considerations are purely hypothetical, yet, although the real 

 processes of decomposition and organization are in many cases vastly 

 more complex, they possess great interest in a survey of the economy 

 of vegetation. 



For the elaboration of the albuminoids, a source of nitrogen must be 

 present to the plant. This essential element is supplied, so far as the 



