CHEMICAL PROCESSES IN CELLS. 139 



late carbon, hydrogen, nitrogen, and various inorganic substances. We 

 will attempt to give a general idea as to the processes by which these 

 substances are absorbed by plants, and the way in which in their tissues 

 they are combined to form organic compounds. 



First, as regards carbon. The carbon of plants is without doubt 

 derived from the C0 2 of the atmosphere, or in solution in rain-water 

 which is absorbed by the leaves and roots, which under the influence of 

 the sun is broken up in the body, and whose oxygen is liberated. The 

 oxygen given off in the day-time by plants has been found to be some- 

 what less than that which is contained in the C0 2 which has been 

 absorbed. This would seem to indicate that C0 2 is only reduced to CO, 

 since it is known that part of the oxygen comes from the decomposition 

 of water. This hypothesis is further rendered more probable by the 

 readiness with which CO combines with other bodies. Thus, it unites 

 with CI at the ordinary temperature, and combines directly with hydro- 

 gen to form formic acid. Doubled,— that is, united with itself,— the 

 radical oxide of carbon, or carbonyl CO, constitutes the oxalic radical 

 C 2 2 , or oxalyl. The acid which contains this radical, — that is, oxalic 

 acid, — can be formed by an incomplete reduction of C0 2 and H 2 in the 

 presence of mineral bases ; various organic acids may thus originate in 

 vegetable cells. Taking the simplest cases, the important acids, formic 

 and oxalic, may be formed in this way, the one with one atom of carbon, 

 the other with two. Thus, with one molecule of water 



C0 2 + H 2 — O = CH 2 0, — formic acid. 

 2 C0 2 + H 2 — O = C 2 H 2 4 = oxalic acid. 



Developing this idea, Liebig has shown that the organic acids once 

 formed may give rise to aldehydes by a subsequent reduction. Formic 

 aldehyde represents formic acid less one atom of oxygen ; oxalic alde- 

 hyde, or glyonal, oxalic acid less two atoms of oxygen, thus : — 



CH 2 2 — = CH 2 = formic aldehyde. 

 C 2 H 2 4 - 2 = C 2 H 2 2 = glyonal. 



The formation, therefore, of aldehydes in vegetables represents a 

 certain stage of reduction of C0 2 and H 2 0. Even more complex 

 substances, but less rich in oxygen, will result from further decompo- 

 sition. 



The examples above given, especially in the case of formic aldehyde, 

 are particularly important, as there is scarcely any doubt that formic 

 aldehyde plays an important role in vegetable synthesis. Thus, six 

 molecules of formic aldehyde will form one molecule of glucose : — 



6 CH 2 = CgHj 2 6 = glucose. 



Again, on the other hand, by the dehydration of aldehydes resins may 

 be formed, as it is well known how r readily ordinary aldehydes become 



