ORIGIN AND FORMATION OF ORGANIC MATTER IN PLANTS. 909 



carbon, but the proportion of oxygen which it contains remains 

 unchanged. This is not true, however, when we determine the changes 

 in composition which the air in which a plant is living undergoes. 

 Schloesing found that under such conditions the volume of oxygen 

 evolved was larger than the volume of carbon dioxid absorbed. This 

 indicates that the evolution of oxygen is not due simply to decomposi- 

 tion of carbon dioxid. Doubtless the greater part of this excess of 

 oxygen is due to reduction of nitrates which the plants take up from 

 the soil, but, as is shown below, a study of plant respiration reveals 

 another source of oxygen. 



PLANT RESPIRATION — THE FORMATION OF SUBSTANCES RICH OR 

 POOR IN OXYGEN — FATTY SUBSTANCES, RESINS, AND VEGETABLE 

 ACIDS. 



By the term respiration we understand the phenomena of the absorp- 

 tion of oxygen and the evolution of carbon dioxid. Respiration occurs 

 in all plant organs and is a function of such importance that when 

 interfered with by the exclusion of oxygen the death of the plant 

 results. If the roots, buds, moistened seeds, and branches of a plant 

 be placed in a flask and a current of air free from carbon dioxid be 

 passed over them and then led through a solution of barium hydrate, 

 the latter will become milky, due to the formation of barium carbonate, 

 thus showing that the vegetable matter has evolved carbon dioxid. 

 The leaves throw off carbon dioxid except when they are exposed to 

 the light and respiration is masked by assimilation. During the night 

 or in dense shade they throw off carb.on dioxid. When the relation 

 between the oxygen absorbed and the carbon dioxid evolved is care- 

 fully determined, it is found that this relation is considerably modified 

 by the temperature to which the leaves are exposed. In a low tem- 

 perature the oxygen absorbed is usually greater than the carbon dioxid 

 evolved, while in a high temperature the reverse is true, that is, there 

 is more carbon dioxid evolved than oxygen absorbed. Since one vol- 

 ume of carbon dioxid contains exactly one volume of oxygen it is evi- 

 dent that when the volume of carbon dioxid evolved is greater than the 

 oxygen absorbed the plant is losing oxygen. This explains how the 

 glucoses which are found in the pods of colza are transformed into 

 the fatty substances of the seed and how inosite and its derivatives 

 formed by chlorophyll action in caoutchouc trees give a resin which is 

 devoid of oxygen. The nature of this transformation has not yet been 

 explained, but the above observations indicate that these substances, 

 like starch, cellulose, and sugar, are derived from formic aldehyde, 

 which, as already explained, is assumed to be the primary substance 

 from which all plant substances are built up. 



The formation of acids in plant tissues is explained more easily than 

 that of fatty substances and resins. When starch or sugars are sub- 

 jected to the action of dilute nitric acid, oxalic acid is produced. Simi- 



