RESTORATION OF THE INGREDIENTS OF THE SOIL. 103 



decay during winter, and thus furnish to the young plant a new 

 source of carbonic acid. The soil, therefore, is not exhausted of 

 its humus by the cultivation of these plants. 



It may also be deduced from theoretical considerations, that the 

 soil receives during the life of the plants as much, or perhaps 

 even more, of carbonaceous materials as it yields to them ; and 

 that the soil is enriched in these matters by the process of secre- 

 tion proceeding at the surface of the fibres of the root ; the mat- 

 ters thus secreted enter into decay during winter, and pass into 

 humus. 



Physiologists differ in their opinions with regard to the pro- 

 cesses of secretion and excretion by plants ; some affirm that these 

 processes do exist, while others deny their existence; so that, at 

 this moment, the opinions are divided on the subject. But still, 

 no one denies that the oxygen separated by the leaves and green 

 parts of plants ought to be considered as an excrement. In the 

 act of vital activity, the plants assimilate the carbon of carbonic 

 acid, and the hydrogen of water, making them constituents of their 

 organs, while they separate part of the oxygen with which these 

 elements were combined. 



In the blossoms we find volatile oils, compounds rich in carbon 

 and in hydrogen, and which are not further employed in any of 

 the vital processes : out of the bark we see flowing resin, balsam, 

 and gum ; and out of the leaves, sugar and mucilaginous sub- 

 stances. 



Oxygen is not separated from the surface of the bark, roots, or 

 other parts that are not green ; but, on the contrary, these parts 

 separate substances rich in carbon which have been generated by 

 the vital processes of the plant, but have not experienced any 

 further change. 



When we compare the barks of the fir,* pine, beech, or oak, • 



* Ashes of Wood of Ashes of the fit k 



the Fir. of the Fir. 



Hertwq. Hertwig. 



1000 wood gave 323 ashes. 1000 bark gave 17 - 85 ashea 



{Carbonate of soda - - 742 

 Carbonate of potash -1130 Soluble eaits 2* 95 

 Chloride of sodium ) -, 

 Sulphate of pofci h 5 ' races 



