352 Proceedings of the 



the exhaustion of the nutritive principle of the earth in which they 

 were planted, the suction of the roots no longer supplied any alimen- 

 tary carbon, the trees produced only white or pale-yellow leaves. 



The same phenomenon may be established by remarking in spring 

 the difference of colour between grain growing in a rich soil, and that 

 growing in a poor soil. The moment the sap which nourishes the 

 plants cannot derive a sufficient quantity of carbon from the earth, 

 the leaves become paler and paler, until the carbon is quite exhausted, 

 and they are then produced quite white. In the decoloration of 

 leaves from want of light, there may be plenty of carbon in the plant, 

 but, instead of being fixed in the tissue, it is dispersed under the form 

 of carbonic acid ; while in that arising from the exhaustion of the 

 soil, the carbon, which is the essential colouring principle, is wanting, 

 and, therefore, the brightest rays of the sun produce no effect. Cold 

 is a third cause of the change of colour in the leaves of plants ; this 

 results both from the obstacle opposed by the lowness of the tempe- 

 rature to the nutrition of the leaves, and also from the age of those 

 organs. Those plants which have the greatest vigour of vegetation 

 will always resist the longest the influence of the cold, which tends 

 to suspend their nutrition, and, therefore, to change the colour of 

 their leaves. This is remarkable in wheat and other grains ; the cold 

 of winter does not affect them, they grow up in the spring with green 

 leaves ; but if, afterwards, cold weather comes, the nutrition is sus- 

 pended, and the leaves become yellow. If, however, in the same 

 field, some parts have been better manured than others, and, conse- 

 quently, have stronger nutritive principle, the grain growing on those 

 parts will preserve the colour of the leaves long after that of the other 

 parts has given way to the influence of the cold. Hence we find that 

 a certain depression of the temperature occasions in plants, during 

 their development, a suspension of the fixation of the alimentary 

 carbon ; and, consequently, (as they are constantly losing some of 

 their carbon in the form of carbonic acid,) a change in the colour of 

 the leaves, but that the effect of this lowness of temperature will be 

 in a great measure resisted by plants which possess in a considerable 

 degree strong principles of nutrition. 



All these observations tend to prove that it is from the soil that 

 plants principally derive the alimentary matter by which they exist ; 

 this alimentary matter is an extractive, soluble in water, existing in 

 various proportions in the different vegetable earths. It is very 

 abundant in the offal of animal and vegetable matter, whence dung 

 and manure are formed. It appears from the experiments of M. de 

 Saussure, that the oxygen of the atmosphere combines with the 

 carbon of this extractive, and changes it into carbonic acid, which, 

 being dissolved by the sap, is transported into the leaves, where it is 

 decomposed by the action of the light ; the carbon is fixed in the 

 vegetable tissue, and contributes to form the green colouring matter, 

 and the oxygen is discharged. 



All carbon which is susceptible of being converted into carbonic 



