FUNCTIONS OF THESE SUBSTANCES. 9 



larger proportion in the grain or seeds as the season of ripen- 

 ing approaches. Fluorine has as yet been little sought for. It 

 is present in very minute quantity, and is probably associated 

 with the phosphoric acid. 



The relative proportions in which these several mineral sub- 

 stances enter into the composition of the several parts of plants, 

 and the variations which these proportions undergo, in different 

 circumstances, are very interesting branches of study, upon 

 which it would be out of place here to touch. They are treated 

 at length in my other published works. I only here remark of 

 the oxide of iron, that, though often present in small quantity 

 only, it is never absent from a plant, and that the oxide of 

 manganese is frequently met with in so minute a proportion as 

 to be incapable of estimation, and is therefore, like fluorine, 

 often omitted in the detail of the substances found when the 

 ash of the plant is subjected to analysis. Why these substances 

 are assumed to be always present in plants, even when they are 

 not to be sensibly detected, and especially, why iron in notable 

 proportion must be contained in all vegetable food, will appear 

 in a subsequent section. 



4, Functions performed in plants by their organic and 

 inorganic or mineral constituents. 



The substances mentioned in the preceding section as occur- 

 ring more or less abundantly in all plants, perform in them 

 at different times, and in different parts of the plant, three 

 general functions. 



1. They all form, more or less constantly and abundantly, a 

 portion of the fixed and solid matter of the plant, taken as a 

 whole. They may not all be found in any one part of the 

 plant, when separated carefully from the rest ; but in the solid 

 parts of the plant, taken as a whole, they are all and always to 

 be met with. When thus deposited, they become for the most 

 part dormant as it were, and for the time cease to perform an 

 active chemical function in the general growth, though as vessels 

 or cells they may still perform a mechanical function. 



2. They undergo various chemical changes in the interior, 

 chiefly while circulating or contained in the sap, by which 



