AND OF THE MINERAL CONSTITUENTS. 11 



found in most cases in those parts of the plant in which potash 

 and soda abound, and deposits a portion of its sulphur wherever 

 the compounds of nitrogen form a notable part of the substance 

 of the plant. 



Phosphoric acid exercises also much influence over the chemical 

 changes of the sap, and finally fixes itself in greatest abundance 

 in the seeds and other reproductive parts of the plant. 



Lime is very important to healthy vegetable growth, as 

 practical experience has long testified. Among other duties, it 

 appears to accompany the phosphoric acid in the sap of plants, 

 and to deposit itself in combination with organic acids (oxalic, 

 &c.) in the leaves and bark, and with phosphoric acid in some 

 seeds and roots. 



Magnesia appears also to attach itself very much to phosphoric 

 acid in the sap, and fixes itself, in combination with this acid, 

 principally in the seed. 



Chlorine, the chemical function of this substance in the sap, is 

 less understood even than that of the other substances above 

 mentioned. It exists chiefly in combination with soda, and is 

 much more abundantly present in some plants, and in some 

 parts of plants, than in others. Though, as I have said, its 

 immediate chemical function in the plant is not understood, we 

 shall see in a subsequent section that it forms a most important 

 constituent of the plant, in so far as the after uses of vegetables 

 in the feeding of animals are concerned. 



Silica exists in the sap in a soluble form, and deposits itself 

 chiefly in the exterior portions of the stems and leaves of plants. 

 It is supposed there to serve as a defence to the plant against 

 external injury, and to give strength to the stem, in the case of 

 the grasses and corn-yielding plants; but what chemical functions 

 it performs, if any, in directly promoting vegetable growth, we 

 can scarcely as yet venture even to guess. 



5. Functions of the several parts of plants. 



The functions of the several parts of plants are of two kinds 

 external and internal, or mechanical and chemical, as they may 

 also be called. These two kinds of functions are by no means 

 equally well understood. 



