38 



Introduction to Botany. 



for the manufacture of plant food, and it further serves 

 the plant in affording strength and rigidity to the tender 

 herbaceous parts. 



25. Elements Necessary to Plants. — There are certain 



chemical elements necessary to 



the nutrition of plants, 

 which must be taken 

 from the soil particles ; 

 these are calcium, mag- 

 nesium, potassium, sul- 

 phur, phosphorus, and 

 iron. Nitrogen is taken 

 in part from the com- 

 pounds of nitrogen in 

 the soil, and in part in- 

 directly from the free 

 nitrogen of the atmos- 

 phere by means of mi- 

 croscopic organisms 

 which reside chiefly in 

 the root tubercles of 

 leguminous plants (see 

 Fig. 13). Were it not 

 for the fact that these 

 elements, for the most 

 part, are in the form of 

 compounds insoluble in 

 water, they would soon be washed away by the percolating 

 water after heavy rains. They are, however, slowly rendered 

 soluble by acids excreted by the root hairs (see Experiment 

 44), by carbon dioxide dissolved in the soil water, and by 

 the oxygen of the soil atmosphere. It must be remembered 

 that there are no openings in the root hairs, and only sub- 

 stances in solution in water can be absorbed by them. 



Fig. 13. 



D, root of a leguminous plant bearing Tuber- 

 cles ; E, a cell from a tubercle containing 

 bacteria, highly magnified ; F, some of the 

 bacteria more highly magnified ; G, a cell 

 from a tubercle after the bacteria have, in 

 part, evidently been absorbed by the plant. 

 After Frank. 



