HOW THE PLANT GROWS 



compounds in the soil, chiefly the nitrates. Bacteria living in nodules 

 on the roots of legumes, such as clover, alfalfa, and peas, are able to take 

 nitrogen gas from the air and pass it on in combined form to the host 

 plants. Thus, the legumes are able indirectly, thru the aid of these bac- 

 teria, to use the nitrogen of the air as food. 



Oxygen, which is a part of all plant compounds, is nearly all obtained 

 from water and carbon dioxid, and not from the free oxygen gas of the 

 air. Some oxygen gas is, however, being continuously absorbed by all 

 green plants and is necessary for their growth. 



The mineral substances required by plants are taken from the soil 

 thru the roots. They include sulfur, phosphorus, potassium, calcium, 



<t 16 



Left PLANT CELLS, MAGNIFIED 350 TIMES 



A, Cell wall; b, nucleus, or life center of cell; c, strands of protoplasm; d, 

 spaces filled with cell sap; e, chlorophyll bodies. (After Strassburger.) 



Eight SECTION OP LEAP, MAGNIFIED 400 TIMES 



A, Stoma, or opening on under side of leaf thru which air enters; b, chloro- 

 phyll bodies in leaf cells; d, lower epidermal cells of leaf; e, upper epidermal 

 cells of leaf. (After Strassburger.) 



magnesium, and iron. The plant can not use these mineral elements in 

 an uncombined form for food, but only when built up into mineral salts, 

 such as sulfates, phosphates, nitrates, and chlorides, of potassium, cal- 

 cium, magnesium, and iron. 



Sulfur, in small amount, is a component of plant proteins. Phos- 

 phorus, likewise in small amount, is present in the life-holding protein 

 of the leaf cells and also abounds in the protein of seeds. Potassium is 

 necessary in the formation of starch and sugar by plants. Magnesium is 

 an essential part of chlorophyll, the green coloring matter of plants, 

 which is necessary for their growth, and iron is also needed in its forma- 

 tion. Calcium is vital to plants, tho its use is not well understood. So- 



