PLANTS. 59 



plant in the form of a gaseous compound. When, 

 however, charcoal is raised to the kindling point, it 

 burns, or oxidizes, and returns again to the atmos- 

 phere whence it came. 



Besides starchy substances, leaves build up 

 another substance called albumen. It contains not 

 only carbon, hydrogen, and oxygen, but an addi- 

 tional element nitrogen. The nitrogen is not 

 taken from the air directly, but is brought up 

 from the earth by the roots as a nitrate dissolved 

 /n water. There is far more nitrogen than any 

 other substance in the air, yet leaves do not have 

 the power of absorbing it. Nitrogen is very 

 essential in the formation of albuminous sub- 

 stances, so that soils poor in its compounds yield 

 very light crops of certain kinds of plants. Some 

 plants the seeds especially are richer in albumen 

 than others, and of course require more nitrogen in 

 the soil. The sticky part of wheat flour is albu- 

 men, and there is a larger proportion of it in the 

 grains of wheat than in Indian corn. Hence a 

 good wheat-growing soil must necessarily be differ- 

 ent in composition from soil in which corn is best 

 grown. Wheat grows well in soil where clover was 

 grown the previous season. The roots of clover 

 have the especial power of extracting free nitrogen 

 from the air and fixing it as nitrates in the soil, 

 whence the wheat plants take it up and build it into 

 albumen of the seeds. 



