26 THE SOILS AND CEOPS OF THE FAKM. 



Plants can exercise this power of assimilating car- 

 bon from the air only in the light, while what we may 

 properly call the breathing of plants goes on continu- 

 ously. They take in and use much more carbon 

 than they give off. They thus make the air more 

 suitable to be breathed by animals, carbonic acid in 

 any considerable quantity in the air being injurious 

 to animal life. The breathing of animals, the burn- 

 ing or slow decay of vegetable or animal matter, in- 

 creases the supply of carbonic acid in the air, thus 

 better fitting it for the use of plants. 



Leaves are believed to absorb some water, especial- 

 ly in times of drouth, when the soil supplies less than 

 the plant needs. They also absorb some nitrogen in 

 the form of ammonia. They giveofP very large quan- 

 tities of wafcsr, chiefly through small openings on their 

 under sides. These openings are called stomata. Al- 

 though the shading of the surface by plants tends to 

 decrease the evaporation of water from the soil, th*9 

 leaves give off so much water that the soil is usu- 

 ally drier when covered with a growing crop thai 

 when it is free from vegetation. The evaporation of 

 water from the leaves greatly aids the upward flow of 

 the sap. 



Much of the work of the leaves is dependent on 

 aanlight, and much of it is done by means of the green 

 coloring matter of the leaves, called .chlorophyll. 



The production of flowers and seeds is not essen- 

 tial to the growth of the individual plant. The seeds 

 may be useful to man in many ways and may be nec- 

 essary for a future supply of plants, but not to the 

 growth and health of the plant producing them. In 

 many plants maturing the seed is accompanied or fol- 

 lowed by the death of the plant. 



