152 PLANT LTFE. 



sodium, chlorine, and silicon. Of these seven, the first four 

 are found in the ash of all plants, and the remaining three 

 are very common. In addition to the elements enumerated, 

 about 25 others are known to occur in the ash of plants, but 

 only in minute quantities. 



A. The water in the plant. 



193. Necessity. — Since water forms such a large percent- 

 age of the weight of fresh plants, it is manifest that it must be 

 supplied in relatively large quantities, if the plant is to con- 

 tinue in an active condition. A portion of this water may 

 be used up in the chemical changes occurring in the body, 

 but it is not possible to discriminate between this and the 

 water which is necessary to furnish the proper physical con- 

 ditions of life. Water is the great solvent by which materials 

 of various kinds are carried into the plant body, and by 

 which a still greater variety within it are transported from 

 place to place. Before discussing the food of plants, there- 

 fore, the relation of water to the plant may be examined. 



194. Air, water, and land plants. — Some plants are not in 

 contact with water except at irregular intervals. These are 

 called air plants, and include some algae, liverworts, mosses, 

 fernworts, and seed plants. All these, however, are able to 

 live only in an atmosphere containing large quantities of 

 water vapor, or in those regions where they are frequently 

 sprayed with water. Water plants float upon the water, or 

 are submerged in it. As distinguished from both air and 

 water plants, are those which normally have the root system 

 and sometimes a portion of the stem buried in the soil, con- 

 tinually or intermittently in contact with liquid water, while 

 the shoot system is occasionally sprayed by rain. Such may 

 be called land plants. 



195. Solutions in water. — In no case, however, is the 

 water in which plants are immersed, or with which they 



