NUTRITION. 125 



sium, potassium, iron, 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. 



162. 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 

 continue 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 

 conditions of life. Water is the great solvent in which 

 materials of various kinds enter the plant body, and by which 

 a still greater variety within it are transported from place to 

 place. Before discussing the food of plants, therefore, the 

 relation of water to the plant may be examined. 



163. 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 algse, 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 have the root system (and some- 

 times a portion of the stem buried in the soil) continually or 

 intermittently in contact with liquid water, while the shoot 

 system is occasionally sprayed by rain. Such may be called 

 land plants. 



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

 water in which plants are immersed, or with which they are 

 sprayed, pure water. It always holds in solution substances 



