152 PLANT LTFE. 



sodium, chlorine, and silicon. r)f these se\en, the first four 

 are found in the ash of all ]ilants, 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 su( h a large percent- 

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

 supplied in relati\'ely large quantities, if the plant is to con- 

 tinue in an acti\e condition. A portion of this water may 

 be used up in the chemical clianges occurring in the bodv, 

 but it is not possilile to discriminate between this ami the 

 water which is necessary to furnish the proper ])lnsical con- 

 ditions of life. Water is the great solvent b\- which materials 

 of various kinds are carried into the plant l)odv, and by 

 which a still greater variety within it are transported from 

 place to i)lace. llefore discussing the ibod of plants, there- 

 fore, the relation of water to the pdant maybe examined. 



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

 contact with water except at irregular inter\-als. These are 

 called air ]jlants, and include some algre, li\'er\vorts, mosses, 

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

 live onlv in an atmos|)here containing large quantities of 

 water vapor, or in those regions where thev are frequentlv 

 spra\'ed u'ith water. Water plants float upon the M'atcr. or 

 are sulimergcd in it. .\s distinguislied from both air au'l 

 water ]ilants, are tliose «hich normalb" ha\e the root system 

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

 tinually or internnttently in contact with li(]uid water, ^\■hile 

 the shoot system is occasionally spra\ed bv rain. Such may 

 be called land jilants. 



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

 water in whi( li ]ilants are immersed, or with which thev 



