498 PRACTICAL BOTANY 



(3) A soil or other root foothold which does not retain water, 

 such as dry sand, bare rock surfaces, or the bark of trees. 



(4) A soil which contains a good deal of water, but is 

 physiologically dry, that is, does not yield water freely for 

 absorption by the roots. 



As regards (1) and (2), it is easy to see that regions like 

 some of the Arizona deserts, with only about a half inch of 

 rain during the entire year, can hardly support any perennial 

 seed plants other than extreme xerophytes. Countries like 

 those which border portions of the Mediterranean, with a 

 total rainfall for the year of 30 to 40 inches, but one which 

 for four months of summer sometimes falls as low as one 

 half inch, with a maximum daily summer temperature in the 

 sun of 130 F. and an intensely dry atmosphere, are better 

 suited to support xerophytes than mesophytes. 



A plant under the conditions mentioned in (3) may be del- 

 uged by violent rains during much of the year and yet in a few 

 hours after each rain be wholly cut off from any water supply. 

 Most lichens (Figs. 190-193 and 370) and many tropical epi- 

 phytic seed plants (Fig. 20), as well as our own Spanish moss 

 (Figs. 367 and 368), live under such conditions as these. 



It is difficult at first sight to recognize what is meant by 

 physiologically dry soils, such as are mentioned in (4). Ordi- 

 narily water cannot be absorbed from soils below a certain 

 temperature, which varies in different kinds of plants. Hence 

 soils in a frozen or nearly frozen condition are physiologically 

 dry, although much water may be present in the form of ice. 

 Soils -(or waters) containing much of the humous acids derived 

 from decaying vegetation are perhaps not physiologically dry, 

 but the acids which they contain are injurious to most plants 

 and it is necessary that species which are found in such sit- 

 uations shall be able to live without absorbing much water. 

 Hence many marsh plants, rooted in very wet mud, have xero- 

 phytic leaves. Finally, soils containing much saline matter 1 are 



1 As common salt, magnesium chloride, sodium carbonate, sodium sul- 

 phate, magnesium sulphate, or mixtures of these salts. 



