ABSORPTION OF WATER BY AEROPHYTES 95 



however, inside a cell or water tube the water tends to sweep the 

 solutes along in its currents. 



It appears that so long as the osmotic conditions are good the 

 water can pass freely into the plant a,nd from cell to cell; but this 

 is not true of all solutes, for the plasmatic membranes will not let 

 all kinds pass, although many kinds are allowed to enter that are 

 apparently of no use. As has been said it is not understood just 

 how the membranes act in discriminating between the solutes; 

 but the size of the molecules of the latter apparently is not always 

 a decisive factor, for large molecules are known to enter when 

 smaller ones are held back. The plasmatic membranes are, how- 

 ever, subject to change and may at one time allow a certain sub- 

 stance to pass and not at another. 



Effect of Temperature of Soil, and Character and Amount 

 of Solutes upon Absorption. — A warm soil is conducive to 

 rapid absorption, while a cold soil hinders and may practically 

 stop it; although a perceptible amount of absorption may in 

 some cases still take place at or below o° C. The coldness of the 

 soils of arctic and alpine regions acting in this way has a very 

 great influence on the stunted growth and economical use of 

 water of the plants native to these places. 



If the soluble salts in the soil reach a concentration above .5 

 per cent, they hinder the flow of water into the plant either by 

 their influence on osmosis or by a poisonous effect on the absorb- 

 ing organs. Thus it comes about that plants along the strand 

 or in salt marshes have reduced transpiring surfaces correlated 

 with the slow intake of water, as seen in the Russian thistle and 

 garden asparagus, both of which are native to the salty soil of 

 .seacoasts. Humic acid also, which is formed by the decay of 

 vegetation in boggy places, retards the absorption of water enough 

 to call forth a reduction of the transpiring surfaces. 



Absorption of Water and Solutes by Aerophytes. — The 

 aerophytes, epiphytes, or air plants, send no roots into the soil, 

 nor parasitic roots into the plants on which they have found lodg- 

 ment; but they derive all of their supplies from the atmosphere, 

 which contributes its moisture in the form of rain or dew, pro- 



