ROOTS AND RHIZOIDS 



493 



The influence of external factors upon absorption. — The amount of 

 absorbed water decreases as the soil becomes desiccated, because of the 

 increasing concentration of the soil solutions. Similarly, the absorption 

 of water and solutes is reduced greatly at low temperatures, frozen soils 

 being physiologically as dry as those of deserts. In both dry and frozen 

 soils the root hairs even may exude water instead of absorbing it. Plants 

 differ widely in regard to absorp- 

 tion, s me carrying on their ac- 

 tivities in spite of the almost 

 perpetually low temperatures of 

 polar soils. High temperatures 

 favor maximum absorption, if 

 the water supply is adequate. 

 Another important factor is the 

 degree of concentration of the 

 medium, strong solutions of 

 sodium chlorid and similar salts 

 greatly retarding absorption ; 

 probably it is for this reason that 

 salt marsh plants often have an 

 inadequate supply of water.' 

 Absorption seems also to be 

 difficult in peat bogs, though the 

 reason is less obvious. Pos- 

 sibly the presence of deleterious 

 substances in the bog water is 

 a factor of importance. 



Soil exhaustion and root ex- 

 cretions. — The migration of root 

 hair zones facilitates the invasion 

 of new areas by growing roots. 



Fig. 705. — Root hairs of a lettuce seed- 

 ling (Lactuca sativa)^ which have developed 

 in the soil; note that the hairs are somewhat 

 sinuous outgrowths (thus contrasting with 

 air-grown root hairs) from the lower portion 

 {i.e. toward the root tip) of the epidermal 

 cells, and that not all epidermal cells give 

 rise to hairs; note also the attached soil 

 particles (s) ; highly magnified. 



' It is important to note, however, that the osmotic pressure of the cell sap of plants 

 varies widely with the habitat, being least in submersed hydrophytes and greatest in 

 xerophytes and in salt marsh plants. The sap of salt marsh plants may have a 

 pressure of twenty atmospheres, as against four or five atmospheres in pond aquatics; 

 hence, the former may be as able to absorb water from a concentrated solution as are 

 the latter from a dilute solution. Many desert plants, especially shrubs, have a pressure 

 of thirty-five to one hundred atmospheres, and thus are better able than are other plants 

 to utilize the scant water of dry soils. Furthermore, it has been demonstrated that in 

 the cell sap of various salt water plants, the osmotic pressure varies considerably from 

 time to time, corresponding with similar variations in the medium. 



