Ch. V, 4] 



ABSORPTION BY HOOTS 



225 





progress in plants, such reactions rarely occurring except 

 in solution. Fifth, it provides a medium of transport, 

 in form of solution, for substances through the plant. Sixth, 

 it is needed to compensate the incessant loss by transpira- 

 tion. These are the /^ 

 reasons why plants 

 must have plenty of ^ 

 water. 



The water used by 

 ordinary plants is 

 wholly a b s o r b e d 

 through their roots, 

 and none is taken 

 through leaves or 

 stems. Further, the 

 actual absorption is 

 known to take place 

 in the young parts of 

 roots, and mainly 

 through the root hairs. 

 The hairs are thus 

 effective, not through 

 any special power de- 

 nied to other cells of 

 ^:the young root, but 

 simply through the 

 great surface they 

 spread. It is because 

 these hairs, tightly 

 adherent to the soil, 

 are mostlj' torn away 

 when roots are lifted 

 from the soil, tliat 

 plants commonly wilt 

 on transplanting, and 

 recover only after 



Fk;. 106. — A plan of a root as an absorbing 

 in(?chauisin, arranged as in Fig.s. 11 and 105, 

 with similar signs for water, protoplasm, and 

 sugar. At the tip the growing point; at the 

 left, pith ; a duct ; two rows of corte.x ; the root 

 hairs. Note that hairs and cortex contain 

 protoplasm and sugar, but the duct contains 

 neither. 



