224 



A TEXTBOOK OF BOTANY 



[Ch. V. 4 



stems. Indeed, except for the relics of their earlj' root 

 anatomy deeplj^ buried within their tissues, and their some- 

 what greater compactness of texture, such roots are stems, 

 both structurally and physiologically, despite their under- 

 ground position. 



4. The Absorption of Water, and Other Functions 

 OF Roots 



Typical roots perform one primary function, viz. absorp- 

 tion of water and mineral matters ; one secondary function, 



\dz. anchorage for 

 the stem ; and one 

 or two minor func- 

 tions supplemen- 

 tarj' to these. 



Water is the 

 most necessary of 

 all the materials 

 absorbed by plants, 

 in which it is used 

 for six purposes. 

 First, it forms an 

 essential constit- 

 uent of the photo- 

 synthetic food 

 (page 21). Second, 

 it constitutes a 

 large proportion of 

 the composition of 

 plants, amounting 

 (as, shown by com- 

 parative weighings 

 of fresh and dried material) to more than 90 per cent in 

 most herbaceous parts. Third, it holds the soft parts tensely 

 spread by high sap pressure within the cells. Fourth, it is 

 a necessary solvent for the many chemical reactions in 



Fig. 165. — Typical root hair, and cortical cells, 

 in a longitudinal section of Radi.sh. (After a wall 

 diagram by Frank and Tychirch.) 



