170 



Science, of Plant Life 



in the death of the outermost layers of the root and the 

 formation of a bark very similar to that bf tree trunks. 



Absorption. The 

 passage of water and 

 other substances into a 

 body is called absorp- 

 tion. Of all the pro- 

 cesses that take place 

 in plants, absorption is 

 the one most commonly 

 associated with roots. 

 In addition to water, 

 roots absorb the min- 

 eral substances found 

 in plants, and they 

 absorb a part, or all, 

 of the oxygen needed 

 for respiration by their 

 own cells. Water and 

 other substances not 

 only enter the external 

 cells of a plant by ab- 

 sorption, but they pass 

 from cell to cell within 

 the plant by the same 

 process. A dead root 

 in the soil may take 

 up water and become 

 saturated ; but only a 

 living root can absorb water rapidly enough to furnish an 

 adequate supply to the living parts above the soil. The 



Epidennis 



irtex 



Ibod-con- 



ductine 



tissue 



Water-con- 



ductinS 



vessel 



Growing 

 point 



RDot cap 



Fig. 99. Diagram of a root tip, showing the 

 tissues and their arrangement. 



