CHAPTER FOURTEEN 



THE STRUCTURES AND PROCESSES OF ROOTS 



In preceding chapters we have learned that leaves manu- 

 facture food in the presence of hght ; that their exposure to 



air facilitates the entrance and 

 exit of carbon dioxid and oxy- 

 gen ; and tliat by being raised 

 and displayed on erect stems 

 their efficiency is increased. 



The support of leaves by 

 stems makes necessary the de- 

 velopment of mechanical tissue 

 in the stems. The display of 

 leaves high above the water 

 supply of the soil requires a 

 conductive system capable of 

 Fig. 95. Dand.'iion plant, showing the raising watcr and mineral salts 



primary tap root and its branches, the fj-Q^l the rOOtS and of Carrying 



secondary roots. r i r i i 



food away from the leaves. 

 Consequently plants that expose great numbers of leaves to 

 the hght must develop large and strong stems. The stems, 

 in turn, must be firmly anchored in the soil, and they must 

 also be supplied with the water and mineral substances that 

 press up through them to the leaves. Anchorage and ab- 

 sorption are the particular functions of roots, though they 

 carry on other processes also, such as conduction of water, 

 transfer of food materials, accumulation of food, respiration, 

 and growth. 



Classification of roots. The root of a well-developed bean 

 seedling will show the essential features of roots. There is 

 the primary root, extending downwards from the base of the 



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