CHAPTER TWENTY-ONE 



THE FORMS AND STRUCTURES OF ROOTS 



In preceding chapters we have learned that leaves manufac- 

 ture food in the presence of light ; that their exposure to air facili- 

 tates the entrance and exit of carbon dioxide and oxygen ; and 

 that their efficiency is increased by being raised and displayed on 

 erect stems. 



Tall, erect stems with sufficient strength for the maximum 

 display of leaves are made possible by the development of 

 mechanical tissue in the stems. 

 The display of leaves high 

 above the water supply of the 

 soil requires a conductive sys- 

 tem capable of raising water 

 and mineral salts from the 

 roots and of permitting the 

 movement of food away from 

 the leaves. 



Stems, in turn, must be 

 firmly anchored, and they 

 must be supplied with water 

 and mineral substances from 

 the soil. 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, ac- 

 cumulation of food, respira- 

 tion, assimilation, and growth. 



Primary and secondary Fig. 102. stages in the development of a com 



roots. The root system of a ^^^^^i^g- -^ is the primary root, 5 a secondary 



'' root, and A an adventitious root from the base 



well-developed bean seedling of the stem. 



177 



