306 TEXTBOOK OF BOTANY 



shaped layers of tissue ( Fig. 125 ) . The innermost cone is formed by the 

 growth of the primary vascular cambium. The others are formed bv the 

 growth of a series of anomalous secondary cambiums, which may origi- 

 nate from parenchyma cells in the phloem or in the pericycle. The thick- 

 ened portions of turnips and conical radishes are largely xylem; but in 

 parsnips the tissues external to the xylem are most prominent. Increase in 

 thickness of bark, especially of roots of trees and shrubs, is partly the 

 result of the de\'elopment of cork from cork cambiums. 



Fig. 125. Concentric rings of tissues visible in cross sections of beet roots 

 formed by the development of a succession of secondary cambiums in pericycle 

 and phloem parenchyma. Photo by E. F. Artschwager, Journal of Agricultural 

 Research. 



The size and extent of root systems. The primary root of a seedling 

 continues its downward extension into the soil. Root hairs develop 

 within a few hours, and lateral secondary roots within a few days. If 

 the plant has a taproot system, the primary root continues growth for 

 weeks or months, and under favorable conditions may penetrate the soil 

 to a depth of 5, 10, 20, and even 30 feet. The roots of most plants, how- 

 ever, develop in the upper 3 to 5 feet of soil. In regions of abundant 

 rainfall and high water table, root systems in general are nearer the 

 surface than in regions subjected to periodic drought (Fig. 126). In 

 deserts there are many species of cacti with widespreading root sys- 

 tems less than a foot below the surface, and species of yuccas and desert 

 shrubs nearby with roots which penetrate deeply. 



