THE ROOT FUNCTION AND STRUCTURE 



45 



FIG. 48. THE MARK- 

 ING OF THE STEM 

 AND ROOT. 



cells forming the root-cap are older and 

 tougher and are suited for pushing 

 aside the soil that the root may pene- 

 trate it. 



Region of most Rapid Growth. The 

 roots of a seedling bean may be marked 

 at equal distances by waterproof ink or 

 by bits of black thread tied moderately 

 tight. The seedling is then replanted 

 and left undisturbed for two days. 

 When it is dug up, the region of most 

 rapid growth in the 

 root can be deter- 

 mined. Give a reason why a root 

 cannot elongate throughout its length, 

 whether there is anything to pre- 

 vent a young root from doing so. 



In Fig. 48 is shown a germinating 

 scarlet runner bean with a short root 

 upon which are marks made with 

 waterproof ink ; and the same root 

 (Fig. 49) is shown after it has 

 grown longer. Which part of it 

 did not lengthen at all ? Which 

 part lengthened slightly ? Where 

 is the region of most rapid growth? 

 Geotropism. Roots turn to- 

 ward the earth, even if the seed 

 is planted with the micropyle up. 

 This phenomenon is called posi- 

 tive geotropism. Stems grow away 

 from the earth. This is negative 

 FIG. 49. THE RESULT. geotropism. 



