32 RELATIONS OF PLANTS TO MECHANICAL FORCES 



dinal channels down opposite sides, by means of a small saw, and 

 examine the tissues of the enclosed portion with those of an un- 

 treated part of the stem. * 



47. The Influence of Curvatures upon the Origin and Arrange- 

 ment of Secondary Roots. Fill a root-cage consisting of a narrow 

 vessel with glass slides with sand or loose porous soil. Put a 

 number of germinating beans, or seeds of lupine on the surface of 

 the soil in such manner that the roots will pass down through the 

 soil near the glass sides. After the roots have attained a length 

 of 5 cm. adjust the root cage so that it will be tilted at an angle 

 of 45 degrees, and allow it to remain in this position until a fur- 

 ther growth of equal amount has been made, then tilt in the oppo- 

 site direction until the same elong- 

 ation has been made. Continue 

 this alternation of the position of 

 the cage until several curvatures 

 have been made, and numbers of 

 secondary roots have been formed. 

 The main roots will be found to 

 have an undulating outline as a 



result of geotropic curvatures. 

 FIG. 15. Root- cage filled with sand 

 or sawdust Note the position of the secondary 



roots with respect to these cur- 

 vatures. These organs will be found to have arisen on the con- 

 vex surfaces of the roots alone. Determine the radius of curva- 

 ture of the main root. If a number of tests are made it will be 

 possible to determine the radius of curvatures necessary to pro- 

 duce this special arrangement of the secondary roots. 



This special response of the root to the form in which it is 

 placed is due to the possession of a form of irritability which en- 

 ables the organism to control its external form or stature. The 

 suppression of the secondary roots on the concave side of the 

 main root and their accelerated development is not a direct re- 



1 Newcombe, F. C. The regulatory formation of mechanical tissue. Bot. Ga- 

 zette, 20 : 441. 1895. 



