CHAP. III. SENSITIVENESS OF RADICLES. 129 



CHAPTER III. 



SENSITIVENESS OF THE APEX or THE RADICLE TO CONTACT AND TO 

 OTHER IRRITANTS. 



Manner in which radicles bend when they encounter an obstacle in 

 the soil Vicia faba, tips of radicles highly sensitive to contact 

 and other irritants Effects of too high a temperature Power of 

 discriminating between objects attached on opposite sides Tips of 

 secondary radicles sensitive Pisum, tips of radicles sensitive 

 Effects of such sensitiveness in overcoming geotropism -Secondary 

 radicles Phaseolus, tips of radicles hardly sensitive to contact 

 but highly sensitive to caustic and to the removal of a slice Tro- 

 pscolum Gossypiuin Cucurbita Raphanus JSsculus, tip not 

 sensitive to slight contact, highly sensitive to caustic Quercus, 

 tip highly sensitive to contact Power of discrimination Zea 

 tip highly sensitive, secondary radicles Sensitiveness of radicles 

 to moist air Summary of chapter. 



IN order to see how the radicles of seedlings would 

 pass over stones, roots, and other obstacles, which they 

 must incessantly encounter in the soil, germinating 

 beans (Vicia faba) were so placed that the tips of the 

 radicles came into contact, almost rectangularly or 

 at a high angle, with "underlying plates of glass. In 

 other cases the beans were turned about whilst their 

 radicles were growing, so that they descended nearly 

 vertically on their own smooth, almost flat, broad upper 

 surfaces. The delicate root-cap, when it first touched 

 any directly opposing surface, was a little flattened 

 transversely ; the flattening soon became oblique, and 

 in a few hours quite disappeared, the apex now point- 

 ing at right angles, or at nearly right angles, to its 

 former course. The radicle then seemed to glide in 

 its new direction over the surface which had opposed 



