(i(K> 



SENSORY SYSTEM 



have been tested from this point of view though to a much slighter 

 extent. The very marked geotropic sensitiveness of the root-tip which 

 was first discovered by Ch. Darwin is obviously a very useful property 

 in view of the conditions to which roots are ordinarily exposed ; it enables 

 the root to return as quickly as possible to its stable position, when it 

 is diverted from its natural line of progression through the soil by the 

 opposition of solid obstacles. 



According to Nemec's well-founded view, it is the central portion 

 of the root-cap (the so-called " columella ") which always contains 



Fig. 247. 



Median L.S. through the root-cap of an adventitious root of Roripa amphibia 

 [Nasturtium amph&biwm]. All the starch-grains in the columella are resting on the 

 (physically) lower cell-walls. After Nemec. 



numerous falling starch-grains (Fig. 247) that represents the actual 

 geotropic sense-organ of the root. This columella is shaped roughly 

 like a truncated cone, and consists of several parallel or slightly 

 divergent cell-layers arranged symmetrically on either side of the 

 median plane. The individual cells are often slightly elongated ; apart 

 from the youngest, which are still in the meristematic condition, they 

 all contain numerous falling starch-grains. In many plants {e.g. in 

 certain Conifers and Crtjciferae, in the aerial roots of Monstcra deliciosa, 

 etc.), the columella is sharply marked off from the rest of the root-cap ; 

 in other cases the adjoining root-cap cells also contain some falling 



