154 SENSITIVENESS OF THE EADICLE. CHAP. Ill 



curve upwards, this movement will be opposed by geo- 

 tropism acting only at a very oblique angle, and the 

 irritation from the card will be strengthened by its 

 previous action. We may therefore conclude that the 

 initial power of an irritant on the apex of the radicle 

 of the bean, is less than that of geotropism when 

 acting at right angles, but greater than that of geo- 

 tropism when acting obliquely on it. 



Sensitiveness of the tips of the Secondary Radicles of the 

 Bean to contact. All the previous observations relate 

 to the main or primary radicle. Some beans suspended 

 to cork-lids, with their radicles dipping into water, had 

 developed secondary or lateral radicles, which were 

 afterwards kept- in very damp air, at the proper low 

 temperature for full sensitiveness. They projected, 

 as usual, almost horizontally, with only a slight 

 downward curvature, and retained this position 

 during several days. Sachs has shown* that these 

 secondary roots are acted on in a peculiar manner by 

 geotropism, so that if displaced they reassume their 

 former sub -horizontal position, and do not bend verti- 

 cally downwards like the primary radicle. Minute 

 squares of the stiff sanded paper were affixed by 

 means of shellac (but in some instances with thick 

 gum-water) to the tips of 39 secondary radicles of 

 different ages, generally the uppermost ones. Most 

 of the squares were fixed .to the lower sides of the apex, 

 so that if they acted the radicle would bend upwards ; 

 but some were fixed laterally, and a few on the upper 

 side. Owing to the extreme tenuity of these radicles, 

 it was very difficult to attach the square to the 

 actual apex. Whether owing to this or some other 

 circumstance, only nine of the squares induced any 



* ' Arbeiten I3ot. Inst., Wiirzburg,' Heft iv. 1874, p. 605-617. 



