Chap. III. SUMMARY OF CHAPTER. 196 



But the most interesting evidence of the delicate 

 sensitiveness of the tip of the radicle, was afforded by 

 its power of discriminating between equal-sized squares 

 of card-like and very thin paper, when these were 

 attached on opposite sides, as was observed with the 

 radicles of the bean and oak. 



When radicles of the bean are extended horizon- 

 tally with squares of card attached to the lower sides of 

 their tips, the irritation thus caused was always con- 

 quered by geotropism, which then acts under the most 

 favourable conditions at right angles to the radicle. 

 But when objects were attached to the radicles of and 

 of the above-named genera, suspended vertically, the 

 irritation conquered geotropism, which, latter power 

 at first acted obliquely on the radicle ; so that the 

 immediate irritation from the attached object, aided 

 by its after-effects, prevailed and caused the radicle 

 to bend upwards,' until sometimes the point was 

 directed to the zenith. We must, however, assume 

 that the after-effects of the irritation of the tip by an 

 attached object come into play, only after movement 

 has been excited. The tips of the radicles of the pea 

 seem to be more sensitive to contact than those of the 

 bean, for when they were extended horizontally with 

 squares of card adhering to their lower sides, a most 

 curious struggle occasionally arose, sometimes one 

 and sometimes the other force prevailing, but ulti- 

 mately geotropism was always victorious ; neverthe- 

 less, in two instances the terminal part became so 

 much curved upwards that loops were subsequently 

 formed. With the pea, therefore, the irritation from 

 an attached object, and from geotropism when acting 

 at right angles to the radicle, are nearly balanced 

 forces. Closely similar results were observed with the 

 horizontally extended radicles of GuourUta ovifera, 



