CHAP. III. SUMMARY OF CHAPTER. 191 



sible, or even probable, that this tendency to reversion 

 may have been increased, as it is manifestly of service 

 to the plant. 



SUMMARY OF CHAPTER. 



A part or organ may be called sensitive, when its 

 irritatioii excites movement in an adjoining part. Now 

 it has been sho\vn in this chapter, that the tip of the 

 radicle of the bean is in this sense sensitive to the 

 contact of any small object attached to one side by 

 shellac or gum-water ; also to a slight touch with dry 

 caustic, and to a thin slice cut oft* one side. The 

 radicles of the pea were tried with attached objects 

 and caustic, both of which acted. With Phaseolus 

 multiflorus the tip was hardly sensitive to small squares 

 of attached card, but was sensitive to caustic and to 

 slicing. The radicles of Tropaeolum were highly sen- 

 sitive to contact ; and so, as far as we could judge, 

 were those of Gossypium herbaceum, and they were 

 certainly sensitive to caustic. The tips of the radicles 

 of Cucurbita ovifera were likewise highly sensitive to 

 caustic, though only moderately so to contact. Ea- 

 plianus sativus offered a somewhat doubtful case. 

 With ^Esculus the tips were quite indifferent to 

 bodies attached to them, though sensitive to caustic. 

 Those of Quercus robur and Zea mays were highly sen- 

 sitive to contact, as were the radicles of the latter 

 to caustic. In several of these cases the difference in 

 sensitiveness of the tip to contact and to caustic was, 

 as we believe, merely apparent ; for with Gossypium, 

 Raphanus, and Cucurbita, the tip was so fine and 

 flexible that it was very difficult to attach any object 

 to one of its sides. With the radicles of ./Esculus, 

 the tips were not at all sensitive to small bodies 

 attached to them ; but it does not follow from this 



