CHAPTER XVII. 



SENSITITENESS AND lEEITABILITT OF PLANT ORGANS. 



General Illustrations — Peotoplasmic Ireitabilitt. — Having 

 now stated on what grounds I believe that the cohesions and 

 adhesions between them, as well as the forms of floral struc- 

 tures have arisen — namely, in response to the irritations set 

 up mainly by insect agencies, coupled with the effects of 

 nutrition, atrophy, hereditary influences, etc., — it will be 

 desirable to show briefly, not only how remarkably sensitive 

 almost all parts, both vegetative and reproductive, are to the 

 action of stimuli, but how they exhibit even visibly respon- 

 sive effects, both in the protoplasm of the cells and in the 

 tissues which are composed of them. 



The sensitiveness of living protoplasm is one of its most 

 marked and well-known phenomena. It exhibits changes in 

 its distribution within the cell as well as motions, which are 

 the direct result of external stimuli. These may be very 

 various, such as light, heat, electricity, or a merely mechani- 

 cal irritation, as well as organic and inorganic solutions. 



Of the effects of stimuli upon the protoplasm, some may 

 be beneficial, and partake of the nature of nutrition, as may 

 be witnessed in the protoplasmic " aggregation " of insec- 

 tivorous plants.* Very similar appearances follow electrical 



• See Darwin's Insectivorous Plants, fig. 7, p. 40. 



