484 The Physiology of Plants BOOK in 



known insectivorous plants, exhibits sensitiveness to 

 contact and to chemical stimulation. The edges of the 

 leaves curl over towards the centre and pour out a secretion 

 resembling that of Drosera when an insect crawls upon 

 their upper margins. A similar differentiation to that of 

 Dionaea is observable. Darwin found that a mechanical 

 stimulus causes the infolding of the leaf-edge, while stimula- 

 tion by a drop of solution of ammonium carbonate causes 

 only an outpouring of the viscid digestive juice. 



The possession of contact irritability was shown by 

 many observers to be possessed by the lowlier plants. 

 Mention may be made of observations by Wortmann 

 in 1887, Errera in 1884, De Bary in 1886, Ward in 1888, 

 Biisgen in 1893, Borge in 1895, and Goebel in 1900, which 

 show it to be shared by both fungi and algae. 



An observation made by Frank in 1872 may be mentioned 

 here. The petioles of various aquatic plants, whose leaves 

 normally float on the surface of the water, show a curious 

 modification of their growth as the height of the water 

 varies, elongating as much as is necessary to save the leaf 

 surface from submersion. This seems to be a form of irrita- 

 bility, showing response to the strain of traction. There 

 is, however, some doubt about this interpretation, as Frank 

 found that the increased growth can be induced by covering 

 the leaf with a thin film of water, avoiding any strain. 



CHEMOTROPISM AND CHEMOTAXIS 



The tentacles of Drosera, the leaves of Pinguicula, and 

 those of Dionaea were observed by Darwin in 1875 to 

 respond to stimulation caused by placing on their surfaces 

 certain chemical substances, such as carbonate of ammonia. 

 The completion of the movements and the secretion of 

 a true digestive juice was found to be largely influenced by 

 the absorption of some of the products of protein decom- 

 position. The power of appreciating chemical stimulation 



