I INTRODUCTION 43 



by hairs or bristles exists, though it is not so general, 

 in "the vegetable as in the animal kingdom. 



Haberlandt has written several interesting memoirs 

 on what he calls the Sense Organs of Plants.^ Some 

 plants have papillae, which, projecting above the 

 general surface, localise and thus intensify the effect of 

 any pressure. In other cases there are special stiff 

 bristles supported by a cellular base, at the side of 

 which is a sensitive tissue, which is, of course, com- 

 pressed if the bristle is pressed against it, even only 

 slightly. In fact, the concentration of impression is the 

 main principle of these organs. The feeler bristles of 

 Aldrovanda were described by Cohn more than thirty 

 years ago. 



Sensitive organs are as yet known in a minority 

 of plants only ; perhaps because the majority have no 

 need for any such special arrangements. It is not, 

 however, too much to say that the potential power is 

 there, ready to spring into action if the need arises. 

 The sensitiveness of plants is indeed in some cases 

 astonishing. 



Darwin ^ showed that the tentacles of Drosera were, 

 affected by a tiny bit of woman's hair xtjItd" of an inch in 

 length, and weighing y^yws ^^ ^ grain ! — a weight 

 quite imperceptible to our nerves. 



The tendrils of climbing plants are amongst the 

 most sensitive organs in the vegetable kingdom. They 

 will react to a piece of cotton-wool or silk weighing- 

 only "0002 of a milligram, while the most sensitive part 

 of our skin would perceive nothing until the weight 

 reached "002 of a milligram. The sensitiveness of the 

 tendril is therefore much greater than ours. 



Many of the lower Algae have a red eye spot. It is 

 probable that the red substance is not the actual 

 perceptive element, but that this property rests rather 

 in the associated colourless plasma, and that the red 



^ His last memoir is "Die Sinnesorgane der Pflanzen," Ges. Deut. Natur- 

 forscher und Arzt, 1904. 

 ^ Insectivorous plants. 



