.MOVEMENTS OF IRRITATION. 453 



similar ones with Vicia seedlings have not been sufficiently 

 numerous to justify this. In experiments with Vicia Faba, more- 

 over, I found that during the first twenty-four hours the decapi- 

 tated roots grew just as actively as the intact ones. The various 

 questions here under consideration must not as yet, according to 

 my own experience, be considered solved. The latest work of 

 Pfeffer I have been obliged to leave out of consideration. 



In going through the literature respecting the function of the 

 root tip one is surprised at the number of contradictions in the 

 statements as to its significance in longitudinal growth and 

 geotropic curvatures. It appears to me that many of the experi- 

 menters in their investigations have not paid sufficient attention 

 to certain considerations. (1) It is possible that the root tip has 

 not the same importance as regards the growth and geotropism 

 of the root in different species of plants. (2) It is necessary, in 

 order to attain trustworthy results, always to work with a large 

 number of objects. (3) It is not a matter of indifference whether 

 we remove lengths of 1, 1'5, or 2 mm. from the end of the root, 

 since frequently, when the decapitation is inconsiderable, the 

 whole of the root tip, by which, according to Darwin, the stimulus 

 is conveyed, will not be removed. (4) In prolonged investiga- 

 tions, attention must be paid to processes of regeneration taking 

 place in the decapitated root. 3 (5) With reference to the nuta- 

 tion of roots (see 157), we have to lay stress on the position in 

 which the intact and decapitated roots are disposed. 



1 See Darwin, Movements of Plants. 



- See Wiesner, Bewegungsvennogen der Pflanzen, 1881"; Detlefsen, Arleiten 

 <L botan. Inst. in Wiirzburg, Bd. 2 ; Kirchner, Programm. zur 64. Jahresfeier 

 der Akadcmie Holicnheim, 1882 ; see further statements by Krabbe, Molisch, 

 Wiesner, and Brunchorst, in the first and the second Jahrgang of the Berichte 

 der Deutschen botan. Gesellschaft. 



1 In this connection see Prantl, Arbeiten d. botan. Inst. in Wilr.burg, Bd. I. 



176. Experiments with the Clinostat. 



The clinostat, which was introduced by Sachs, 1 is one of the 

 most important instruments for making investigations in plant- 

 physiology. It enables us to prevent heliotropic and geotropic 

 curvatures in plant structures. 2 An excellent form of clinostat 

 was designed by Pfeffer, and this can be obtained at a price of 



