THE CAUSES OF AUTONOMIC MOVEMENT 25 



movements in general is uncertain. Stahl's suggestion that they serve mainly to 

 favour transpiration is in the highest degree improbable l . 



Dutrochet 2 was the first to distinguish between spontaneous (autogenic) and 

 induced (aitiogenic) movements, and also held correct views as to the importance of 

 the external conditions as regards growth and movement. It is, however, often 

 forgotten that a movement can still be considered autogenic when, by the activity of 

 the plant itself, an external agency is used for purposes of orientation 3 . 



Methods. A hemispherical glass vessel, or a plane sheet of glass, may be placed 

 over the plant, and the position of the growing apex, as seen vertically above, marked 

 on the glass with indian ink, or an oil pencil 4 . This gives a projection of the move- 

 ment, but unless the growing apex and the glass are near together the error of 

 parallax becomes considerable. Photographs may also be taken in one or two planes 

 and afterwards compared 5 . Slight movements may be measured by means of a vertical 

 microscope containing a micrometer ruled in squares 6 . The attachment of a glass 

 thread is inadvisable, since this may readily produce disturbances of growth 7 . 



SECTION 5. The Causes of Autonomic Movement. 



It is often the case that a change in the properties of the organism 

 or in the irritability of certain portions may cause constant external con- 

 ditions to act as stimuli producing response. Thus if the geotropic 

 irritability of an organ alters from positive to negative, a corresponding 

 curvature will be produced in the growing zone. Diffuse stimuli may also 

 be utilized in the same way. Thus supposing that at a particular stage 

 of development one of two opposed tissues is excited to more active growth 

 by the existing temperature than the other, then we should have a curvature 

 produced although the temperature remained constant. Curvatures pro- 

 duced in this way under constant external conditions are spontaneous or 

 autogenic, those produced by variations in the external conditions are 

 aitiogenic. In the former case we have to decide whether a constant external 

 agency is made use of in the manner indicated for directive purposes, 

 or whether the stimulus is of purely internal origin 8 . The importance of 

 autogenic and aitiogenic changes of irritability for tropic movement will 



1 Pfeffer, Druck- und Arbeitsleistungen, 1893, p. 362 ; also Stahl, Bot. Ztg., 1897, p. 98. 



2 Cf. Sachs, Flora, 1863, p. 449. 



3 [If this were strictly applied, all physiological movements would be autogenic, since without 

 the activity of the plant, only the direct mechanical action of physical agencies could produce 

 movement.] 



4 Darwin, Climbing Plants, 1876, p. 86. 



5 Dewevre and Bordage, Revue ge"n. d. Bot., 1892, T. iv, p. 65. 



6 Fritzsche, Ueber die Beeinflussung d. Circumnutation durch verschiedene Factoren, 1899, p. 6. 



7 On th'e methods of magnifying the movement cf. Darwin, The Power of Movement in Plants, 

 1880, p. 5 ; Wiesner, Bewegungsvermogen, 1881, p. 158. 



8 Movements resulting from the accumulation of secreted products or from the growth of an 

 organ into a dissimilar medium may be classed as aitiogenic. 



