440 TRANSFORMATION OF ENERGY 



at an angle of 135° with the vertical and pointing upwards, i. e, 45° above the 

 horizontal. [Several authorities, Fitting (1905) especially, have shown that 

 the optimum stimulus is given in the horizontal position.] In both types of 

 organ, however, it may be determined that in addition to the normal position 

 the inverse position is also a position of rest; certainly roots bend downwards 

 in a short time when inverted and shoots upwards. 



These reactions are, however, consequent on small curvatures induced 

 by internal factors, by which a deviation from the position of rest is brought 

 about. If a plant turned upside down be mechanically prevented from per- 

 forming any autonomous curvature a geotropic curvature never takes place as an 

 after-effect on the klinostat. There is always one noticeable difference between 

 the two positions of rest, the normal position is stable but the inverted position 

 is labile. Any organ which is inclined somewhat from the inverse position 

 does not bend back again into that position but attempts to assume the normal 

 direction. The only point we have to deal with here is the transference of the 

 stable into the labile position, and vice versa; we have only to alter the end by 



Fig. 136. Two slioots of Physos/egia, fixed horizontally in. wet sand and in a moist atmosphere, one (tin- right ) 

 by its base, the other (the leftl by the apex. Both show geotropic curvature. (Slightly reduced). 



which the plant is fixed (Frank, 1868 ; Noll, 1892). If we employ isolated 

 branches and fix them horizontally by their apices the reaction which takes 

 place is the same as that seen when the branch is fixed in the normal position, 

 that is to say, the side facing the ground exhibits growth acceleration, but the 

 further results are quite distinct, for the base bends upwards and reaches the 

 stable rest position in the inverted lie (Fig. 136). 



Any attempt to discover why a geotropic curvature follows when the plant 

 is in certain positions, while other positions may be described as positions of rest, 

 at once suggests the question, what is the initial effect of gravity on the plant ? 

 Researches which have been carried out during recent years show more and more 

 clearly that between the application of the stimulus and the movement a whole 

 series of processes takes place, whose existence is rendered especially prominent 

 if the application of the stimulus and the performance of the movement affect 

 distinct and widely separated parts of the plant. Under such circumstances at 

 least three different operations may be distinguished ; (i) the appreciation or 

 perception of the stimulus by the receptive or sensitive organ ; (2) the reaction of 



