DIV. II 



PHYSIOLOGY 



341 



horizontal position (No. 1). The. growth 

 in the stem of the seedling is strongest 

 just below the cotyledons, and gradually 

 decreases towards the base. The curva- 

 ture begins accordingly close to the coty- 

 ledons, and proceeds gradually down the 

 stem until it reaches the lower, no longer 

 elongating, portions. Owing to the down- 

 ward movement of the curvature, and 

 partly also to the after effect of the 

 original stimulus, the apical extremity 

 becomes bent out of the perpendicular 

 (No. 7), and in ^this way a curvature in 

 the opposite direction takes place. For 

 two reasons this excessive curvature must 

 again diminish (13-16); the stem is now 

 exposed to another geotropic stimulus in 

 the opposite direction to the first, and this 

 is combined with a tendency to straighten, 

 which is termed AUTOTROPISM ( 10 ). 



Every geotropic curvature flattens out or dis- 

 appears when the plant, before full growth has 

 taken place, is caused to revolve on the klinostat. 

 Since in this case the geotropic stimulus is want- 

 ing, some other cause must underlie the straighten- 

 ing. It appears, in fact, that every change in 

 the condition of curvature of an organ, whether 

 resulting from geotropism or from some other 

 cause, acts as a stimulus. The plant works 

 towards a restoration of the original condition, 

 and this tendency is termed autotropism. An 

 organ which was originally straight thus tends 

 by autotropism to return to this condition when 

 curved in any way, either by growth or by 

 mechanical bending. Similarly a curved organ 

 tends to regain its original form when this has 

 been for any cause lost. 



In some cases negatively geotropic curvatures 

 may take place in full-grown ( I01 ) shoots, i.e. in 

 such as no longer exhibit growth in length when 

 not geotropically stimulated. Thus in woody 

 stems and branches the growth in length of the 

 cambium of the lower side may bring the organ 

 into the erect position as a result of geotropism. 

 The greater the resistance of the parts which 

 have to be passively bent the more slow and in- 

 complete will this response be. The so-called 

 nodes of grasses, which in reality are leaf-cushions, 



FIG. 278. Different stages in the pro- 

 cess of geotropic movement. The 

 figures 1-16 indicate successive 

 stages in the geotropic curvature of 

 a seedling grown in semi-darkness : 

 at 1, placed horizontally ; at 16, 

 vertical. For description of inter- 

 mediate stages see text. (After 

 NOLL. Diagrammatic.) 



