352 Morphogenetic Factors 



the electron microscope show as to the character of such cell walls laid 

 down under pressure? 



Biicher found that when a growing shoot, negatively geotropic, is 

 forcibly kept horizontal the cells of the upper side are smaller and 

 thicker-walled than those of the lower. This phenomenon he called 

 geotrophism and explained it on the assumption that since the lower 

 side grows more rapidly (tending to bend the stem upward) it must 

 be under compression and the upper side under tension. To test the 

 relative effects of gravity and bending, he inverted a plant of Ricinus 

 and held its tip bent at 90° to the axis (and thus horizontal). Campto- 

 trophism and geotrophism should thus be brought into opposition. Actu- 





Fig. 16-5. Effect of vertical compression on hypocotyl of Ricinus. At left, young 

 seedling, protected by sheath and cotton plug, held in two blocks of plaster and there 

 subjected to compression from weight of plaster. At right, bast bundle from upper part 

 of such a hypocotyl ( above, under compression ) and from the normal control ( be- 

 low), showing increase in cell size and reduction in wall thickness as a result of com- 

 pression. ( From Biicher. ) 



ally, the plant reacted as it would to gravity, with smaller cells on the 

 upper, concave side. Perhaps in such a case it is the difference in degree 

 of tension that determines the result. 



Vochting ( 1908 ) placed the stem of a potted plant in a horizontal po- 

 sition and put a support under it at some distance from the pot. On the 

 free end he hung weights and observed a considerable increase in the 

 cross section of the stem, especially of its vascular system. This was most 

 marked near the point of support, where the strain was greatest, and on 

 the upper and lower sides. Growth of the stem in length had ceased, 

 so that these changes resulting from mechanical stimulation were evi- 

 dently in the secondary tissues. Haerdtl (1927) found similar results. 



