MOVEMENTS OF IRRITATION. 



517 



cotyls are bound to rods standing in the soil of the flower-pots. 

 The seedlings are now arranged so as to present the broad faces 

 of the cotyledons towards the window in front of which the box is 

 placed. After a time it will be found that both cotyledons have 

 placed themselves perpendicular to the incident rays of light. The 

 front one must in so doing have described an arc of 135 down- 

 wards, the back one an arc of 45 backwards. If we wished 

 to explain these movements as due to the action of gravity (and 

 other forces) we should have to make the at least unwarrantable 

 assumption that the geotropic irritability of the two cotyledons 

 of a plant is different. The following experiments, however, 

 serve well to prove that the light position of the leaf-blades can 

 be brought about solely by the action of light. 



We grow seedlings of Phaseolus in flower-pots. They are ready 

 for use when the primordial leaves have unfolded, but are still in 

 very vigorous growth. If we illuminate the plants from above, 

 then the leaves take the position indicated in Fig. 173. If now 

 we make the light fall not vertically from above, but horizontally, 

 and nearly vertical to the plane of insertion of the leaves, a torsion 

 of about 90 is effected by the pulvini of leaf-stalk and blade, and 

 the blades then stand 



once more at right Fig. 173. 



angles to the inci- 

 dent rays of light 

 (see Fig. 175). Now 

 plants with the 

 leaves in the posi- 

 tion represented in 

 173 are slowly 

 rotated in such a 

 way that their shoot 

 axes move in a ver- 

 tical plane, while 

 the light falls fairly 

 vertically to the 

 plane of insertion 

 of the leaves. The 

 effect of gravity is 

 eliminated, and the 

 epinasty of the pul- 

 vini of the leaf- stalk 



173 - 176 '- YoUD S ^aseolus Plants. (After Schwen- 



