184 



manner that when their tip receives the light lliej acquire positive- 

 geod'opisin and bend downward; wiien tiie tip pierces the ground 

 again and is tlius no longer iiiunained, tlie transverse-geotropisin 

 reappears 



Tiiese movements too may he explained in the simplest way 

 from our suppositions. In the normal rhizome, as we have seen 

 (page 6), the static apparatus should lie against the lowest longi- 

 tudinal wall; if, under the influence of light, the apparatus is displaced 

 again towards the source of the light, i. e. in the direction of 

 the apical transverse wall, the lip will have to bend downwards in 

 order that the starch-grains ma}' again reach the middle-field, and 

 (his is just the movement that we see the rhizome make. When 

 again in the dark the apparatus, and therefore the rhizome too, 

 will resume its former position, as in the preceding case. 



c. The sleep-movements of leaves, as will be known, are influenced 

 by light to such a degree that it was long believed that light alone 

 was the cause of them. Later, however, exhaustive I'esearches, in 

 particulai' tiiuse of PkkI''1'KK, showed that gravitation has also a share 

 in them. This has been most clearly demonstrated for instance in 

 the experiment with Phaseolus, in whicli the petioles of the two 

 tirst leaves were secured during the day in theii' normal position, 

 so that only the leaflets could make the sleep-movement. When 

 the plant was then turned upside down, the nyctitropic movement 

 took place at night, but showed exactly the reverse of what in the 

 normal position occurred, i. e. in the light the leaves now stood 

 vertically upright, whereas in the dark they were spread out 

 horizontally. Thus, vvitii respect to gravitation the leaves moved in 

 the same direction as before, with regaid lu light however in a manner 

 exactly contrary to the normal way, from which it is evident that 

 it was the gravitation in the first place which governed the nycti- 

 tropic movement and determined the equilibrium of the leaf. 



As practically nothing is known regarding the position of the 

 statocysts in the leaves (see page J 79), it is still difficult to express 

 here any opinion with respect to the eventual shifting which the 

 static apparatus might undergo here under the influeiu'e of light, 

 the more so because there are so many varieties of nyctitropic 

 movement. Important in this respect for an explanation in the 

 sense as meant here, however, is the fact that it proved the pre- 

 sence of the principal auxiliary, namely the static apparatus itself, 

 in leaves which show sleep-movement. 



(/. What has been written concerning the sleep-movements is 

 really also applicable to the movements which cause the leaves to 



