176 



(where the beiuliiig occurs) and in tlie case of the stem against the 

 one that is turned towards it. 



If in the cenlrifugal-experiment, tlie statolitlies are moved outwards 

 in the statocyst, then, tor the same reason as given above, the stem 

 must react by bending towards the centre, whereas the root will 

 curve away from it. 



h. Normal horizontally-growing plant-organs, such as rhizomes 

 and some rootlets of epiphytes, can only maintain their position of 

 ei|iiilihriuni and continue growing in the same direction if the 

 middle-field lies at the lowest longitudinal wall of the statocyst, for 

 the same reason again that it is only in this position that the starch- 

 grains will come in contact with this middle-field. Whether these 

 organs also attain their quickest growth in this position has still 

 to be investigated. 



c. Besides lying against the transverse and longitudinal walls, 

 the middle-field might also lie between the two, i.e. slanting; in 

 such cases the organ should also exhibit a slanting position of 

 equilibrium, the size of the angle it makes with the perpendicular 

 depending upon the |)Osition of the apparatus with regard to the 

 axis of the statocyst. This would explain the fixed position which 

 the lateral branches and lateral roots of the first order always 

 assume, and which is so different in different plants (cf. e.g. Araucavia, 

 the common foliage trees, Populus piramidalis). 



II. Variable position of the static apparatus. 



Various organs of plants undergo a change in equilibrium during 

 their normal development which could be ascribed now to a shifting 

 of the apparatus at a certain moment, that is to say, if it can be 

 demonstrated that gravitation-stimulus or longitudinal growth plays 

 a part in the phenomenon. 



The shifting may take place eithei' at a certain moment or he 

 continuous; moreover it may occur autonomously or as a result 

 of some outside cause. According to this we may distinguish the 

 following cases : 



a. The position of the apparatus changes, autonomously, at a 

 certain moment. 



During the germination of the seed of a twining plant the young 

 stem is at first vertical, but very soon the summit assumes a more 

 or less horizontal position and at the same time the twining com- 

 mences. It is possible that this transition from negative to transversal 

 geotropism were preceded by an autonomous displacement of the 



