GEOTROPISM. 11 



447 



CzAPEK (1895) has also proved that no reaction takes place if the lateral root is 

 turned at right angles upwards or downwards, but if it be only a little removed 

 from these rest positions a curvature upwards or downwards, according to cir- 

 cumstances, takes place, which ceases when the limiting angle is again reached. 

 This latter position only is, however, the stable rest position, the other two posi- 

 tions must be regarded as labile. Rhizomes correspond to lateral roots so far 

 as regards the labile rest positions, but differ from them as to the stable rest 

 position, for that in the case of the root is directed obliquely downwards, 

 while in the rhizome it is horizontal. One would naturally expect that the 

 subaerial lateral organs, e. g. many flowers, lateral branches, would find 

 their stable rest positions when directed obliquely upward ; as a matter of 

 fact, branches turn back again to their oblique position if they be forced 

 upwards or downwards. We shall return to this point later, but meanwhile 

 we may note that many flowers, e. g. Narcissus pseudonarcissus, exhibit another 

 form of diageotropism (Vochtixg, 1882). The peduncle is bent over hori- 

 zontally on the orthotropous scape and if it be taken out of this lie and placed 

 pointing obliquely upward or vertical it returns to the horizontal once more ; 

 it is remarkable, however, that 

 each reaction ceases if the 

 flowers are directed obliquely or 

 directly downwards. 



While the stable rest posi- 

 tion of the orthotropic plant 

 organ is quite constant and is 

 coincident with the perpen- 

 dicular, we find that plagio- 

 tropic rest positions undergo 

 variations not only when 

 different organs are considered 

 but also in one particular organ. 

 Thus we meet with very marked 

 differences in a selected ex- 

 ample, which are due to internal 

 and ex ternal factors . There are 

 especially two internal factors, 

 which we cannot always keep 



distinct : those which depend on the influence of the state of development, 

 the ' ripeness ' of the plant, and those which depend on the relationships of 

 the parts to each other and to the whole (correlations). If we study the lateral 

 roots of a bean (Phaseolits) which have been grown in uniformly moist soil we 

 find that from above downwards they form successively the following angles 

 with the chief root, viz. 130°, 80°, 80°, 90°, 90", 65°, 75°, 75°, 40°. 



Apart from the individual peculiarities of single roots one notices a decrease 

 in the size of the angle as the apex of the chief root is approached. Still more 

 remarkable than these differences between lateral roots is the case where 

 a single organ in the course of time changes its reaction. Let us study the 

 development of the horizontal rhizome more closely, taking Adoxa as our 

 example. The seedling above the level of the cotyledons consists of an ortho- 

 tropic stem which reaches the light owing to negative geotropism, but later 

 on this stem bends back and buries itself in the soil. The shoot thus exhibits 

 a complete inversion of the normal geotropic reaction, since it now behaves like 

 a plagiotropic secondary root or an orthotropic but positively geotropic organ. 

 After it has reached a certain depth the rhizome takes a horizontal direction 

 and produces scale leaves, but it again comes to the surface, acting in a negatively 

 geotropic manner, when the formation of foliage leaves and flowering shoots 



Fig. 139. Formation of tlip. ihizome of Adoxa ■mnschatelUiia. 

 Alter A. Braun (Das Individuum, 1853, Plate II, Fig. 3). 

 cnt, cotyledon ; A', axis of tlie seedling. 



