SUPPLEMENT 137 



the principal treatises are : BACH (1907) ; CZAPEK (1906) ; DARWIN (1903, 

 1904 a, 1904 b) ; FITTING (1905, 1907) ; Gius (1905) ; HABERLANDT (1902, 

 1903, 1905, 1906 a, b) ; JOST (1902) ; LINSBAUER (1907) ; NMEC (1902, 1904, 

 1905) ; NOLL (1902, 1905) ; SAMUELS (1905) ; SCHRODER (1904) ; TISCHLER 

 (1905) ; ZACHARIAS (1905). Were we to attempt to state the pros and cons 

 in full detail, we should have to devote a whole lecture to the subject. 



The change in position of the starch-grains is not the only change which 

 has been observed in geotropic organs, for NEMEC (1901) has shown that 

 peculiar rearrangements take place in the protoplasm of geotropically stimu- 

 lated cells, but this phenomenon can scarcely be regarded as a direct effect 

 induced by gravity. Chemical as well as histological research on this subject 

 has led to certain results. G. KRAUS (1880, 1884) was the first to show that 

 the amount of sugar present in the under side of horizontally laid shoots at 

 first increased, while the amount of acid decreased ; later on both decreased. 

 Neither phenomenon occurs in absence of oxygen, so that they may perhaps 

 be connected with the process of curving, and not with the perception of 

 gravity, for the perception may take place in absence of oxygen. CZAPEK 

 (1898, 1906) has recently attempted to show that a phenol derivative accumu- 

 lates in geotropically stimulated root apices, the identification of which with 

 homogentisinic acid has at least not been confirmed (ScnuLZE, 1907). The 

 accumulation of this substance may arise from the fact that its destruction 

 under normal conditions by an enzyme is inhibited by an anti-enzyme, but the 

 fact itself is as yet not perfectly established (comp. FITTING, 1907), and its 

 interpretation opens up still greater difficulties. Since this same substance 

 increases also after phototropic and hydrotropic stimulation, it is obvious 

 that it can have no special relationship to the perception of the gravitational 

 stimulus. Again, it can have nothing to do with the curving, for it takes 

 place above all in apices which remain straight. 



445, Lecture XXXV is XXXIV in 2nd German Edition. 



446, 1. 15, for bent read directed 



447, 1.12, for branches read many branches 



448, 11. 28-9, for especially is evidence . . . compensated read further, 

 PORTHEIM'S (1904) results do not altogether support WIESNER'S view. 



1. 36 P. 450, 1. 16, for A corresponding experiment . . . basal nodes 

 read SACHS has carried out a corresponding experiment on the root. As in 

 other correlations so here also it is seen that an alteration in the lateral organs 

 takes place not only after removal of the chief axis, but also after inhibition 

 of its functions. Thus the principal root is replaced by a lateral one when 

 the growing point of the former is encased in gypsum (BRUCK, 1904). 



The question now is whether the plagiotropism of the lateral branches is 

 an inherent character of these branches, or whether the rest position is the 

 result of two directive forces, so that, though naturally orthotropic, they are 

 diverted from the perpendicular by influences emanating from the chief axis. 

 The influence of the chief axis is most clearly demonstrated when geotropism 

 is excluded by rotation on a klinostat. Under these conditions the lateral 

 root does not assume casual and irregular orientation in relation to the chief 

 axis, but forms a definite angle with it known as the ' special angle '. In roots 

 this ' special angle ' is, in general, greater than the limiting angle induced 

 by positive geotropism : the lateral roots, that is to say, when on the klinostat 

 stand out more at right angles from the primary root. The limiting angle 

 observed when the roots are subjected to the influence of gravity is not 

 constant, but may be reduced by employing greater centrifugal force (SACHS, 



