A Theory of (jeotropisni. 61 
Since the action currents at the angles given in the diagram 
(Fig. 5) would be somewhat similar in strength (cp. Newcombe* 
14, for primary roots), and their effects would be neutralised by the 
same current strength (PD *5 in Fig. 5) from both upper and under 
surfaces; these angles would be the angles of balance and C would 
grow upwards and D downwards. If C had a downward angle the 
same as that of D it should theoretically grow downwards but it 
arises on the upper side and is thus directed upwards before it has 
developed the motor region which is necessary for geotropic 
curvature. 
Secondary and tertiary stems can be treated theoretically in 
the same way, always keeping in mind the point that the action 
current in the lateral branches and the normal polarity current in 
the main stem are to be considered as reversed in their direction 
and arrangement as compared with those in the root, so that the 
secondary laterals take up an upward angle of balance. The action 
of heliotropic stimulation interferes chiefly with the angles of the 
tertiary branches, because in these the geotropic action currents 
are weak compared to those of the main stem and secondary 
branches. 
The action of heliotropic stimulation becomes altogether 
predominant in the leaf, which may be regarded as neutral or non* 
polarised as far as gravity is concerned. Buds, flowers and fruits 
show gravity responses (13, etc.), but the responding region is 
usually the stalk which can be regarded as a secondary or tertiary 
stem structure. Food supplies also may interfere in such cases by 
altering the osmotic pressure, apart from permeability changes, as 
has been suggested previously (20, pp. 197, 295). 
Although this theory of geotropic stimulation and curvature in 
stem and root thus explains the chief points in the orientation of 
stems and roots in normal and in experimental conditions, and the 
orientation of their secondary and tertiary laterals, and although 
it is also in complete accord with the known facts of the physico¬ 
chemical phenomena of protoplasm and cellular tissues, it is 
recognised that a large field for experimental work is opened up in 
the verification of details and in the development of the new 
measure of control over the plant organism which is suggested. 
As a theory the suggested differentiation in hydrogen ion 
concentration of root and stem has very wide applications to 
cytolysis, acid-tolerant and calcifuge species, immunity from and 
liability to attack by bacteria and fungi, the development of 
