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James Small . 
obtained. Proceeding as before a number of experiments were 
carried out, with the result that the stems grew down in the acid 
atmosphere as shown in Figs. 6, 8 to 11, Plate I. 
These experiments may be taken, therefore, as further 
preliminary confirmation of hypothesis 6 in so far as the stem or 
coleoptile is concerned. The stem has proved to be more 
satisfactory in the readiness with which it responds to an acid 
atmosphere, and because fungi do not grow so rapidly under such 
conditions. It is^obvious that further confirmation is desirable and 
experiments are in progress. 
Fio. 5. Diagram to show secondary and tertiary roots. 
Theoretical Conclusions. 
After these experiments there seems to be nothing 
unreasonable in considering that the stem is of an “ alkaline ” 
nature with electronegative particles in the disperse phase of the 
plasma membrane and that the root is of an “ acid ” nature with 
electropositive particles. 
Explanation of Orientation of Secondary and 
Tertiary Roots and Stems. 
The previous theories of curvature apply only to main stems 
and roots. It is known that secondary roots grow at a more or less 
definite downward angle to the vertical under geotropic stimulation, 
while tertiary roots grow more or less in all directions. Secondary 
and tertiary stems grow in much the same way, although the 
