68 REGENERATION 



in the stem. Schimper has shown that the nitrites are present 

 in the leaf when the plant is kept in the dark, but that they dis- 

 appear in the light, provided that the leaf contains normal, 

 non-etiolated chlorophyll. Baudisch^ found that when an 

 aqueous solution of potassium nitrite and methyl alcohol is 

 exposed to ultraviolet light, formhydroxamic acid is formed. 

 Baly, Heilbron, and Hudson^ have found that solutions of 

 nitrite or nitrate containing formaldehyde also form form- 

 hydroxamic acid in the Hght. It is possible that from the 

 formhydroxamic acid amino-acids may be formed as suggested 

 by Baudisch as well as by Baly, Heilbron, and Hudson. The 

 nitrites are therefore carried in the ascending sap. On the other 

 hand, the sugars are synthetized in the presence of light from 

 CO2 in the leaves containing chlorophyll, and the sugars required 

 for the synthesis of the material for roots must be carried by the 

 descending sap. There exists then unquestionably a difference 

 in the chemical nature of the ascending and descending sap in a 

 normal plant exposed to light. The question is whether this 

 and other possible differences {e.g. of Ph) are the cause of the 

 polar character of regeneration in the stem or whether the polar- 

 ity exists regardless of such differences. To test this it is 

 necessary to study the influence of the leaf on the polar character 

 of regeneration, since the sap in the leaf contains all the sub- 

 stances (specific and non-specific) required for the growth of both 

 roots and shoots. This is no mere surmise, but is demonstrated 

 by the fact that from each notch of an isolated leaf both roots 

 and shoots can arise, and that the mass of shoots and roots pro- 

 duced varies in proportion with the mass of the leaf. This 

 leaves no doubt that the same tissue sap can give rise to entirely 

 different organs and that the cause of the difference must lie in 

 differences in the chemical and physical constitution of the cells 

 from which the different organs arise. It will be shown in this 

 chapter that the shoots or roots produced in a small isolated 

 piece of stem containing one or more leaves originate almost 

 entirely from the sap sent out by the leaf. If it is intended to 

 explain the polar character of the regeneration in this case on 

 the basis of a chemical difference of the sap sent out by the leaf to 

 the two opposite poles of the stem it will be necessary first to 



1 Baudisch, O.: Ber. d. d. chem. Ges., vol. xliv, p. 1009, 1911. 



2 Baly, E. C, Heilbron, I. M., and Hudson, D. P. : Trans. Chem. Soc, 

 vol. cxxi, p. 1078, 1922. 



