360 ~ PHYSIOLOGICAL BASIS OF POLARITY. I 



the stem, since it can be shown that if once a shoot is caused to grow, it 

 increases with the mass of the leaf, no matter whether the shoot is sit- 

 uated at the base or the apex. The leaf has a powerful inhibitory effect 

 upon the development of basal shoots. If we assume this inhibitory- 

 influence to be due to inhibitory substances we must further assume 

 that not more than mere traces of these inhibitory substances reach 

 the apex which are not suflEicient to interfere with the growth of shoots. 

 At the moment we cut out the piece of stem from a plant the stem con- 

 tains throughout a sufficient quantity of these inhibitory substances 

 to prevent shoots from growing, and these inhibitory substances will 

 continue to flow in the descending sap towards the base of the stem. 

 The most apical buds in the stem will hence be the first ones to become 

 sufi&ciently free from inhibitory substances to be able to grow and the 

 regeneration of shoots will start at the apex of the piece of stem. 

 As soon as the shoots are beginning to grow at the apex they in turn 

 act like a leaf so that now the further growth of shoots at the base 

 is permanently inhibited. On the other hand, the influences which 

 inhibit shoot formation at the base are associated or identical with 

 influences favoring root production. Hence the leaf will favor root 

 formation at the base of the stem and shoot formation at the apex. 

 This gives an idea how the leaf may contribute by its "internal secre- 

 tion" to the establishment of the polar character of regeneration. 



If it could be shown that plants possess a closed circulatory system 

 comparable to that of animals, all these facts might become easily 

 intelligible if we assume that inhibitory substances for shoot formation 

 (and favorable substances for root formation) are carried in the de- 

 scending sap from the leaf to the root, where they are retained or 

 altered, so that the ascending sap becomes practically (but not abso- 

 lutely) free from these substances and contains only the nutritive 

 material for the formation of shoots. 



The assumption that the inhibitory influence of a leaf upon shoot 

 formation in the basal part of a stem is due to inhibitory substances, 

 is not without analogy in biology. It is known that when twins in 

 cattle have different sex the female is in the majority of cases sterile, 

 and Lillie^ has shown that there exists an exchange of blood between 



6Lillie, F. R., /. Exp. ZooL, 1917, xxiii, 371. 



