354 PHYSIOLOGICAL BASIS OF POLARITY. I 



Correlation between Inhibitory Effect of a Leaf on Shoot Production and 

 the Opposite Effect on Root Production in a Stem. 



The writer has already called attention to the correlation existing 

 between the inhibitory effect of a leaf upon shoot formation and the 

 opposite effect on root formation. This is expressed among others in 

 Fig. 15. The stems on the left side, with a full size leaf at the apex, 

 formed a considerably larger mass of roots in the same time than the 

 stems on the right whose leaves are reduced in size. The larger the 

 apical leaf the greater the mass of roots produced by the basal part 

 of the stem in the same time and under equal conditions; and the 

 greater the inhibitory effect upon the shoot production in this part of 

 the stem. 



A striking demonstration of this correlation is given in the upper 

 row of drawings in Fig. 16. One-half of the leaf and one-half of its 

 petiole were cut off. The leaf was at the base of the stem which 

 contained only one node in front of the leaf. Generally only one of the 

 two buds in the node situated apically from the leaf grew into a shoot; 

 namely, the one on that side where half of the leaf was removed. 

 The growth of the bud on the side where the half leaf was preserved 

 was retarded or suppressed. At the basal end of the stems roots 

 developed, but at first only on that side of the stem where the leaf 

 was preserved.^ Hence the leaf behaves as if it sent out, in addition 

 to the material needed for regeneration, substances retarding shoot 

 formation and favoring root formation. 



In the lower row of stems (Fig. 16) the leaf was preserved at the 

 apical end of the stem. In this case the inhibitory effect of the half 

 leaf on shoot formation is much greater than when it is at the base 

 of the stem (upper row). Most of the stems in the lower row have 

 not yet formed any shoots, but where a shoot was formed (as in 

 Illfl) it was formed on the opposite side from that where the half 

 leaf was preserved, while the roots were on the same side with the leaf. 



In Fig. 16 the half leaf was above. In Fig. 17 the half leaf was 

 always below. The result in Fig. 17 is the same as in Fig. 16, i.e., 

 on the side where the half leaf is preserved the shoot formation is 

 always retarded compared with that on the other side. The inhibi- 

 tion is more complete when the leaf is at the apex (lower row) than 

 when the leaf is at the base (upper row). 



■* Later on roots may form on both sides of the basal end of the stem. 



