350 PHYSIOLOGICAL BASIS OF POLARITY. I 



When the leaf is above (left side of Fig. 11) none of the buds in 

 the first node will grow out. If any buds grow out, they are either the 

 second or fourth on the lower side of the stem. These shoots grow out 

 with much delay compared with the growth of shoots in stems where 

 the apical leaf is on the lower side of the stem. 



The inhibitory influence is therefore greater when the apical leaf is 

 on the upper than when it is on the lower side of a horizontally sus- 

 pended stem. This influence of gravity supports the idea that it is 

 the sap sent out by the leaf which produces the inhibition. The 

 diagrammatic Figs. 12 and 13 make this clear. In Fig. 12 the leaf is 

 below and the path of the conducting vessels from the leaf is marked 

 by black lines. The two buds of the first node lie on the upper edge of 

 the sap flow containing the hypothetical inhibitory substances The 

 buds of the first node may or may not receive enough of these substances 

 to prevent their growth. When, however, the leaf is above (Fig. 13) 

 seepage frorti the vessels will cause the buds in the first and third 

 nodes to be flooded with the sap and the inhibitory substances con- 

 tained in it, thus preventing their growth. The lower bud in Node 2 

 (or Node 4) is outside the direct path of the conducting vessels of the 

 leaf and hence the lower bud of Node 2 as well as of Node 4 may de- 

 velop. Through the influence of gravity traces of the sap may pos- 

 sibly reach the lower bud of the second or fourth node. This may 

 account for the fact that growth of these buds is usually retarded. 



The reader will notice that these facts give us a neat method of re- 

 stricting the growth of shoots to the lower side of a horizontally sus- 

 pended stem, contrary to the general rule that in such cases shoots 

 arise on the upper side of the stem. When we remove the lower half 

 of such a horizontally suspended stem (containing one leaf at the 

 upper side of its apex) leaving on the lower side only the region of the 

 second node (Fig. 14), regeneration of a shoot will occur only from 

 the bud on the under side of this second node. The growth of the 

 buds in the intact upper half of the stem is completely suppressed 

 and the growth of the bud on the under side of Node 2 is slow for 

 reasons stated. No growth will occur on the upper side, except after 

 the leaf is wilted or conduction of its sap through the stem 

 is interrupted. 



