128 FOUNDATIONS OF BOTANY 



this, with other causes, may force them to develop and to 

 grow into branches. 



Sometimes the tree altogether fails to produce buds at 

 places where they would regularly occur. In the lilac the 

 terminal bud usually fails to appear, and the result is con- 

 stant forking of the branches. 



139. Adventitious Buds. Buds which occur in irregu- 

 lar places, that is, not terminal nor in or near the axils of 

 leaves, are called adventitious buds ; they may spring from 

 the roots, as in the silver-leafed poplar, or from the sides 

 of the trunk, as in our American elm. In many trees, for 

 instance willows and maples, they are sure to appear after 

 the trees have been cut back. Willows are thus cut back 

 or pollarded, as shown in Plate II, in order to cause them 

 to produce a large crop of slender twigs suitable for 

 basket-making. 



Leaves rarely produce buds, but a few kinds do so when 

 they are injured. Those of the bryophyllum, a plant allied 

 to the garden live-for-ever, when they are removed from 

 the plant while they are still green and fresh, almost always 

 send out buds from the margin. These do not appear at 

 random but are borne at the notches in the leaf-margin and 

 are accompanied almost from the first by minute roots. 



Pin up a bryophyllum leaf on the wall of the room or 

 lay it on the surface of moist earth, and follow, day by day, 

 the formation and development of the buds which it may 

 produce. 



This plant seems to rely largely upon leaf-budding to 

 reproduce itself, for in a moderately cool climate it rarely 

 flowers or seeds, but drops its living leaves freely, and from 

 each such leaf one or several new plants may be produced. 



