K.— BOTANY tgj 



the strong lateral shoots the basipetal impetus to cambial activity moves 

 strongly down to the base of the shoot and on to the main stem, so that 

 the shoot is subsequently firmly bound to the main axis by differentiated 

 and lignified vascular elements common to them both. In weakly growing 

 shoots basipetal cambial activity is weak and carries over little, if at all, 

 into the main stem, to which this shoot has as a result but little lignified 

 vascular attachment. Then as the parent axis enlarges under the 

 vigorous impulse to radial gro^vth reaching it from the vigorous terminal 

 shoot and contributed to by the more vigorous branches above, the weaker 

 lateral twigs, as they continue to make little or no lateral growth, are forced 

 off by a perfectly natural process of abscission which leaves a clean scar 

 on the surface of the parent axis. These twigs thus gradually disappear 

 from below upwards, leaving in the leafy crown a scaffold of stronger 

 branches. 



Later in life, probably after thirty or forty years of vegetative growth, 

 flowering begins on lateral shoots in which only the apical bud continues 

 a relatively weak vegetative growth. The buds immediately below this, 

 and all other buds except perhaps a few of the most basal ones, develop 

 flowers, and these flower-producing buds contribute nothing to the 

 cambial activity of the axis ; in fact, as they draw food from it they 

 probably diminish the vigour with which the basipetal impulse to cambial 

 activity travels down the stem from the terminal bud. Such shoots 

 usually make a most inadequate vascular connection with the axis that 

 bears them. The parenchyma amongst the woody tissue is in excess and 

 contributes to a swollen base which, strained by the girth expansion of 

 the more vigorous axis it joins, is abscissed after some years of flowering, 

 even though the branch thus abscissed is fifteen to twenty years old. 

 In some species of poplars and oaks, in England, the ground beneath the 

 trees is thus carpeted each autumn with a crop of still fresh, sappy 

 branches, self-pruned from the distal branch system, the rounded bases 

 of the abscissed branches and the cup-shaped scars on the branches that 

 bore them bearing witness to the natural manner in which they have 

 separated. 



II. Structure. Cambial Growth and Vascular Differentiation 

 IN Softwood and Hardwood. 



It is clear that the branch system of the tree is mainly determined by 

 the close relation that exists between shoot growth and radial growth. 

 Only when the bud is still making extension growth and producing new 

 leaves will the woody stem beneath it increase in thickness and remain 

 a functional member of the woody crown. The slender twig still shows 

 evidence, in contour and leaf scars, of the series of nodes and inter- 

 nodes laid down during shoot extension, but in later years the smooth 

 addition of each new radial increment, spreading basipetally downwards 

 from new leafy shoots above and independent of any local influence, 

 gradually obliterates all trace of node and internode, whilst the repeated 

 cracking of the bark as the stem thickens may make it impossible to trace 

 the original leaf scars. This region of the woody shoot now forms an 



