K.— BOTANY 191 



point beautifully. When a branch is pruned it is the invariable rule that 

 the cut is made just above a bud, not just below one. As a result no 

 piece of stem is left projecting beyond the uppermost bud on the pruned 

 branch. The reason is that practical experience has shown that any such 

 projecting length of stem, above the influence of any bud, makes no further 

 growth but withers or rots into an unsightly ' snag ' — clear evidence that 

 cambial activity from the buds is only basipetal and that it cannot re- 

 commence in a region of the woody axis which has no living bud above it. 

 All forestry practice is really based upon this fundamental fact. When 

 growth starts in the tree, the buds in the light are moving first, and if 

 their growth is sufficiently vigorous, buds on lower branches shaded by 

 neighbouring trees may never resume growth. Such lower branches 

 fail to make any radial growth also and lose their supplies of water and 

 food to the vigorously growing regions of the crown and trunk. It is the 

 tacit recognition of this fact that underlies the system of close planting 

 to obtain straight-shafted timber — a system which meets a rather different 

 problem in the different branching habits shown by softwood as com- 

 pared with hardwood trees. 



Branching in Softwood and Hardwood Trees. — In the softwoods, as in 

 Abies, Picea, or young pines, the branching habit of the tree is usually 

 singularly regular, a whorl of branches starting each year from buds left 

 in the axils of some of the uppermost leaves on the shoot of the previous 

 year. In the spring, growth activity begins in all the apical buds of leader 

 and branches at about the same time. In the leader and the youngest 

 branches radial growth is thus stimulated and progresses down the stem 

 very rapidly and at about the same rate ; but in older branches, and 

 progressively as the branches grow older, the downward propagation of 

 cambial activity becomes slower and slower. As a result, when in the 

 main stem radial growth is already well advanced — because differentiation 

 of new wood, when once begun, proceeds very rapidly — the bases of the 

 lower branches joining the stem still show no signs of radial growth. The 

 new wood formed on the main stem runs downwards in a loop closely 

 encircling the previous year's wood of the branch, which runs radially 

 inward through it, the two tissues having no continuity at all. Later, 

 when new wood formation begins at the base of the branch the new wood 

 is continuous with the downward running elements of the new layer of 

 wood still forming on the axis. Still lower on the main axis, especially 

 when shaded by neighbouring trees, branches will be found in which the 

 radial growth of the branch does not recommence because the apical bud 

 does not grow. These lower branches lose their sap to the growing 

 trunk. Around the dry wood of such branch-bases flows increment after 

 increment of the wood formed on the main axis. But the branch is usually 

 set at an angle to the main stem ; each new layer of wood is forming from 

 above downwards, and expanding outwards with irresistible force against 

 the dried and brittle base of the branch, lifting up the dead bark which 

 clothes it, throwing this into folds around the base of the branch to which 

 the bark is firmly fixed, and ultimately straining the dry tissue so much 

 that the branch is broken off. New increments of wood still flow around 

 the base of the broken stump of wood until they cover it over completely. 



