ABNORMAL TISSUES. 



135 



in the normal course of the stem-fibres at the junction of 

 the branch with the stem. The properties of wood that 

 depend on straightness of fibres are thus impaired. 



When the branch dies its base is still nourished for several 

 years and a callus is formed around it, as is the case with 

 any foreign body in the stem, such as a nail. The longer 



the interval before i 



the dead branch falls 

 from the tree, the 

 longer is the base of 

 the dead branch sur- 

 rounded by occlud- 

 ing tissue. The swel- I 

 ling of the wood 

 (between 3 and 4, 

 Fig. 51), causes a cir- 

 cular depression of 

 the bark round the 

 dead branch, which 

 remains moist for a 

 long time and li- 

 ens its decomposi- 

 tion. At length tin; 

 1) ranch breaks off 



I 



L 



Fig. 



rial section through a verticil 

 of a conifer. The knots, a a, firmly connected 

 with the wood, showing the beauty of sue^woocl ; 

 ft, a small intermediate knot that will fall out when 



the plank is dry. 



and falls, owing to 

 its weight acting as 

 by a lever on its 

 rotten base. If this 

 lever is shortened, 

 <?.</., by men break- 

 ing off dead branches, 

 for firewood,, a stump, or snag remains, all of which becomes 

 occluded gradually, to the deterioration of the timber. The 

 earlier this breakage of branches begins in a crop of trees, 

 the more knotty and inferior is the timber it produces. 



If a plank be taken from the region 1 3 (Fig. 51) of the stem 

 it includes the hard enclosed wood of the branch, which, being 

 specifically heavier than that of the stem, is liable to warp and 

 crack (Fig. 52 a). If the saw cuts between 8 and 4 (Fig. 51), 



