224 DISEASE IN PLANTS. 



of shoots from adventitious or dormant buds at 

 the base of old tree trunks, or in the starting of 

 the same process where a branch has been broken 

 off. The new bud begins to develop a shoot, but 

 soon dies at its tip owing to paucity of food- 

 supplies to the weak shoot, while new buds at its 

 base repeat the process next year with the same 

 result, and each of these again in turn, and so on. 

 The consequence is an extremely complex nest 

 of buds, all capable of growing in thickness and 

 putting on wood to some extent, but not of 

 growing out in length. In course of time this 

 mass may attain dimensions measurable by feet, 

 forming huge rounded and extremely hard- 

 knotted burrs, the cross-section of which shows 

 the vascular tissues running irregularly in all 

 directions, and, owing to the very slow growth, 

 extremely dense and hard. The dark spots in 

 such sections e.g. Bird's-eye Maple are the cut 

 bud-axes all fused together, as it were. On old 

 Elms such burrs are common at heights on the 

 stem which preclude the assumption of any coarse 

 mechanical injury, and similar structures occur on 

 the boles of other forest trees suddenly exposed 

 to light by the felling of their companions, which 

 suggests that these epicormic shoots result from 

 some disturbance due to the action of light. 



Witches' Brooms are irregular tufts of twigs often 

 found among the branches of trees such as Birches, 

 Hornbeam, etc., where they look like crows' nests, 

 and similar structures are to be found on Silver 

 Firs and other conifers. In the former case they 



