_Burrs_ or _Knauers_ are irregular excrescences, principally woody, with gnarled and warted surfaces. They are frequently due to some previous injury, such as the crushing or grazing of cortical tissues by cart-wheels. The excitation of the tissues thus wounded results in the development 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 are due to _Exoascus_, in the latter to _Aecidium_, fungi which are perennially parasitic in the shoots, and stimulate the twiggy development of a number of buds which would normally have remained in abeyance, or not have been formed at all, and only do so now in a fashion different from that of normal branches. Rosette-like formations, depending on similar disturbing causes on the part of insects, occur in conifers--_e.g._ _Gastropacha Pini_. Dense tufts of twiggy shoots may be developed on many trees by pruning in such a way as to stimulate the shooting out of basal buds which would otherwise remain dormant, _e.g._ Elm, Ash, and thus it occurs that injuries such as frost, insect bites, etc., may induce the production of such tufts in a tree crown. The dense nests of stool-shoots thrown up from felled tree-stumps are of essentially the same nature--partly adventitious and partly dormant buds being enabled to grow out because they can now be supplied with materials previously carried beyond them while the trunk was still there. Suckers, if repeatedly cut down, may also behave similarly.