EXCRESCENCES. 225 



are due to Exoasciis, in the latter to Aecidhuii, 

 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. GastropacJia 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 adven- 

 titious 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. 



Wood-nodules or Sphaeroblasts are curious marble- 

 like masses of wood which protrude with a cover- 

 ing of bark from old trunks of Beeches, etc., and 

 can be readily dug out with a knife. The nodule 

 has arisen by the slow growth of the cambium of 

 a dormant bud, the base of which separated at 

 an early date from the wood beneath ; the cam- 

 bium then closed in over the base and laid on 



p 



