147 



HHitcJw § rooms. 



By C. R. Stbaton, F.E.S. 

 [Head at the Salisbury Meeting of the Society, 1896.] 



URIOUS plant structures, which go by the name of witches' 

 brooms, are frequently seen growing on Birch, Abele, 

 Hornbeam, and Silver Fir. They are not unlike bird's nests, or 

 bunches of mistletoe ; they are, however, not parasites like mistletoe, 

 but distorted parts of the tree itself. When a Birch tree is affected 

 a bud will be found here and there larger and looser than the 

 others ; if the loose scales be shaken off it will be seen that the 

 contained shoot is stunted and a circle of buds surrounds the un- 

 developed central bud. Each bud of this circle undergoes the same 

 development, without waiting for the returning seasons of growth, 

 and crop after crop is thrown out until the work of five or six years 

 has been crowded into one. The leaves and shoots dwindle, but 

 the woody base goes on increasing. If one of these brooms be 

 tapped gently over a sheet of paper a number of small gall-mites 

 may be shaken out. These phytopti are not, properly speaking, 

 insects, but belong to the same class as spiders. Their cylindrical 

 bodies are J- of an inch in diameter, and they have four short legs 

 placed close to the head. Their eggs are found under the scales of 

 the bud. It is the influence of the phytoptus that produces this 

 rapid bud formation, and as a result an enormously increased supply 

 of food for its young. I need not enumerate the many trees which 

 gall-mites tuft in this way. Sometimes the flower bud only is 

 attacked, and many of those flowers that " run back" to green leaves 

 owe this peculiarity to the presence of gall-mites. Witches' brooms 

 are not, however, always due to animal interference. The Silver 

 Fir bears brooms of great size which are due to the influence of a 

 cluster cup fungus (Peridermium elatinum). All the twigs forming 



