THE DISEASES OF CONIFERS. 



137 



owing to the leaves of the Silver Fir dropping off in the second 

 year, or even earlier, instead of persisting for several years as 

 in the normal course of events. 



Another remarkable case is that of the " Witches' brooms," 

 very common in Europe, and by no means rare in this country. 

 I have myself found these on A. Fi?isapo, as well as on A. 

 jpectinata, in Windsor Great Park. 



"Witches' brooms" are curiously tufted masses of twiggy 

 branches, which take their origin from parts of the stem 

 attacked by the mycelium of Mcidium elatinum, the Uredo- 

 form of which is as yet unknown, and possibly does not exist. 

 The life-history was worked out very thoroughly by the late 

 Prof. De Bary.* The hyphse so irritate the growing tissues 

 of the young shoots that the latter gain enormously in dia- 

 meter, and put forth numerous shoots which alter their whole 

 character. Thus, instead of growing outwards in a nearly 

 horizontal plane, they turn vertically upwards, and branch 

 copiously in a fastigiate manner ; then their leaves are smaller, 

 and arranged in regular spirals round the erect twigs. These 

 leaves are infested by the mycelium, and eventually bear the 

 ^cidia, and fall prematurely. 



This mycelium is perennial in the cortex, cambium, and 

 wood of the stems, and does much damage by stopping the 

 leaders, and paving the way for rot-fungi. It happens not un- 

 frequently in this country that the mycelium simply sojourns in 

 the stems, and does not lead to the full development of the 

 Witches' broom," but only causes tumour-like swellings of 

 the axis. 



The treatment of infected trees resolves itself into careful 

 pruning and removal of the monstrous organs. It would be well 

 worth the time of some capable investigator to undertake further 

 researches into the nature of this disease. 



This malady, by the way, has nothing to do with the 

 " Witches' brooms " developed on Birches, Cherries, Hornbeams, 

 and other Dicotyledons, which are due to the ravages of various 

 species of Exoascus, curious ascomycetous fungi allied to the 

 one that causes " Bladder-plums." 



Farlow has found JEc. elatinum on Picea concolor (Gordon)']" 



* 'Bot. Zeitung," 1867. 



f "A Provisional Host-Index of the Fungi of the United States," 

 Part III., 1891, pp. 158-170. 



