( (-^7 ) 



X. 



WITCHES' BROOMS. 

 By James Saunders, A.L.S. 



Jiead at S/. Alhans, 9th Jannnry. 1906. 

 PLATES IV-VI. 



The abnormal growths, consisting of dense tufts of twigs, 

 which sometimes occur on trees, are often assiuned to be badly- 

 constructed birds'-nests, and the resemblance in some cases is so 

 misleading as to require close examination to detect the dilf erence. 

 They are, however, always due to a diseased condition of the 

 plant affected, and although in the case of birch-trees the 

 general health of the plant is apparently not injured, yet in 

 the wild cherry large limbs are sometimes killed, and the organism 

 as a whole is evidently enfeebled. 



These growths are known as Witches' Brooms, Witches' 

 Besoms, and Witches' Knots, names that have probably 

 originated in the supposition that some occult influence, and 

 therefore malevolent, had given rise to the al>normal develop- 

 ments. The following agents are those by which these growths 

 are usually prodviced : — 



A. Parasitic fungi, such as Taphrina (^Exoascus) and 



^cidium. These are the most potent exciting causes. 



B. Phytophagi, and possibly other insects. 



C. Qnarling, i.e. an excessive development of leaf-buds on the 



branches or on the main stem ; an obscure disease, the 

 proximate cause of which is doubtful. 



The following notes refer chiefly to the instances which have 

 been observed in Beds and Herts, or more precisely within 

 a radius of ten miles from Luton. 



It is probable that some species of trees are affected by more 

 than one agency, for further particulars of which attention is 

 directed to the valuable work on ' Diseases of Plants induced by 

 Cryptogamic Parasites,' by Karl von Tvibeuf (English edition 

 by Dr. William G. Smith). In this work, on pp. 152 and 

 153, a list is given of " species (of Exoasceae) which cause 

 Witches" Brooms," and of which " the asci are produced on the 

 leaves." The eniuneration includes both foreign and British 

 deciduous trees which were known to be thus affected. They 

 are : Alnus incana (Alder) *Betula vermcosa (JMrch.) * B . pubescens, 

 B. odorata, *B. nana, *Carpinus Betulus (Hornbeam), *Prunus 

 Cerasus (Cherry) , *P. Avium (Wild Cherry) , *P. institia (Bullace) , 

 *P.domestica (Plimi), P . pennsylvanica, Acer platanoides (Maple), 

 ^sculus calif ornica (American Horse Chestnut). 



Those marked with an asterisk (*) are native in the British 

 Isles or have become naturalised. 



In the district under consideration the following species 

 bearing Witches' Brooms have been I'ecorded within the last 



