Abnormal Branch of Prunus Avium. 
BY 
A. W. BORTHWICK, D.Sc., 
LECTURER ON FOREST BOTANY IN THE UNIVERSITY 
OF EDINBURGH. 
With Plate LII. 
Witches’ Brooms are of fairly common occurrence in our 
woodlands, parks, and policies. Their twiggy, compact crow’s 
nest-like appearance readily attracts attention. They cannot 
by any means be called ornamental, and in the majority of cases 
their presence disfigures the natural beauty of the tree. 
These abnormal growths are caused by various agencies. 
Often they are due to the attack of a fungus or an insect. The 
irritation set up by the invading organism at the point of attack 
stimulates the tissues in the immediate vicinity to very rapid 
and abnormal development, which results in formation of the 
hypertrophies called Witches’ Broom In many cases the exact 
nature and cause of the malady has been definitely ascertained, 
but there still remain a good many “Brooms” about which 
practically nothing is known. In fact, the list of woody plants 
which bear Witches’ Broom appears as yet to be anything but 
complete. This list has, however, been considerably added to 
within recent years as the result of an article by Professor 
Tubeuf,! in which he pointed out that nothing was known as to 
the nature or cause of Witches’ Broom which he had observed 
on the following species—beech, spruce, Scots pine, Weymouth 
pine, larch, elm, silver fir (a different one from that caused by 
A&cidium elatinum), Robinia, Prunus spinosa, poplar, and others. 
1 Tubeuf in Naturwissenschaftliche Zeitschrift fur Land und Forstwirtschaft, July 
1904, p. 295. 
{Notes, R,B.G., Edin., No. XX, March 1909.] ' 
