TREES AND SHRUBS 



sacs, are quite exposed, instead of being enclosed in a perithecium. The 

 asci arc developed in spring and summer on the under surface of the leaves, 

 which curl up, lose their fresh green colour, becoming sickly in appearance, 

 and covered with a velvety, greyish-white hoariness. The contained sporidia 

 are globose. 



Witches' Broom of Hornbeam is produced by Exoascus Carpi ni. 



Witches' Rroom of Pines {Peridermium {JEcidium) elatinum). This fungus 

 is parasitic on the Stone Pine and various species of Silver Firs. The 

 mycelium causes fusiform swellings or canker knobs, from which often arise 

 the deformed shoots, bearing pale green swollen needles. It is perennial in 

 the cortical and bast tissues of the stem, and even penetrates the cambium 

 and wood ; in the latter case giving rise to canker-swellings, with or without 

 the witches' broom. The leaves of the latter remain yellowish, and towards 

 autumn they bear two rows of whitish recidia or " cluster-cups " on their 

 underside. After the discharge of the Eecidiospores, the leaves die and fall 

 off. Each year the mycelium advances into the new shoots, and the 

 witches' broom increases in size, its growth continuing for perhaps twenty 

 years. In young woods all trees that show cankerous swellings should be 

 removed. 



lxxviii 





