JUNE 149 



growth was the result of the operations of some species 

 of gall insect ; but it is not so. The true explanation is 

 given in a paper on ' Fungoid Pests of Forest Trees/ con- 

 tributed to the Royal Horticultural Society's Journal for 

 1905 by him whom, had we the figurative imagination of 

 Orientals, we should reverence as ' the Father of Toad- 

 stools 'Dr. M. C. Cooke. The cause of ' Witch's Broom/ 

 it seems, is not to be traced to an insect, but to a minute 

 fungus rejoicing in the sounding title Exoascus turgiduft. 

 The naked sacs containing the sporidia or germs of this 

 fungus appear in spring or summer upon the under 

 surface of the leaves, which curl up and fade, becoming 

 covered with a dry hoariness. Each sac has a stem cell 

 at the base, through which the mycelium, by means of 

 which the fungus absorbs and stores nutriment, pene- 

 trates the epidermis of the leaf. After it has exhausted 

 the supply of nourishment in the leaf, it invades the 

 stem and twig, setting up therein an irritation which 

 causes a dense growth of stunted twigs, which die in 

 time, though perhaps not for many years, and persist 

 so long as the tree lives, or until the branch falls. 



The common birch, as we know it, consists of two well- 

 known varieties, which Ehrhart and some other botanists 

 regard as distinct species. One of these, known to 

 nurserymen and gardeners as the weeping birch, is the 

 prevailing kind in Strathspey, and appears to be almost 

 immune from the attacks of Exoascus. It is certainly 

 far the more desirable of the two kinds of birch. 



To another fungus, Rhytisma acerinum, are due those 

 black spots on the leaves of the sycamore (the plane, as 

 we call it in Scotland). The parasite is so universally 

 distributed in British woodlands, that many people 



