152 



DISEASES OF TREES 



of their concave surface, a small roundish aperture is formed. 

 Numerous perithecia, with very small tubes and hyaline spores, 

 form in the dead cortex underneath the lower surface of the 





FiG.Sy. The upper left-hand figure represents a portion of a branch of Alnus viridis. 

 The periderm has been ruptured at four places by the stroma of Valsa oxystoma, 

 on which the necks of the perithecia are visible. To the right are seen the asci 

 and spores. The upper right-hand figure represents a piece of a smaller branch 

 whose periderm has also been ruptured by the stromata, but on which the peri- 

 thecia have not yet developed. The central figure shows the microscopic section 

 of one of the stromata of the upper left-hand figure, and the lowest figure a similar 

 section taken from the branch shown in the upper right-hand corner. 



lenticular tubercles. The perithecia push a long stout flask- 

 shaped neck through the lenticular stroma. On account of the 

 black necks finally protruding to some extent from the stroma, 

 each tubercle shows a considerable number of them. 



