174 



DISEASES OF TREES 



infected at a, but which, with the part of the crown situated 

 above the point of infection, only succumbed in its eighty-fifth 

 year. The top of a diseased tree is specially liable to die during 

 a dry warm summer, when the wood, having for the most part 

 undergone resinous degeneration, is unable to allow sufficient 

 water to pass to compensate 

 for the rapid evaporation from 

 the crown. 



For the most part aecidia 

 are formed only in that re- 

 gion of the cortex which has 

 become diseased during the 

 preceding year. They break 



FIG. 104. Portion of the stem of a 

 young pine showing the vesicular 

 aecidia of P. Pint corticola break- 

 ing through the cortex. At three 

 places which are darker shaded 

 spermogonia are situated beneath 

 the periderm. Natural size. 



FIG. 105. A pine-branch which has been 

 infested by P. Pint corticola for several 

 years. The branches on the left side, 

 which were the first to be attacked, are 

 already dead. From these the mycelium 

 has spread backwards on to the main 

 branch and the other lateral branches. 

 One fifth natural size. 



through the outer dead cortical layers in the months of May and 

 June as hemispherical, oblong, sausage-shaped, yellowish white 

 vesicles filled with reddish yellow powdery spores (Fig. 104). 

 Amongst these one recognizes with difficulty the flat spermo- 

 gonia which are about the size of a pea. These consist of 

 innumerable fine sterigmata which are situated between the 

 deepest periderm-layer and the living cortex, being arranged at 

 right angles to the former. The small spermatia are abjointed 

 fronV the apex of these sterigmata. 



Branches and twigs of the crowns of the older classes of trees 



