INJURIES INDUCED BY PLANTS . 181 



originated without any connection with witches' brooms (Fig. 

 109), and on such swellings no formation of spores ever occurs. 



More frequently infection occurs on or in the immediate 

 neighbourhood of a bud. When the bud proceeds to grow it 

 forms a young witches' broom that is to say, a shoot whose 

 cortex is stimulated to growth by the advancing mycelium, and 

 whose young leaves are so affected by the parasite that they 

 remain small, are somewhat round in cross-section, and show 



FlG. in. Branch of a silver fir, with a witches' broom two years old, a. The 

 mycelium in advancing through the tissues of the branch has so stimulated a 

 dormant eye that it has developed into a shoot a year later, b. The portion of 

 the branch that has been invaded by the mycelium is much swollen. 



scarcely any chlorophyll. They remain yellowish, and in the 

 beginning of August two rows of secidia appear on their under 

 side, which open and shed their spores in the end of the 

 month (Fig. in). Soon afterwards the leaves die and fall off. 

 The witches' broom is consequently deciduous. Each year 

 the mycelium advances into the new shoots, to produce the 

 phenomena already described. The twigs of branches which 

 show this peculiar symbiosis * anastomose abundantly, and for 

 the most part incline upwards, so that they appear amongst 

 the ordinary branches of the silver fir like perfectly independent 



* [Symbiosis signifies the fact that two organisms are living together a 

 dual existence.] 



