204 TEXT-BOOK OF FUNGI 



and only reveals itself during the last stage of the life of 

 the host, as a sooty mass of spores in the anthers, ovary, 

 etc. As a rule, when an ear of oat is smutted, all the grains 

 are destroyed, but sometimes a few grains escape and 

 develop in a perfectly normal manner, thus proving that 

 notwithstanding the presence of the parasite from its 

 birth, the host has been able to run its normal course of 

 life, even to the production of perfect seed. Obviously 

 an infected oat plant has to work a little harder than an 

 uninfected one, as it has to provide food for its parasite 

 as well as for itself. This condition of things appears to 

 be a leading up to symbiosis, falling short owing to the 

 fact that there is no obvious benefit derived from the union 

 by the host, but the reverse ; hence the parasite is as yet 

 dominant. 



In the remarkable growths often present on the branches 

 of trees, more especially conifers, but also on cherry, birch, 

 etc., known as 'witches' brooms' or 'birds' nests,' the 

 suggestion of symbiosis is yet more evident. Witches' 

 brooms possess some common characteristics. When a 

 branch of a tree is infected, most frequently by some 

 member of the Uredineae, the stimulation exercised by 

 the parasite results in an excess of food being concen- 

 trated at the point of infection. This leads to the forma- 

 tion of a dense mass of branches, themselves usually 

 copiously branched, being produced at this point and 

 constituting a witches' broom, which may equal in size and 

 appearance a crow's nest, and in many instances where such 

 growths are of considerable age this size is much exceeded. 



The general habit and mode of life of a witches' broom 

 is in all cases very different from those of the parent plant 

 from which it springs. Its branches are always vertical 



