16 INTRODUCTION [CH. 



and of the dandelion and other angiosperms by Synchytrium; in the latter 

 instance the cells surrounding the seat of infection are also enlarged. 



In many cases fresh cell formation as well as enlargement of the host 

 cells takes place; this may be limited to the neighbourhood of the infected 

 spot so that the host organ becomes locally deformed; thus "peach leaf curl" 

 and many similar abnormalities are formed by Exoascus and its allies; and 

 irregular rose-coloured blisters are produced on the leaves of Ericaceae in- 

 fected by Exobasidium. More elaborate deformations are produced by some 

 of the Ustilaginales, and, in the case of "witches'-brooms," by the rusts and 

 Exoascaceae. 



A witch's-broom is a bunch of modified twigs, caused usually by insects, 

 but sometimes by fungi. In the latter case it is the product of a lateral bud, 

 which, stimulated by the presence of the fungus, or by the food material 

 which the cells of the fungus deflect from its proper course, grows out to 

 form an abnormally dense bush of twigs; its leaves are produced earlier than 

 those of healthy branches and, even in the case of normally evergreen conifers, 

 are deciduous, falling off at the end of each season. Here the specialized 

 shoot, in spite of its contained parasite, appears to flourish, though to the 

 detriment of the rest of the tree; it may indeed be suggested that something 

 approaching symbiosis has been established, but in this relationship the 

 fungus is clearly the dominant partner 1 . 



SYMBIOSIS 



The physiological conditions under which the thallus of a lichen is built 

 up are somewhat similar; the algal cells appear healthy and are capable of 

 vegetative multiplication, but the fact that the fungus alone is concerned in 

 the development of the fructification sufficiently indicates its supremacy. 

 In the case of Thermutis orlutina the alga is devoured by the fungus in 

 preparation for the formation of the fruit. 



In mycorhiza, that is in the structures formed by the association be- 

 tween the mycelium of a fungus and the roots (or other organs) of one of the 

 higher plants, the advantages of the symbiotic relation often belong less to 

 the fungus than to its partner so that the vascular plant may actually become 

 dependent on its fungal associate and unable to develop in its absence. The 

 mycelium may be either endotrophic, forming a skein of branched and 

 interwoven filaments in the cells of the host and sending comparatively few 

 hyphae to the exterior; or it may be exotrophic, that is to say mainly ex- 

 ternal in development. 



Endotrophic Mycorhiza. An extreme case of obligate symbiosis has 

 been described by Rayner for the ling, Calluna vulgaris, which grows in 

 association with a fungus resembling the members of the genus Phoma in 



1 For bibliography, see p. 19. 



