INTRODUCTION 7 



flower, and Kerner thought it possible that double-flowered 

 plants provided with many stamens, which have been long 

 grown in gardens, and which produce such flowers when 

 propagated by seeds, may have acquired this characteristic 

 in the first place under the influence of the mites. (See 

 notes in Chapter VII.) 



Fungus galls are caused by the presence of a parasitic 

 fungus in plant tissue. The fungus obtains entrance by 

 means of the spore (the equivalent of the seed in the higher 

 plants) which may be carried by wind currents or other 

 agency to the host, and, germinating thereon, pierces its 

 outer cells. In most cases entrance is probably more 

 easily effected in weakly plants, but plants that are to all 

 appearances quite healthy and vigorous may also be 

 attacked successfully. The results of infection are very 

 varied. The mycelium may permeate the host, produce its 

 spores only at a late stage, and cause no appreciable over- 

 growth or gall formation. In some plants atrophy may 

 result, as in Bent Grass infested with Tilletia decipiens; it 

 is much dwarfed and assumes the form known as pumila, at 

 one time regarded as a distinct variety. Occasionally the 

 entire habit of the plant becomes altered, as in the case of 

 Wood Spurge attacked by Endophyllum euphorbiae, when the 

 shoots are longer and the leaves shorter and wider than 

 those of normal plants. Modifications of habit are, however, 

 outside the scope of this book ; it is localized hypertrophy 

 alone that comes under our definition of a gall. In some 

 cases the overgrowth is slight, in others it is very marked. 

 The fusiform swelling caused by the presence of Peridevmium 

 elatinum (the aecidial stage of Melampsora cerasiii) in 

 branches of Silver Firs, is a good example of the latter. 



Plowright observed that the leaves of our native species 

 of Dock [Rumex), when attacked by Uvomyces rumicis, often 

 " retain their original green colour long after the unafiected 

 portions of the leaves have become yellow from age." 

 Retention of the chlorophyll is seen also in many leaves 

 bearing insect galls — e.g., the hairy tubular outgrowths 



