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 Tillctia 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 euphorbias , 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 Peridermium 
elatinum (the aecidial stage of Melampsora cerastii ) 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 Uromyces rumicis, often 
“ retain their original green colour long after the unaffected 
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 
