THE LICHENS 291 



frequently occur on the leaves of Alpenroses, the 

 Alpine Currant, and many other plants. 



In other cases galls are produced by the stimulus 

 of parasitic fungi. Galls of this nature may often be 

 observed on the leaves of the Alpenroses, A large 

 spongy body, spherical or oval or somevrhat irregular in 

 shape, arises from a portion of the leaf, and may grow 

 almost to the size of an apple, though it is frequently 

 much smaller. This gall is of a pale yellow colour, 

 often tinted with rose on the side turned towards the 

 sunlight. These galls have been called Alpenrose 

 Apples, The Fungus which stimulates these new 

 formations is known as Exohasidium rhododendri. 



The Lichens, 



It is often imagined that the large Lichens, especi- 

 ally the greyish-green Beard Lichen ( Usnea barbata) 

 (Plate XLIV,, Fig. 2), which festoon the branches of 

 the Spruce and other Conifers in Alpine Switzerland, 

 are parasitic upon theso trees. This is not, however, 

 correct. The Lichens simply attach themselves to the 

 trees, to which they do little or no harm. Many plants, 

 especially in the tropics, though much more rarely in 

 temperate climates such as that of Western Europe, 

 are what is called epiphytic. That is to say, while 

 they attach themselves to other plants, they do not live 

 parasitically at their expense. They neither penetrate 

 nor destroy their tissues, nor rob them of their 

 food-supply. The attached plant remains entirely 

 self-supporting. 



