THE DISEASES OF PLANTS 289 



its needs. Thus the gi-avity of the attack is often 

 minimised, and the disease restricted to local areas. 



The well-known "Witches' Brooms," frequently 

 seen on Cherry, Spruce, and many other trees, afford 

 an excellent example of abnormal growths in response 

 to the attack of a Fungus. These shoots are always 

 quite erect, however drooping the ordinary branches 

 may be. They arise from diseased buds which grow 

 abnormally. Such shoots never produce flowers or 

 fruits. They are branches specialised for the fungus 

 in which the parasite finds a home suited to its needs, 

 and thus the disease is locaHsed, and does not spread 

 throughout the whole plant. 



Diseased organs are thus frequently dissimilar in 

 appearance to healthy structures. This is often well 

 seen in the case of the House-leeks {Sempervivum) 

 (see p. 98). If we examine the rocks in summer-time, 

 in a locality where these plants are abundant, we 

 shall often find that some rosettes occur in which the 

 leaves are either very pale or red in colour. They 

 are also distinctly larger, as well as three or more 

 times as long as those of the healthy rosettes {cf, 

 Plate XXIV., Fig. 2, with Fig. 1 of same Plate. These 

 rosettes are attacked by an internal parasitic Fungus, 

 known as Endophyllum sempervivi, and in the large 

 size of the diseased leaves we see an adaptation to 

 the invader. 



Another excellent example of dissimilarity between 

 a healthy and a diseased plant may bo frequently 

 observed in the case of a Spurge {Euphorbia rj/paris- 



T 



