290 ADAPTATIONS AMOxNG ALPINE PLANTS 



sias, Linn., which is a very abundant plant in the 

 Alps, especially in dry places. The healthy shoots are 

 much branched, and bear numerous large umbels of 

 complicated inflorescences which resemble flowers, 

 the golden-yellow bracts of the inflorescence simulat- 

 ing a corolla. Frequently among the clumps of this 

 plant we shall find small unbranched shoots, bearing 

 pale leaves, sometimes slightly larger than the leaves 

 of the healthy plant, and often rust coloured on the 

 lower surface. These shoots never bear flowers. A 

 dense, little rosette of leaves is often found at the 

 apex. These are shoots speciahsed to a parasitic 

 Fungus {Uromyces pisi), allied to the Rusts of 

 wheat, which lives within them, and thus the parasite 

 is for the most part localised to such shoots, and 

 does not spread indiscriminately through the whole 

 plant. 



A familiar instance of response to disease is the 

 production of galls. These may be caused either by 

 plant invaders or by parasitic animals, chiefly insects. 

 The tissues of the part of the host plant attacked by 

 the parasite, be it a leaf or be it a branch, are pro- 

 foundly modified by the presence of the invader, and 

 stimulated to renewed, and often excessive, growth, 

 and the formation of new tissues or organs of unusual 

 shape and size, which are called galls. The well- 

 known "Eobin's Pin-cushions" on Rose leaves, and 

 the *'Oak Apples" on Oak twigs, are two common 

 examples out of many thousands of varieties of 

 galls, due to insect attacks. Galls of various forms 



