22 



REACTION OF HOST TO PARASITIC ATTACK. 



of Other examples, since many of these will be referred to again 

 in other chapters, particularly when hypertrophy is under consi- 

 deration. 



§ 5. EFFECT OF PARASITIC FUNGI ON THE FOEM OF 

 THE HOST-PLANT. 



1. Arrest of growth, and Atrophy. While a large number 

 of fungi produce more or less extensive enlargement of parts 

 of their host, others cause arrest of organs, crippling, impoverished 



nutrition, and even atrophy of 



an extreme kind. Incompletely 

 developed organs of this kind 

 may originate even where the 

 fungus in possession produces 

 only local hypertrophy. In- 

 teresting examples are presented 

 by many species of SyTwhytrium, 

 {e.g. S. taraxacum and S. ane- 

 mones). The former is endo- 

 phytic in Taraxacum, and exerts 

 a stimulus resulting in increased 

 growth, not limited to the 

 single cell attacked, but ex- 

 tending to neighbouring cells, 

 partMatrophy which, in conscquence, multiply 

 and form a ring-like swelling 

 round it. The leaves as a 

 whole, however, are poorly developed, so that the lamina in 

 veiy extreme cases may be represented only by the midrib and 

 narrow margin (Fig. 4) ; while on leaves attacked on one side, 

 that side alone is stunted, the other is normal. Taraxacum 

 leaves badly attacked by Pucdnia are not at all deformed, 

 whereas those of Anemone show striking arrest of growth (Fig. 

 190). Leaves of Cirsium attacked by Pticcinia suaveolens exhibit 

 an arrest of the same kind, remaining less divided and of 

 softer texture (Fig. 186). 



Flowers affected by parasitic fungi present many striking 

 malformations. Magnus ^ describes such a case in Anemone 



^ Magnus, " Einfluss v. Parasiten auf d. Ausbildung d. befallenen Fflanzen- 

 theiles." Naturviiss. Rundschau, 1891, No. 25. 



Fig. 4. — Synchj/triwmtaraxaci. 

 of laminae of Taraxacum, officinale. About i 

 natural size. (t. Tubeuf phot.) 



