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 FORM OF 



THE HOST-PLANT. 



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

 of I'un^i produce more or less extensive enlargement of parts 

 of th.-ir 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 Synchytrium 

 (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- 



__J tending to neighbouring cells, 



which, in consequence, multiply 



i f nrrn fl r i 11O ' liVp swpllincr 

 101 111 8, RUg-llkC SW6. ing 



round it. The leaves as a 

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

 very 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 Puccinia are not at all deformed, 

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

 190). Leaves of Cirsium attacked by Puccinia 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 1 describes such a case in Anemone 



M<_'iuis, " EinHuss v. Parasiten auf cl. Ausbildung d. befalleneii Pflanzen- 

 theiles." Nafnrmss. Jiundnrhau, 1891, No. 25. 



Fio. 4. Syncliiiti-'iiui turuxaci. Partial atrophy 

 of laminae of Taraxacum officino.le. About i 

 natural size. (v. Tubeuf phot.) 



