24 



REACTION OF HOST TO PARASITIC ATTACK. 



True atrophy is best seen in those cases where iiower-iornia- 

 tion is suppressed. This effect of parasitic fungi on their host is 

 by no means uncommon, the fungus alone reproducing itself, 

 while the assimilating host-plant remains sterile. This atrophy 

 is found not only in annual plants, but also in those where the 

 symbiosis iniglit be designated as perennial. The last-mentioned 

 case is exemplified in Accidium clatinum, the witches' broom of 

 which never bears flowers ; again, by witches' brooms of Ejonscus 



Fig. G. — Euphorbia Cj/parissias. A healthy flowering normal plant compared 

 with the attenuated non-flowering form inhabited by Aecidium euphorbiae. 

 (V. Tiibeuf phot.) 



ccrasi (Fig. 5), which bears only leaves when the rest of the 

 tree is in blossom. Another perennial symbiosis behaving thus 

 is shown in Eupliorhia Cyparissias attacked by Accidium 

 euphorbiae ; year after year the diseased shoots produce only 

 leaves, which assist in the reproduction of the fungus (Fig. 6). 

 Similarly with many other Uredineae. 



Arrest of the seed occurs in ovaries of species of Pnnius under 

 the influence of Exoascl (Fig. 7). In flowers attacked by Ci/sfopu,s 



LiBRABX 





