24 



REACTION OF HOST TO PARASITIC ATTACK. 



True atrophy is best seen in those cases where flower-forma- 

 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 might be designated as perennial. The last-mentioned 

 case is exemplified in Aeeidium datinum, the witches' broom of 

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



Fig. 6. — Euphorbia CyparUsias. A healthy flowering normal plant compared 

 with the attenuated non-flowering form inhabited by Aeeidium euplurrbiae. 

 (v. Tubeuf phot.) 



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

 tree is in blossom. Another perennial symbiosis behaving thus 

 is shown in Euphorbia Gyparissias attacked, by Aeeidium 

 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 Prunus under 

 the influence of Exoasci (Fig. 7). In flowers attacked by Cystopus 



