EFFECT OF PARASITIC FUNGI ON THE FORM OF HOST-PLANT. 29 



at the same time fleshy and of a bright rose-red colour ; the 

 ovules are sterile or abnormally formed. Wakker, however, 

 found no very marked change in the anatomical structure of 

 such flowers. 



The species of the Exoasceae also produce striking hypertrophy 

 of flowers. Thus there are the sac-like outgrowths of the 

 catkin-scales or ovaries of poplar caused by Taphrina Johansonii 

 and T. rhizophora (Fig. 52), and the "pocket-plums" or "fools" 

 due to Exoascv^s pruni (Figs. 49 and 51). In these last- 

 mentioned cases, the outer layers of the ovary become thick and 

 fleshy, sometimes remaining green, while the stone and kernel 

 remain rudimentary. The alder, under the influence of Exoascus 

 alni incanae, has the catkin-scales much enlarged, deprived of 

 chlorophyll, and of a red colour (Fig. 53). 



Mummification, or the transformation of the fruit into a fungal 

 resting-body or sclerotium, is not unfrequent. In some respects 

 this process resembles the change in ovaries brought about by 

 TJstilagineae. Here, however, we have to do neither with hyper- 

 trophy of the fruit, nor yet with its complete destruction. The 

 best-known sclerotium is that of Claviceps purpurea (Fig. 84). 

 It first fills up the base of the ovary, then kills it and grows 

 out as a large horn-like sclerotium. The sclerotium of the oak 

 {Sclerotinia BatscJiiana) completely replaces the acorn, leaving 

 only the outer covering enclosing it. Likewise, in the mummified 

 berries of bilberry, cowberry, crowberry, cranberry, and others, 

 one finds the normal parts almost wholly replaced by the 

 resting-mycelium of some species of Sclerotinia. 



Formation of new Organs. 



Although parasitic fungi commonly induce hypertrophy of 

 existent organs and development of normal latent structures, 

 they are seldom associated with formation of new organs. As 

 such, however, we must regard the formation of adventitious 

 buds on the fronds of Pteris quadriaurita, Eetz, and Aspidium 

 aristatum, Sw., under the influence of Taphrina Laurencia, 

 and T. Cornu ce'rvi, respectively.^ Buds or bulbUs of this 

 kind occur normally on several species of ferns ; but in those 

 just mentioned they appear only as a result of the parasite, and 

 develop into structures reminding one of a witches' broom. 

 ^ Giesenhagen, Flora, 1892. 



