EFFECT OF PARASITIC FUNGI ON THE FORM OF HOST-PLANT. 25 



the ovules become atrophied, whereas the rest of the flower 

 is hypertrophied. Similarly witli 

 fiowers of cowberry deformed Ijy 

 Exohasidiain. 



2. Hypertrophy. — Alany para- 

 sitic fungi cause abnormal enlarge- 

 ment or other malformation of 

 plants which they attack. The 

 simplest case of hypertrophy is 

 seen in the enlargement of a uni- 

 cellular plant as a result of an 

 endophytic parasite, ejj. Pi/oho/ i/s 

 Kh'inii with Plcufmclirhis. 



The same example is also the 

 simplest possible case of a gall 

 caused by a plant, and distin- 

 guished by the name of " fungus- 

 galls " or Mycocecidia, from Zooce- 

 cidia, the galls caused Ijy animals. 

 Larger galls occnir on leaves 

 attacked by Synchjjtriiim, where 

 not only the single cell attacked 



becomes enlarged, but also the surrounding cells ; these galls, 

 however, form but tiny points on diseased leaves. Similar 

 small and local enlargements of the leaf-cells, accompanied 

 frequently by cell multiplication, are caused by many other 

 fungi, ejf. species of Exoascm. More extensive malformation may 

 embrace some part or even the whole leaf, so that it is more 

 or less enlarged and beset with blister-like outgrowths, as with 

 other Exoasceae (see Figs. 62 and 64). Other gall-forms are 

 presented by ExohasicUum on the alpine-rose (Fig. 259), where 

 the gall is always localized to a small area of the leaf, and 

 on the cowberry, where the gall may extend over whole leaves, 

 and even include the shoot (Fig. 256). 



Hypertrophy of the whole shoot, resulting in elongation and 

 thickening of the twigs, is a phenomenon frequently met with 

 in the " witches' brooms," to be referred to later. And just 

 as entire branch-systems may become hypertrophied and elon- 

 gated, so may whole plants, if the mycelium, instead of remaining 

 localized, spreads throughout the plant. Examples of this will 



Fig. 7. — Fruit of plum deformed by 

 Exoa$cus pruni ; the stone is shrivelled 

 and abortive. 4 natural size. (v. Tubeuf 

 phot.) 



