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



the ovules become atrophied, whereas the rest of the flower 

 is hypertrophied. Similarly with 

 flowers of cowberry deformed by 

 Uxobasidium. 



2. Hypertrophy. — Many 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, e.g. Pilobolus 

 Kleinii with Pleotrachelus. 



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 by animals. 

 Larger galls occur on leaves sxoascus . 



^^111 a 7 J ■ T_ and abortive. 4 natural size. (v. Tubeuf 



attacked by Synchytriwm, where p^ot.) 

 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, e.g. species of Exoascus. 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 Exoaseeae (see Figs. 62 and 64). Other gall-forms are 

 presented by Exohasidium 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 



Fruit of plum deformed by 

 jirwni \ the stone is shrivelled 



