166 



ASCOMYCETES. 



This is supported by Smith's investigations, in which an an- 

 atomical comparison of diseased twigs of peach and almond 

 showed no difference in the pathological effects. 



Exoascus crataegi Puck, occurs on Crataegus Oxyacantha^ 

 and causes red swellings on the leaves and flowers, accom- 

 panied by hypertrophy of shoots in which the mycelium 

 perennates. 



Exoascus Tosquinetii (West). The deformation caused by 

 this species is frequent on the black alder (Alnus glutinosa). 

 The thickened, elongated, wrinkled twigs render attacked parts 

 very conspicuous in contrast to the normally developed parts of 



the tree. The leaves may be 

 wholly attacked and much 

 enlarged, or they may only 

 be hypertrophied at places so 

 as to form pustule-like swell- 

 ings. The epidermal and 

 mesophyll- cells of diseased 

 leaves become greatlj' en- 

 larged. 



Exoascus aureus (Pers.). 



The leaves of the black poplar 



{Popidus nigra) attacked by 



this parasite exhibit pustules 



(Fig. 62). The asci are 



formed as a golden coating 



on the concave side of the 



pustules, which is, in most 



cases, the under side of the 



leaf, rarely the upper. The 



cells forming the pustules 



have thicker walls and a 



somewhat different shape from the normal epidermal cells, and 



they are not unfrequently sub-divided by walls of secondary 



origin (Fig. 63). 



According to Smith, the cells of the palisade parenchyma have 

 also thickened walls, as well as being elongated and occasionally 

 chambered ; the cells of the spongy parenchyma are enlarged and 

 have thicker walls ; so also are the cells of the collenchyma of 

 the leaf venation. 



Fjg. 62. — Exoascus auTeiis, Leaf of Po;pulus 

 nigra, showing the pustule-like swellings, (v. 

 Tubeuf del.) 



