HYPOCRELLA AND ASCHERSONIA. 179 



the normal yellow-brown on the other. Aschersonia viridans 

 owes its name to this colour change. 



The situation of the fungus, whether epiphyllous or hypo- 

 phyllous, depends upon the position of the host insect, and is 

 not of specific value. If a given si^ecies of insect were always 

 confined to one position, and a given species of Hypocrella 

 were parasitic only on one species of insect, then the infor- 

 mation might in some instances serve to differentiate between 

 species ; but neither of these conditions is true. 



The Perithecia. 



The perithecia are immersed in the tissue of the stroma. 

 Even in cases where they appear to be more o* less superficial, 

 as in Hypocrella RaciborsHi, there is a region of stromatic 

 tissue surrounding the perithecium proper. In general, the 

 perithecia are flask-shaped, and vary, between species, only 

 in size and the relative lengths of the neck and body. 



The wall of the perithecium is distinctly differentiated 

 from the ground tissue of the stroma. It is, as a rule, from 

 20 to 30 [jt thick, and is composed of more or less parallel 

 hypha?, densely compacted, running lengthwise of the peri- 

 thecium. These hyphtf are narrower than those in the body 

 of the stroma, and their walls are less thickened. The wall 

 of the neck is usually thicker than that of the body of the 

 perithecium and towards the apex it increases still more in 

 thickness, so that the apex is expanded obconically. The 

 upper face of the neck, which constitutes the ostiolum, is 

 either flat or slightly conical, its edge in either case being 

 level with the general surface of the stroma. The term 

 " ostiola projecting," employed in descriptions of these fungi, 

 must be understood to mean that the ostiolum is situated in a 

 slightly elevated projection of the stroma, the elevation of the 

 actual perithecial tissue being almost too shght to be noticed 

 macroscopically. 



The wall of the perithecium contrasts strongly with the 

 tissue of the stroma. It differs, as a rule, from the wall of the 

 pycnidium, and hence there is little difficulty in deciding from 

 sections whether a given specimen is Hypocrella or Aschersonia, 

 even when the perithecia are quite immature. The ostiolum 



