98 DISEASES OF TREES 



get rid of the infected leaves by raking them together and 

 burning or burying them. 



Poly stigma fulvum attacks Prunus Padus and Amygdalus. 

 This parasite is specially destructive to almond-trees, the yellow 

 patches frequently embracing more than half of the whole leaf- 

 surface. As the perithecia are formed on the fallen leaves in 

 the following spring, it is advisable to burn them. 



Polystigma ochraceum is parasitic on the leaves of the wild 

 cherry. 



CLAVICEPS PURPUREA. 1 ERGOT 



Mention may also be made here in a few words of that 

 disease of cereals which, from the appearance of peculiar 

 sclerotia or mycelial tubers, is designated ergot. 



The well-known black bodies that accompany ergot, and 

 which occur on many species of Graminecz, fall to the ground 

 when the crop is reaped. There they pass the winter, and 

 after germinating* in the moist soil in the following spring 

 each sclerotium generally produces a number of long-stalked 

 spherical sporophores. Sunk over the whole surface of these 

 reddish spherical bodies are to be seen numerous flask-shaped 

 perithecia, whose orifices project somewhat above the general 

 surface. Each ascus holds eight filamentous spores, which reach 

 the open air by being pushed out through the orifice. Should a 

 filamentous spore chance to reach and germinate on the flower 

 of a cereal, the germ-tube forces its way into the ovary, where the 

 mycelium develops in the tissues and almost completely con- 

 sumes them. The ovary, which is entirely enveloped in mycelia, 

 displays on its surface brain-like corrugations, which are the 

 gonidial stromata. The gonidia, which are very small, oval, 

 unicellular, and colourless, are imbedded in a sweet mucila- 

 ginous fluid which is secreted by the gonidial stromata, and 

 appears in drops between the parts of the flower, being known 

 as honey-dew. This gonidial form of the parasite was formerly 

 designated Sphacelia segetum. Only after the formation of 

 gonidia is finished does the ergot proper appear, and this it does 



1 Tulasne, Ann. des set. nat., 3rd ser., vol. xx. p. 56. 



