FUNGOID PESTS OF CULTIVATED PLANTS. 215 



with the upper surface smooth and at first soft, white, or greyish, and 

 often brownish as it advances in age, when the brown cuticle often cracks 

 and peels off. The under surface is flat, or a little concave, pierced all 

 over with minute pores ; the margin thick and curved inwards. When 

 young, the substance is soft enough to be cut like cheese, and is per- 

 sistently white ; when older it becomes firmer, but is always rather soft. 

 In size it varies from three to seven or eight inches across. 



Common in Europe, Asiatic Siberia, and North America. 



The fungus should be removed and destroyed whenever found, to 

 prevent the dispersion of the spores. As it is probable that the mycelium 

 is perennial, there is no hope of saving a tree after the appearance of the 

 polypore. 



Sacc. Syll. vi. 5207 ; Mass. PI. Dis. p. 189, fig. 43 ; Cooke, Hdbk. 

 No. 772 ; Soiu. B. F. t. 212 ; Mass. Fun. FL i. p. 248 ; Hart, d Som. 

 Dis. Trees, p. 206. 



WITCHES' BROOM OF BIRCH. 



Exoascus turgidus (Sad.). 



The peculiar bunches of stunted twigs often to be seen on Birch trees 

 are familiar enough, and so is the name of ' Witches' Brooms ' or { Witches* 

 Besoms/ but it is not everyone who knows that it is a disease caused by 

 minute fungi. 



The naked asci, or sacs, which contain the sporidia are developed in 

 spring and summer on the under surface of the leaves, which curl up, lose 

 their fresh green colour, and at length appear covered with a greyish-white 

 hoariness, like hoar-frost. The asci (46-50 x 15 /*) have a stem cell 

 (16 17x15 p) at the base, and they diminish gradually downwards so 

 that they penetrate the epidermal cells. The sporidia, which are enclosed 

 in the asci, are globose (3-4 ^ diam.). 



Known also in Germany, Denmark, Sweden, and Finland. 



Sacc. Syll. viii. 3347 ; Phil. Br. Disc. p. 484 ; Hart. & Som. Dis. 

 Trees, p. 133. 



BIRCH-LEAF BLOTCH. 

 Dothidella betulina (Fries), PI. XX. fig. 27. 



This parasite on the living leaves of Birch has many features in 

 common with the Elm-leaf blotch, with which it corresponds in the 

 mature fruit not being developed until after the leaves have fallen, and 

 are laid on the ground. The blotches are rather small (^ mm.), at first 

 covered by the epidermis, at length naked, prominent, rather angular, with 

 an uneven surface, black and shining, containing white cavities or cells, in 

 which the fruit is matured. The asci are elongated (70 x 10 //), each 

 enclosing eight sporidia, which are ellipsoid, obtuse at the ends, and 

 divided into two nearly equal but sometimes unequal cells (10 x5 fj), of a 

 very pale yellowish colour, discharged, when mature, through the prominent 

 mouth at the apex of the cell. 



Eeported also in France, Germany, Sweden, Finland, Italy, and 

 Asiatic Siberia. 



Sacc. Syll. ii. 5256 ; Cooke, Hdbk. No. 2413 ; Grev. Sc. Crypt. FL t. 

 200, f. 2. 



