in] EXOASCALES 91 



Microthyriaceae 



The aerial mycelium of the Microthyriaceae is dark-coloured and super- 

 ficial ; the flattened perithecia are shield-shaped and only the upper part of 

 the sheath is fully developed ; it consists of a disc of radiating hyphae, 

 which increase by branching as they grow towards the periphery, and are 

 firmly attached one to another along their lateral walls. In this way a 

 continuous layer of pseudoparenchyma is formed, below which the asci 

 develop more or less at right angles to the surface of the leaf The asci are 

 cylindrical "or pyriform, and the spores frequently bicellular. A definite 

 ostiole may be formed, as in Microthyrium, or the perithecium may tear 

 open at the apex as in Asterina. These two, with about forty and ninety 

 species respectively, and Asterella with sixty are the largest genera in the 

 family. The species are mainly tropical with a few representatives in Europe 

 and North America. 



MICROTHYRIACEAE: BIBLIOGRAPHY 



1913 Thiessen, F. Uber einige Mikrothyriaceen. Ann. Myc. xi, p. 493. 



1914 Thiessen, F. Uber Membranstructuren bei die Microthyriaceen. Myc. Centr. iii, 

 P- 273- 



ExOASCALES 



The Exoascales form a group of obligate parasites on vascular plants ; 

 they cause hypertrophy of the infected parts, producing yellow, red or purple 

 discolorations, blisters, curling of the leaves, malformation of the fruit, and 

 sometimes abnormal branching with the formation of tufts of fasciated twigs 

 known as witches'-brooms. The latter peculiarity is by no means always 

 attributable to the Exoascales but is induced also by certain rusts and by 

 the attacks of insect parasites. The Exoascales are responsible for several 

 diseases of economic importance including peach leaf curl induced by 

 Exoascics deformans, a witches' broom on cherries due to E. Cerasi, and the 

 distortion known as pocket plums caused by the presence of E. Pruni, which 

 infects the flesh of the fruit and inhibits the development of the stone. 



Infection apparently takes place at about the time of the opening of the 

 buds of the host, probably by means of "spores deposited on the bud-scales; 

 in cold, moist weather, when the young leaves are in a state of lowered 

 vitality, the fungus readily gains entrance; it can be checked by the use of 

 appropriate sprays. Once in the leaf the hyphae in most cases ramify 

 between the cells of the host, but in Taphrinopsis Laurencia on Pteris 

 biaurita they are intracellular. No haustoria are developed. 



The mycelium may be annual or it may be perennial, hibernating mainly 

 in the cortex and medulla of the young twigs and causing hypertrophy, 

 especially of the hypodermal tissue, so that infected branches appear abnor- 



