366 SUPERFICIAL FUNGI. 



all excluded ; but even then the group consisted of very hetero- 

 geneous elements, and forms which were described as belonging 

 to the same genus, after consideration and careful study, it has 

 been found advisable to place in entirely different families. 



In many of the genera the mycelium closely resembles that 

 of the Perisporwcecc, having one or two-celled hyphopodia at 

 regular intervals, and Theissen makes the presence or absence 

 of these bodies a generic distinction ; but the essential character 

 in the Microthyriacecc is the structure of the perithecial mem- 

 brane. 



In the genus Asterina, which may be taken as typical of the 

 Microthyriacecc, the thyriothecium, as it is called, originates 

 from a medial or terminal cell on the hypha, which divides 

 rapidly, and forms a small clump of cells between the hyphse 

 and the leaf surface. Each of the cells comprising this initial 

 group begins to grow in a radial direction, each radiating hypha 

 being in close contact with its neighbour, and being attached to 

 it by the radial walls. As the disc grows the increase in diameter 

 is provided for by the repeated forking of these radiating hyphae. 

 These are sometimes connected along their entire length forming 

 a compact disc, but more frequently they are free at the circum- 

 ference, giving the thyriothecium a fimbriate appearance. The 

 central part has meanwhile become arched to form the perithe- 

 cial cavity, while the peripheral zone remains appressed to the 

 leaf surface. 



Theissen regards this structure as a development of the com- 

 plete spherical form found in the Perisporiacece; the thyriothe- 

 cium consists only of the basal half, which has been upside down, 

 and hence is called " inverse." This inverse form, together with 

 the radial iprosenchymatous structure oi the membrane, is typical 

 of the thyriothecia of the Micriothryiacecc proper. In the genus 

 Asterina, whicih we have been considering, the spores dehisce 

 through a star-shaped opening formed by the comporent hyphae 

 splitting apart from the centre. 



The pycnidia are exactly similar in form to the thyryiothecia, 

 and in many of the earlier descriptions the conidia have been 

 mistaken for ascospores. 



The fungi with hemispherical perithecia do not always 

 exihibit the radial structure described above ; in some cases, e.g., 

 Micropcltis, a form of which is found on Marafha fraxinea, the 

 initial group of cells develop^, in an entirely different way. 

 Instead of radiating hyp^hse. weak, almost colourless, hyphal 

 branches are developed almost at right angles to one another, 

 these branches repeatedly forming an irregular network, which 

 continually increases in size ; meanwhile secondary branches arise 

 within the meshes of the net, which anastomose and form a com- 

 pact membrance, which later on becomes blue-green or blue- 

 grey. In many species the secondary branches, which are formed 

 witihin the meshes of the network, become curved, and are irregu- 

 larly toothed or indented in various ways, the very youngest 



