440 



BOTANY 



PART II 



To these orders must be added the Exoasceae, in which the asci 

 arise from cells of the mycelium without the formation of any 

 fructification, and the very simple Saccharomycetes or Yeast Fungi. 

 These two groups can be regarded as reduced Ascomycetes. 



The Laboulbeniaceae in which the asci are enclosed in small 

 perithecia occupy an isolated position. 



The genetic connections of these orders are not yet clearly established. 



Order l. Erysibaceae (Mildew Fungi) (" 65 ) 



The small spherical perithecia have a closed investment (peridium) which ulti- 

 mately opens irregularly and liberates the ascospores. The asci stand singly or 

 in a group in the centre of the fruit. 



The Erysibaceae live as epiphytic parasites whose mycelium, somewhat 



an 



FIG. 382. Sphaerotheca castagnei. Fertilisation and development of the perithecium. 1, 

 Oogonium (og) with the antheridial branch (as) applied to its surface ; 3, separation of 

 antheridium (an) ; 3, passage of the antheridial nucleus towards that of the oogonium ; 

 It, fertilised oogonium, in 5 surrounded by two layers of hyphae derived from the stalk cell (st) ; 

 6, the multicellular ascogonium derived by division from the oogonium ; the penultimate 

 cell with the two nuclei (as) gives rise to the ascus. (After HARPER.) 



resembling a cobweb, and ramifying in all directions over the surface, particularly 

 the leaves, of higher plants, sends out haustoria which penetrate the epidermis 

 of the host. In some cases the mycelium also inhabits the intercellular spaces 

 of the leaf. The ripe ascus fructifications (perithecia) are small black bodies 

 provided with peculiar appendages. In the simplest forms (e.g. in the genus 

 Sphaerotheca} the spheroid perithecium encloses only a single ascus with eight 

 spores. It is enveloped by a covering of sterile hyphae, forming a sheathing 

 layer, two to three cells deep. The genera Erysibe and Uncinula, on the other 

 hand, develop several asci in each perithecium, and in Phyllactinia 12 to 25 asci are 

 present. Since all the eight nuclei are not utilised in spore formation the 

 number of spores in each ascus is usually 4, or only 2. The perithecia are 

 irregularly ruptured at their apices and the spores are thus set free. As HARPER 

 has shown, the first rudiment of the perithecium consists of an oogonium and 



