390 



BOTANY 



PAUT II 



the intercellular spaces of the leaf. The ripe ascus fructification! (perithecia) are 

 small black bodies provided with peculiar appendages. In the simplest forms 

 (e.g. in the genus Sphaerothcca) the spheroid perithecium encloses only a sin^lf 

 ascus with eight spores. It is enveloped by a covering of sterile hyphae, forming 

 a sheathing layer, two to three cells deep. The genera Eryripke and Uiicinula, on 

 the other hand, develop in each perithecium several asci, and in Plnjllactinia 

 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 peri- 

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

 HAI:I'KK lias shown, the first rudiment of the perithecium consists of an oogonium 



tt ic- 



Fio. 328. Sphaerotheca caxtaijnei. Fertilisation and development Of the peritheriuiii. 7, Ou-oninm 

 (o0) with the antheridial branch (02) applied to its surface ; S, separation of antheridium (an) ; 

 #, passage of the antheridial nucleus towards that of the oogonimn ; l t , union of the nuclei ; 

 5, fertilised oogonium surrounded by two layers of hypha; derived from the stalk-cell (st) ; 6, 

 the multicellular ascogonium derived by division from the oogonium : the terminal cell with 

 the two nuclei (us) gives rise to the ascus. (After HARPKR.) 



and an antheridium. These are uninucleate cells, separated from the mycelium by 

 partition walls, and stand close together. The male nucleus passes into the 

 oogonium by an opening which forms in the cell walls (Fig. 328, 1-4). After 

 fertilisation the oogonium is surrounded by investing filaments which spring from 

 its stalk-cell or from that of the antheridium (5), and the oogonium itself becomes 

 converted into a multicellular structure (6). In Sphaerotheea the ascus containing 

 eight spores arises from the multinucleate terminal cell, while in Erysiphe and 

 Phyllactinia this cell produces ascogenous hyph; which in turn give rise to the 

 numerous asci. 



Before entering upon the formation of perithecia, tin- .Mildew Fungi multiply 

 by means of conidia abstricted in chains from special, erect hyphae, from the tip 

 downwards. The Mildew Fungus of the Grape-vine occurs on the leaves and 

 berries in America and has appeared in Europe since 1845. This fungus, known as 

 Oidium Tuckeri, is the conidial form of Uiicinula necator (- U. spiralis], the small 

 perithecia of which have appendages spirally' rolled at their free ends and are only 

 rarely found (Fig. 329). 



2. The Perisporieae are closely related to the Erysipln-ae, but are saprophytic 

 and live on decomposing organic matter. To this order belong two of the most 



