298 SPHAEROTHECA. 



first pale blue and then dull green, the colour changes 

 starting at the centre of the patch and spreading to the 

 outside. From the floating mycelium there arise erect 

 hyphae which develop into gonidiophores, and submerged 

 hyphae which grow vertically down into the liquid. 



The mycelium has the same structure as in Eurotium, 

 but the gonidiophores are repeatedly branched, the parallel 

 branches being arranged in a brush and each ending in a 

 chain of gonidia formed by basipetal abstriction as in 

 Eurotium. 



The ascocarps, which are rarely met with, are formed in 

 much the same way as in Eurotium. In Penicillium, how- 

 ever, both sexual organs are spirally coiled round each 

 other, one of them later giving off ascogenous hyphae. 

 The enveloping hyphae form a densely interwoven firm 

 mass of tissue, the outer layers of which are yellow and 

 the inner '(containing the ascogenous hyphae) colourless, 

 and the hard and relatively large ascocarp undergoes a 

 resting period of about two months. Ultimately the inner 

 tissue of the mass is used up by the developing asci, and 

 the spores escape by the breaking up of the hard brittle 

 rind. 



SPHAEROTHECA. 



431. Allied to the Eurotium and Penicillium section of 

 Ascomycetes is an interesting group, the Erysipheae, in- 

 cluding the Mildews which are parasitic on the leaves of 

 various plants, e.g. Hop, Eose, and various other wild and 

 cultivated flowering plants. 



Sphaerotheca, species of which grow on Hop and Eose, is 

 one of the simplest forms of Mildew. The mycelium of 

 Mildews is peculiar in that it creeps over the surface of 

 the infected leaf, forming a web of intercrossing threads 

 the hyphae, which send in short processes, the suckers or 

 hanstoria, into the epidermal cells of the leaf. The 

 hyphae are single rows of cells, each containing a single 

 nucleus. 



During the summer, the mycelium sends up erect 

 gonidiophores, each of which buds off at its tip a chain 



