DOTHIDEALES 



293 



migrate from one cell to the other, unite there in pairs and enter the 

 ascogenous hyphae. 



Meanwhile the cells of the top of this young meristem have flattened ; 

 they gradually thicken their walls, as happened earlier in the basal 

 plectenchyma, and change into a cover layer. 



An interpretation of this life cycle is at present impossible, as Sys- 

 tremma Ulmi is the only ontogenetically investigated species. It may 

 be mentioned, however, that similar relations have been found in Epi- 

 chloe Bambusae (Gaumann, 1927). One must regard the ephemeral cell 

 series as the remains of a solitary ascogonium in which, as in Poly stigma, 

 parthenogamy occurs between two sexually activated cells. 



From the point of view of the morphology of fructifications, the 

 differentiation of the stroma into a conidial ectostroma and ascigerous 

 entostroma is characteristic for Systremma Ulmi; and furthermore the 



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coaoac 



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Fig. 192. — Dothidella Derridis. Section of stroma. (After Theissen, 1914.) 



entostroma is first laid down as a hypodermal sterile basal stroma which 

 only later develops fertile pads at the top. This budding of the ento- 

 stroma is undoubtedly the same process as that which in the Myriangiales 

 has led to the budding of the loculi. 



If one imagines that the fertile hyphae budding from the entostroma 

 no longer unite into pads under the epidermis but emerge from the 

 stomata singly or fasciculately and there intertwine to form a fertile 

 tissue, one has the second tribe, the Leveillelleae. These possess an 

 extensive sterile subepidermal basal stroma, which is connected by 

 numerous hyphae or mycelial strands with the fertile extramatrical 

 ascus stroma. In systematic literature the subepidermal basal stroma 

 is usually called the hypostroma. 



This removal of the fertile ascus stroma from the interior of the 

 leaf to the surface may be regarded as an expression of the asterinoid 

 direction of development which we have observed in many Perisporiales, 



