292 



COMPARATIVE MORPHOLOGY OF FUNGI 



cells cut off unicellular, uninucleate conidia. As the hyphae only pene- 

 trate the deeper leaf tissues late, the injury to the leaf is slight and the 

 disease is only recognized in late summer (the end of August) by 

 discolored spots. After the end of conidial production this subcuticular 

 conidial stroma is carried away; only the hyphae in the interior of the 

 leaf remain and intertwine between epidermis and palisade tissues to a 

 dense plectenchyma. The cells, which in the central and basal layers 

 are uninucleate and especially poor in protoplasm, thicken their walls 



Fig. 191.— Systremma Ulmi. 1, 2. Sexual cells. 3. Plasmogamy. (After Killian, 1920.) 



and turn brown. At the top of the plectenchyma the cells are richer in 

 protoplasm; here they divide rapidly, especially in the spaces between 

 the epidermal cells, so that the walls are usually periclinal to the mass, 

 forming over the plectenchyma numerous small tissue swellings consisting 

 of rows of cells, regularly septate when young. 



In the middle of each of these new pads there appears a darkly stain- 

 ing cell which elongates and abjoints into three to four daughter cells 

 (Fig. 191, 1 and 2). These gradually become 2 to 3 nucleate. Two of 

 the cells develop more strongly and push the others together. Subse- 

 quently they come into open communication (Fig. 191, 3), the nuclei 



