May 6, 1918 



Yellow-Leaf blotch of Alfalfa 



313 



sometimes between the cells. When the tissue is thoroughly invaded, 

 the interior of the larger palisade cells and many other large parenchyma 

 cells become lined with a thick-walled cellular fungus layer. Finally the 

 leaf tissue almost disappears and becomes replaced with a fungus stroma. 



In culture the mycelium develops largely in the substratum, where it 

 forms a dense mat that does not appear to amalgamate into a stroma, 

 though in old cultures it may produce a stroma at the surface of the cul- 

 ture medium. The aerial mycelium usually appears as a close felt of 

 hyphae arising from the submerged mycelium. In case the mycelium 

 reaches a height of about 2 mm., it tends to interweave into numerous 

 little flexuous spires. 



CoNiDiAL, STAGE. — As soon as the mycelium has thoroughly invaded 

 the leaf tissue, conidia begin to appear in cavities among the palisade cells 



Fig. I. — Pyrenopeziza medicaginis: Advanced stage of development of the conidial stage. 



just below the epidermal layer of the upper surface of the leaf. At first 

 these cavities are nearly spherical in form, but they rapidly increase in 

 size, extending for the most part in the direction parallel to the surface 

 of the leaf (fig. i). Thus they become greatly broadened, and often 

 much lobed and convoluted in cross section. The lining of the cavity 

 consists of interwoven hyphae or of a very thin layer of fungus tissue from 

 which the conidiophores arise in a closely packed layer. Next to the 

 epidermis there may be a few large, somewhat circular, dark-colored cells 

 which do not form a complete protective layer. There is no regularly 

 developed ostiolum, the opening being merely irregularly torn through 

 the epidermis. Occasionally when the cavity is large and the weather 

 is moist, the torn ends of the ruptured epidermis become recurved, 

 which exposes the layer of conidiophores (PI. 25). When the leaf tissue 



