580 STATE BOARD OF AGRICULTURE. 



septate, and the cells bulge and increase in size, until they are large spherical 

 bodies, 20 or more microns in thickness. The cell walls are rather thick 

 and have a dark-brown color. When the mycelium comes in contact with 

 glass, it forms a black mass of thich walled cells which closely adheres to the 

 smooth surface; in liquid cultures it forms a black ring immediately above 

 the surface of the culture medium. Old cultures, on favorable media, form 

 black compact masses of densely interwoven threads approaching a pseudo- 

 parenchjTnatous tissue. On dextrose agar, sclerotium-like bodies, several 

 millimeters in diameter, are sometimes produced in the mass and near the 

 surface of mycelial growth. These very much resemble sclerotia of the 

 genus Sclerotinia. Sectioning, however, reveals the same context through- 

 out; the entire structure is composed of dark bro'^ii cells closely compacted 

 and cemented together. Klebahn, in addition to finding these bodies in 

 pure culture, has noted them on diseased plants. 



Pycnidia 



Pycnidia are produced singly, either scattered or clustered on the surface 

 of the host. Mj^celial development around the pycnidium is sparce; nothing 

 approaching a stroma has been observed. Young pycnidia are hyaline; 

 older ones vary from brown to black. A few pycnidia are more or less spheri- 

 cal, but more comm^only they are corsidcrably depressed; all are very sym- 

 metrical. In size thcj' m^easure gO-lCO x lCO-250 microns. The pycnidia 

 walls varj^ in thickness from 6 to 8 microns. The neck is cylindrical or ex- 

 panded at the base and top. Pycnidia usually originate immediately below 

 the epidermis; as they increase in size, they may push outward, until, at 

 maturity, they are firmly seated on the surface with only the base imbedded in 

 the host tissue; or they may remain imbedded in the host tissue Avith only 

 the neck exposed. 



The pycnidium has its beginning in a tangled Aveft of hyphae which rounds 

 out into a globose fruiting body. The first part to darken is the neck of the 

 ostiole which is conspicuous as a black ring, even before any part of the 

 pycnidium becomes superficial. The remainder of the pycnidial wall is 

 hyaline until after the spores are formed. Then there is a deposition of 

 brown coloring matter in the outer layer of cells which produces the dark 

 colored pycnidium. The greater part of the black outer portion of the pycnid- 

 ial wall consists of a layer of large, rather thick walled cells. This is rarely 

 more than one cell in thickness. Near the base of the ostiole, the wall be- 

 gins to increase in thickness, until the wall of the neck itself is two to three 

 cells thick. To this greater number of thick walled cells, is partly due the 

 darker color of the pycnidial neck. Within, covering the internal surface 

 and extending well up into the neck of the pycnidium, is a layer of thin- 

 walled cells filled with dense masses of protoplasm. These give rise to min- 

 ute conidiophores which, in turn, give rise to spores by constriction and for- 

 mation of cross walls near their tips. 



^Spores 



Spores of Phoma apiicola are small rod-shaped, thin-walled, hyaline bodies, 

 measuring 3-3.8 x 1-1.6 microns. Staining discloses the presence of vacuoles 

 which arc surrounded by cytoplasm having a finely granular structure. 

 The size of spores on the host plants does not vary to any considerable de- 

 gree. This uniformity apparently holds true with the organism very gen- 

 erally, as the following table of measurements by different investigators will 

 show. 



