76 Journal of the Mitchell Society [September 



Acervulus — The aeervuli are either isolated or variously grouped 

 and are always subepidermal in origin, Fig. 21. They open to the 

 surface by the rupture of the epidermis in the form of a slit through 

 which the conidia escape, or by an irregularly lacerate opening. 



The stroma upon which the acervulus is seated usuallj' extends 

 3 to 4 host cell laj-ers in depth and is made up of compact pseudopar- 

 anehyma which is at first colorless and becomes with age brown to 

 blackish, Fig. 13. This stroma completely occupies the interior of the 

 host cells without apparent modification of their cell walls. The 

 nourishing mycelium extends radiately from the periphery of the 

 stroma, Fig. 21, and is confined for the most part to the epidermal 

 and hypodermal cells. It has never been observed to be intercellular 

 and cells whose cavities are practically filled with mycelium contain 

 apparently normal intact nuclei. The h.yphae composing this 

 mycelium are also hyaline at first, but darken at maturity. 

 Since the mycelium does not penetrate the xylem tissues there is in- 

 sufficient interference with the conduction of water to prevent the 

 tips of the plant from growing, even when the stems are involved for 

 the greater part of their length. 



The conidiophores arise within the epidermal cells and by their 

 elongation rupture the cuticle which persists for a time as a frayed 

 border at the margin of the acervulus. They are compactly arranged 

 and in section appear palisade-like. They are cylindrical to clavate. 

 in shape and quite variable in size, averaging 20-30 x 6-Sfi. Those 

 which are more slender are believed to be the structures which were 

 interpreted by Atkinson and Edgerton as conidiophores intermingled 

 with the basidia. 



The conidia are borne in a whorl or crown at the apex of the 

 conidiophore. Fig. 1, 6, 9, 10 and 17. They are not formed on sterig- 

 mata, but arise as protrusions from the apical wall of the conidio- 

 phore. As they are abstricted, othei-s are formed in their places, re- 

 sulting in the formation of a mass of conidia which may ooze out 

 of the mouth of the acervulus. The conidia are oblong to sul)elliptical, 

 straight or curved, continuous, hyaline, granular and measure 12-20 

 X 3-3.5/., Fig. 2. 



The setae are irregularh' disposed through the aeervuli. Fig. 13. 

 They are very abundantly present on j^oung vetch stems and can 

 readily be distinguished with a hand lens. They are only sparingly 

 present on old stems, however, and one may not be able to determine 



