19"] WOLF—ACTINONEMA ROSAE 219 



leaf and furnishes nutriment for the subcuticular part. It is 

 connected with the latter by occasional hyphae which penetrate 

 the epidermal cells or pass between them. 



A section of the fruit bodies or acervuli perpendicular to the 

 surface of the leaf shows that they are formed between the cuticle 

 and the outer wall of the epidermal cells. They are consequently 

 flattened. The stroma of the acervulus is seated directly on the 

 epidermal cells and consists of a very thin layer of small, hyaline 

 to yellowish, pseudoparenchymatous cells. It is connected with 

 the internal mycelium below by hyphae which extend either through 

 or between the epidermal cells into the mesophyll. Laterally the 



stroma 



membrane 



There is no 

 •vulus. On 



the upper side of this stroma certain cells are formed 



conidia. These conidiophores are not prominently differen- 



form from 



but are slightly 



elongated upward. The conidia are hyaline, 2-celled, and oval 

 to elliptical in outline. They are usually somewhat constricted 

 at the septum. The conidia are formed on the somewhat pointed 

 upper ends of the conidiophore layer. The great numbers which 

 are produced cause such a pressure that the cuticle is finally rup- 

 tured. The cuticle, which is the only covering for the acervulus, 

 is thus thrown back irregularly, exposing the mass of conidia and 

 permitting their escape. 



While the spots together with the mycelial strands and acervuli 

 appear dark, this color is not due to the fungus, which is almost 

 colorless, but to the disintegration of the cells below the spot. 



Development of acervuli 



It is from the subcuticular mycelium that the acervuli arise. 

 At certain definite points the mycelium begins to form a stroma, 

 which increases in a centrifugal manner, forming a more or less 

 circular stromatic layer. Certain cells oi this stroma which are 

 to give rise to the conidia are directed upward as short stalks. 

 These increase in size, forming a closely aggregated layer standing 

 Perpendicular to the stroma. Meanwhile, the mesophyll tissue 

 directly below the acervulus is being disintegrated and a dense 



