EXPERIMENT STATION BULLETINS. .653 



formation of the secondary spores, except that no definite elongated pro- 

 jection that might be considered as a well differentiated conidiophore is 

 formed. The cells that give rise to the conidia are hyaline, resembling 

 the peridial cells and can be traced to the brown thread from which they 

 have arisen. Spores are produced to such an extent as to burst open the 

 peridium at its weakest point and form an exuding spore mass some- 

 times twice as large as the pycnidium from which it arose. Usually 

 that part of the peridium that is exposed is entirely broken up by the 

 mass of spores. Plate 4, Fig. 1 illustrates this observation. In the pyc- 

 nidium shown, (Plate 7, Fig. 2), the weakest point of the wall must 

 have been submerged in the agar so that the thick part of the wall ex- 

 posed to the air remained intact, while the exuding mass forced itself 

 into the agar. 



However, for complete elucidation of the method of pycnidial forma- 

 tion of this fungus, the development and formation of the fruiting body, 

 must necessarily be studied in the host. Eight plants were inoculated. 

 After three days, just as soon as the infection was visible as a small 

 water-soaked spot, material was killed in Flemming's strong medium. 

 Material was killed every day until the exudation of the spore masses 

 began. The formation of pycnidia and spores was found to be identical 

 in every particular to that described for agar cultures. The develop- 

 ment of the pycnidial wall takes place in an exactly similar manner as 

 in artificial media. After the "balling" up of the mycelium, the hollow 

 case grows larger and larger, after which the spores are formed. These 

 same sections also allowed the nature of the forces acting on the pycni- 

 dium to form the ostiole to be definitely determined. Due to the rela- 

 tion of the pycnidium to the surrounding tissue, the mature pycnidium 

 finally comes to possess a more or less broad ostiole as has been pointed 

 out by previous investigations. The steps in its origin seem to be as 

 follows : 



Soon after the pycnidium begins to form, two forces are in evidence: 



1. That brought about by the enlargement of the pycnidium. 



2. That brought about by the shrinking of the adjoining tissues of 

 the leaf (a condition which can be readily seen with the unaided 

 eye). The enlargement of the pycnidium may cause a rupture of 

 the epidermis of the host by an upward pressure or if the fruit- 

 ing body be completely embedded within the leaf tissue, there is 

 no rupture of the epidermis. 



While this fungous wall (only 3-5 cells) is pushing itself outward, the 

 surrounding tissue is beginning to shrink due to the activity of abund- 

 ant mycelium in the surrounding tissue of the latter, while the epider- 

 mis which had previously remained attached to the top of the pycnidium, 

 begins to exert a dismembering force on all sides of the pycnidium. The 

 great excess of this tension over the strength of the wall results in the 

 pulling apart of the pycnidium and the formation of the so-called "os- 

 tiole." (Plate 7 shows this very clearly.) Along with this, the 

 pressure of the spore mass no doubt, contributes to the rupture by its 

 action at the point of greatest tension. In event, however, the pycnid- 

 ium is formed within the tissue so that it does not protude above the 



