UREDINALES 



557 



cleate, fusion cell is formed (Fig. 374, 1 to 3). Under exceptional cir- 

 cumstances more than two fertile cells fuse, producing a multinucleate 

 fusion cell. Since both fertile cells in Fig. 374, 1 to 3, are equal in form 

 and position, the plasmogamy is isogamous. There also appear in the 

 same sporus many deviations from this isogamous type. Thus cells of 

 different ages may copulate, e.g., one of these may have cut off the sterile 



Fig. 374. — Phragmidium disciflorum. Development of aeciospores. 1 to 3. Fusion 

 of basal cells. 4. Fusion of superimposed cells. 5, 6. Development of aeciospores. 

 (X 1,130; after Moreau, 1914.) 



cell before the other; their position may be different (one somewhat higher 

 than the other); and, finally, the septum may be only incompletely 

 dissolved so that there may be only a pore through which the nucleus 

 of one cell passes into that of the other. Where these forms appear 

 singly, they may have been caused as pathological processes or as arti- 

 facts by the uneven penetration of the fixative; in other forms, e.g., Phrag- 

 midium violaceum in which they appear normally, their heterogamous 

 nature may not be contested. Furthermore, frequently instead of the 

 septum between two basal cells, the septum between one basal cell 

 and the underlying hyphal cell is dissolved (Fig. 374, 4) or the wall 

 between a basal cell and a cell of the neighboring mycelium or, finally, 

 the wall between any two mycelial cells is dissolved. Thus plasmogamy 

 takes place autogamously or pseudogamously. 



