SPH^ESOTHECA. 



I0 9 



preceding somewhat that of the antheridium. Each consists at first of 

 a short oval branch, which is distinguished from the ordinary vegeta- 

 tive hyphae only by its denser protoplasmic contents and by standing 

 at right angles to the surface of the leaf of the host plant. 



As soon as the oogonium has attained a length equal to two or three 

 times its width, and a diameter which is about twice that of a mycelial 

 filament, it is cut off from the parent hypha by a cross- wall. At this 

 stage it possesses a single nucleus which can scarcely be distinguished 

 from the nuclei of vegetative cells. Frequently, before the young 

 oogonium is delimited by the cross-wall, the antheridial branch appears 

 quite near the base of the former, and grows upward, closely applied 

 to its side (Fig. 39, A). The oogonium appears to grow faster than 

 the antheridial branch at first, thereby bending over toward the latter, 



B 



FIG. 39. Sexual organs and fecundation in Sphcerothtc* castagni Lm. (After Harper.) 



A, young oogonial and antheridial branch, oogonium on left. 



B, later stage of same, antheridium delimited by a transverse wall. 



C, copulation of antheridium and oogonium ; the two sexual nuclei in contact in oogonium. 



D, oogonium in which the sexual nuclei have fused. 



E, young ascogoniutn with two nuclei; wall of perithecium is now several cells in thickness. 



and giving the impression that the contiguous walls were grown 

 together, and that the growth of the oogonium was retarded on the 

 side next the antheridium. The antheridial branch is now separated 

 from its mycelial filament by a cross-wall which is higher in position 

 than the corresponding wall of the oogonium. This cell contains also 

 only one nucleus. When the development of the oogonium is com- 

 plete the antheridial branch elongates and its nucleus divides. One of 

 the resulting daughter-nuclei passes into the somewhat attenuated end 

 of the cell, which is cut off from the lower part to form the anthe- 

 ridium (Fig. 39, B). While the stalk cell now elongates and the 

 antheridium increases in size the oogonium experiences little change ; 

 consequently, the antheridium is carried upward, and finally comes to 

 lie as a cap placed more or less obliquely on the top of the oogonium. 

 At this stage the nucleus of the egg-cell is larger than the ordinary 



