C2 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



*J 



between the surface of the spore and that of the epipiasm from which 

 it is drawn back. The surface of the spore is plainly smooth and con- 

 tinuous, while that of the epipiasm is slightly ragged and irregular, 

 indicating that the spongy cytoplasmic reticulum has been cut through 

 without closing the interstices of the epipiasm, either by the rounding 

 out of reentrant cavities by surface tension or by the deposit of a surface 

 layer of material to close such openings. The spore-plasm is evidently 

 inclosed and smoothly enveloped by the material of the polar fibers, 

 while no such substance has been deposited on the corresponding sur- 

 face of the epipiasm. 



After the spore is completely inclosed the remnant of the fibers 

 disappears from the region about the central body (fig. 76). The latter 

 then breaks away from the plasma membrane and the nucleus gradually 

 regains its spherical or oval shape by drawing in the beak-like prolon- 

 gation. The stages in this process are very well shown in Phyllactinia 

 (figs. 77, 78). The central body comes thus to occupy its old position 

 on the surface of the spherical nucleus. The chromatin is apparently 

 at this stage an irregular reticulum, but is always attached to the central 

 body. It is frequently drawn back from the nuclear membrane into an 

 irregular mass on all sides, except where it is attached to the central 

 body (fig. 79). Later a wall is built around the spore and a resting 

 condition ensues which lasts till the bursting of the perithecia in the 

 following spring. The connection of chromatin and center can be 

 observed in favorable preparations in the fully matured spores, but these 

 stages, owing to the presence of the spore wall, are less easily fixed than 

 the earlier ones. In the fully ripened condition a considerable amount 

 of reserve material is deposited in the spore-plasm, which thus becomes 

 quite different in its composition from the surrounding epipiasm. 



The germination of the ascospores into a vegetative mycelium com- 

 pletes the life cycle of the mildew. The nuclei of the mycelial hyphae, 

 as already described, show the same polar structure, with the chromatin 

 directly connected with the central body, which we have traced through 

 the stages in the development of the ascocarp. We thus have a fairly 

 continuous account of the existence of the central body and the main- 

 tenance of its connection with the material of the chromosomes through 

 two nuclear fusions in the oogonium and in the young ascus, through 

 a series of divisions in the ascogenous hyphae, and the triple division in 

 the ascus, and finally through the formation of the ascospores by free 

 cell formation. This constitutes the longest and most varied series of 

 stages through which the central body of a plant has yet been traced. 



