l6 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



it and thus closing the apex of the ascocarp, though, as noted, because 

 of the curved form of the oogonium, this region may not be vertically 

 over the stalk-cell of the oogonium. The further growth of these and 

 other hyphse from the antheridial stalk-cell plays an important part in 

 the development of the perithecial envelopes, but the branches from the 

 oogonial stalk and those from the antheridial stalk become so inter- 

 mingled that it is not easy to distinguish them in later stages. 



While the first enveloping branches are pushing up from the stalk- 

 cells the oogonium is also beginning its further development. From 

 its function in the mature ascocarp we may call this product of the 

 growth of the oogonium the ascogonium. This growth of the oogonium 

 is essentially, as noted, the germination of a fertilized egg which, instead 

 of being set free from the mother plant, continues in unaltered vegeta- 

 tive connection with it. In its growth the ascogonium elongates and 

 increases in diameter. At first, since the antheridial stalk-cell does not 

 elongate and the end of the ascogonium is firmly attached to the anther- 

 idium, the two ends of the ascogonium are held at about the same dis- 

 tance apart as they were at the time of fertilization, and its growth 

 results in a bulging out and increased convexity of its free middle por- 

 tion. The suggestion given by sections of the growing fruits at this 

 time is that of a mutual rivalry in growth between the ascogonium and 

 the enveloping hyphse. The ascogonium tends constantly to burst out 

 of the inclosing hyphse, which latter, by elongation and richly branch- 

 ing, tend to inclose and cover it over its whole surface. 



Whether or not the inclosing hyphse grow more slowly at first and 

 actually exert pressure on the growing ascogonium, it is certain that 

 the ascogonium becomes curved in a very irregular fashion at this 

 stage. The sections (figs. 12-23) are about 5 p, thick, and the asco- 

 gonium bends up and down in the thickness of the section in a fashion 

 which would be difficult to represent without special shading, which 

 would interfere with the representation of its protoplasmic content, etc. 

 Hence, I have here, as in the earlier stages, disregarded in the drawings 

 the vertical bending of the ascogonium, and have, by focusing up and 

 down, shown it in median optical section as if projected in the plane of 

 the section. This has resulted, of course, in making it appear some- 

 what shorter than it really is, and as a further result, in cases of suc- 

 cessive sections in which the ascogonium appears, its parts do not seem 

 to fit together as they would if its actual windings were shown in the 

 figures. Since, however, we are concerned only with the facts of its 

 increased size, the multiplication of its nuclei, its division into cells, and 



