DEVELOPMENT OF THE PERITHECIUM. 9 



The sexual apparatus is formed, as in other mildews, where two 

 hyphae cross or lie close beside each other, and we have thus the oogonia 

 and antheridia arising as lateral branches from separate hyph?e, though 

 it seems fairly clear, from the circular shape of the infected spots in 

 most cases in Phyllactinia, that both these hyphae belong to a single 

 mycelium, which in all probability is the product of the growth of a 

 single spore. 



The oogonium and the antheridial branch seem to arise simulta- 

 neously, and they become closely applied to each other and begin to 

 become slightly spirally twisted at once. The oogonium is thicker and 

 heavier from the start and grows in length more rapidly also, so that it 

 bends around the antheridial branch while the latter remains straighter. 

 The oogonium may make one almost complete turn about the antheridial 

 branch, which tends to stand rather vertically to the surface of the leaf 

 (figs. 2, 3, 10). The oogonium is much thickened in the middle and 

 may taper considerably toward both its base and apex (fig. i). It is 

 densely filled with protoplasm and contains regularly a single nucleus 

 (figs, i, 3-7) . It is cut off from the hypha, on which it arose as a lateral 

 branch, by a cross-wall which is put in a short distance from its point 

 of origin on the mother hypha. The mycelial cell from which the oogo- 

 nium arises is generally of average length, and the oogonial branch may 

 arise at either end or in the middle of such a cell (fig. 2) . 



There is no evidence of any special differentiation in the cell from 

 which an oogonium arises, and the first division of the nucleus of such 

 a cell furnishes one daughter nucleus for the gamete and the other for 

 the cell of the mother hypha. The antheridial branch arises in the same 

 fashion, but is slenderer at the start, and there is apparently no pushing 

 up of the antheridial branch by the side of the already developed oogo- 

 nium, as I have described for Sphaerotheca. Indeed, it seems as if the 

 two branches were firmly attached at a very early stage and that this 

 condition, combined with the more rapid elongation of the oogonium, 

 leads to the bending of the latter around the antheridial branch. This 

 stretching of the oogonium also bends the tip of the antheridial branch 

 to one side and twists it slightly, so that it comes to lie on the upper side 

 01 the end of the oogonium (figs. 3, 4, 5, S, 10). not forming a cap on 

 its apex, as in Sphaerotheca. 



It is to be noted that the unequal tendency to spiral twisting in the 

 sexual branches, together with their unequal size, produces a structure 

 whose appearance varies considerably, according to the side and the 

 angle from which it is viewed, as is well seen by comparing figs, i to n, 

 which serve to show the various possible aspects as well as the different 



