22 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



the same plane as the body of the ascus, and hence does not appear in 

 the section in which the body of the ascus is best shown. In fig. 30 the 

 second and third asci, counting from the right, show the narrowed 

 basal portion, and, as suggested, these figures do not show the body of 

 the ascus so fully as those lying further to the left. 



At this stage the wall of the perithecium in Phyllactinia can be 

 roughly differentiated into three layers. The outermost consists of a 

 single layer of peripheral cells. From these cells in the basal region 

 the secondary mycelial hyphse sprout as noted above. From this same 

 layer in the equatorial region of the perithecium the spine-like append- 

 ages with enlarged bulbous bases arise. These have not yet begun to 

 develop at the stage shown in fig. 30. From the same peripheral layer, 

 in a zone about the apical region of the perithecium, are developed the 

 peculiar penicillate cells which have been so frequently the objects of 

 study in recent years. The first beginnings of these outgrowths are 

 shown in the swollen and protuberant form of certain of the peripheral 

 cells in the upper portion of fig. 30, to the left. The outer layer of 

 perithecial cells in Phyllactinia is especially active in growth. They 

 remain thin-walled and show no tendency to become hardened and to 

 dry out until the perithecium is nearly ripe. 



Beneath the peripheral layer is a zone several cells in thickness, 

 which functions as a protective, mechanical, strengthening layer from 

 a relatively early stage in the development of the perithecium. Its 

 protoplasts contain large vacuoles and its cell-walls undergo a change 

 apparently analogous to lignification. The cells appear as poor in con- 

 tent and with specially strengthened walls. Inside this zone and next 

 to the developing asci are several layers of cells richly filled with proto- 

 plasm and with thin, apparently unmodified walls. These cells consti- 

 tute a close packing about the developing asci and, as has been suggested, 

 are very probably concerned with their nutrition, though it is difficult 

 to bring positive evidence on this point. 



The later development of the perithecial walls consists simply in 

 further differentiation of the parts already indicated. The penicillate 

 cells already referred to are very easily studied in all stages of their 

 development. They arise, as noted, as blunt outgrowths of the periph- 

 eral cells in a zone about the apex of the perithecium. They are either 

 not developed at all, or are relatively small in the apical region itself. 

 The cells which give rise to them push up vertically to the surface of 

 the host-leaf, rather than radially to the surface of the perithecium. 

 They are at first blunt and unbranched projections from the peripheral 

 cells (fig. 50), but after they have reached a height equal to about twice 



