120 FROMME: SEXUAL FUSIONS IN FLAX RUST 
single layer as he found in Phragmidium speciosum. The entire 
sorus at this time is a group of vertically placed hyphal branches 
without pseudoperidium or paraphyses. It cannot be considered 
as a unit in any proper sense but rather as a collection of repro- 
ductive units. The development of the sorus proceeds in a more 
or less centrifugal fashion, the older branches being found at the 
center while the younger are at the outer borders of the group. 
The various stages in the degeneration of the sterile cells are well 
seen in passing from the outer borders toward the center. Christ- 
man’s interpretation of these sterile cells as ‘‘buffers’’ seems 
entirely adequate, as their function is evidently a protective one. 
They relieve the pressure of the epidermis on the underlying 
reproductive cells and then degenerate to provide room for the 
subsequent development of the latter. As noted, in all the forms 
so far investigated only one layer of sterile cells has been observed, 
while in Melampsora Lini two layers of sterile cells are normally 
produced. In his investigation of Melampsora Rostrupi, Black- 
man occasionally found a double layer of short crushed cells 
lying above the fused cells, but owing to the age of his material 
he did not note the method of their formation. In view of the 
facts above described it seems quite reasonable to assume that 
Blackman’s observation shows that a similar condition is found 
in M. Rostrupi. It is possible that all the Melampsoras develop 
their gametes at a deeper point in the sorus than the more super 
ficial caeomas of the Phragmidium type. 
The frequent intimate association between the spermogonia — 
and aecidia, referred to above, is an interesting feature of the 
flax rust. In rusts bearing both spermogonia and aecidia the 
former usually precedes the latter in time of development by 4 — 
period of several days or weeks. In this form, however, a marked — 
difference in time of development does not exist. In general the 
spermogonia appear somewhat earlier than the aecidia, but they 
_ are often found developing simultaneously and intimately associ 
ated. Spermogonia may bound the aecidium on either side % 
may even be included in it. It is very difficult to trace the origi? - 
of the mycelium from which the two sori arise on account of the — 
interweaving of the hyphae, but there is no sharp boundaty 
between the two and they cannot be distinguished in the veg’ 
