338 Blackman . — On the Fertilization , Alternation of 
AECIDIUM-DEVELOPMENT. 
Phragmidium violaceum. 
The so-called aecidium in this genus, like that of Caeoma ( = Melempsora 
in many cases), is characterized by a very simple structure, for unlike that 
of the other genera of the group it is neither definite in shape nor 
bounded by a thick- walled pseudoperidium. In the form investigated 
the aecidium is nothing more than a group, indefinite in extent, of aecidio- 
spore-bearing cells, bounded at the periphery by a number of thin-walled 
paraphyses, which are, however, sometimes wanting. It is clear that 
this aecidium is no more a definite organ than is the sorus of uredospores 
or teleutospores, which has an exactly similar arrangement — a group of 
spore-bearing cells surrounded by paraphyses. The aecidiospores are 
developed immediately beneath the epidermis, and not deeper down in 
the tissues as in the typical aecidium. 
The first beginning of the aecidium is the massing of hyphae beneath 
the epidermal cells of the leaf, usually on the lower side. The hyphae 
form there a layer of uninucleate cells, two or three cells thick. The cells 
immediately beneath the epidermis increase somewhat in size, and soon 
become divided by a transverse wall, parallel with the surface of the leaf, 
into an upper and lower cell, each with a single nucleus. The upper cell 
remains more or less cubical, and shows a vacuolate protoplasm and a small 
nucleus, which has no nucleolus and sometimes remains as a dense 
structure without returning completely to the resting state (Fig. (56). The 
lower cell elongates considerably, and shows abundant granular protoplasm 
and a large nucleus with a well-marked nucleolus and a clear chromatin 
network (Figs. 61, 66, 70). The upper cell is a sterile cell ; its nucleus 
becomes disorganized, and it is soon destroyed by the upward growth 
of the cell below. It is from the lower cell, which may be called the fertile 
cell that the aecidiospores arise ; for after a pause in its development 
it becomes binucleate , and proceeds to elongate (Fig. 61) and cut off 
a series of binucleate aecidiospore-mother- cells (Fig. 60), the pair of nuclei 
dividing together by the process of conjugate division. 
The discovery of a sterile cell is alone a very important fact, for it 
shows that the aecidiospore-producing cell or hypha, at least in Phrag. 
violaceum , is a very specialized reproductive cell and not simply of the 
nature of an ordinary conidiophore. 
The process by which the fertile cell became binucleate was naturally 
closely investigated, but all attempts to observe the single nucleus of the 
fertile cell in any stage of division were quite unavailing, in spite of the 
fact that other divisions were met with fairly frequently, and that in 
passing from the periphery to the centre of an aecidium of an appropriate 
