ACIDIA 3 
but the mycelium produced is very scanty. Their function 
will be discussed in a later paragraph, but it may be mentioned 
here that all attempts to produce infection by means of them 
have uniformly failed. 
THE AXCIDIUM. 
Before the spermogones have completed their development, 
similar but larger conglomerations of hyphe arise on the lower 
surface of the leaf, a little way below the epidermis, which they 
raise up into a rounded dome. These masses enclose a number 
of erect hyphe of two kinds—an 
outer series of parallel closely- 
joined colourless hyphe, forming 
the peridiwm, consisting of more 
or less hexagonal cells, which meet 
above and roof over the dome- 
shaped cavity; and an interior 
series which remain shorter and 
give off from their upper end 
parallel chains of spores called 
aecidiospores, which fill the whole 
of the enclosed space. Each new 
spore is produced beneath the older 
ones, which are thereby pushed 
gradually up. Finally, this struc- 
: 5 cen Fig. 3. Leaf of Nettle (under 
ture, which is called an aecidium, side) showing the scidia 
of P. Caricis on the lamina 
ruptures the epidermis, forces its andl petiole. 99% 
way between the cells, the peridium 
bursts at the summit, the edges become revolute, and there is 
formed a white cup-shaped hollow, its floor covered with erect 
chains of orange spores (see Fig. 1). These spores have a 
rather thin, colourless, finely warted cell-wall, and are filled 
with rich bright-orange granular and oily contents. The 
mycelium which forms the ecidia is continuous with that 
which bears the spermogones, and its cells are uninucleate, but 
the spores themselves are binucleate. The origin and meaning 
of this change will appear afterwards. 
1—2 
