22 1 
and Life-history of Pnccinia Graminis. 
Fig. io. Transverse section of a leaf of barberry infested 
with the Aecidium- form. The section has passed through 
three spermogonia and two aecidia (to the right below). The 
mycelium, ramifying in the mesophyll of the leaf, causes 
hypertrophy — due to the stimulated cells acting as centres 
of attraction for larger shares of food-materials, and then 
growing abnormally rapidly at their expense — whence the 
cushion-like thickening especially on the under side of the 
leaf. After developing in the cushion for about eighteen or 
twenty days, spermogonia begin to form, especially (but not 
only) on the upper surface. All that I can say about their 
earliest stages is that closely-woven balls of hyphae are 
formed below the epidermis, gradually become larger and 
hollow, and burst at the apex through the epidermis. 
After developing several series of spermogonia, the my- 
celium begins to form larger balls of interwoven hyphae 
beneath the epidermis of the lower side of the leaf. These 
gradually increase in size, and form the aecidia — hollow, 
spheroidal cavities filled with the aecidio spores. By this 
time the cells of the mesophyll in the neighbourhood of the 
aecidia are becoming disorganised : the chlorophyll-grains 
lose their firmness of contour, and the walls of the cells 
turn light brown, as shown in the figure. (Zeiss B.) 
Fig. io a. Vertical section through an aecidium much more 
highly magnified. The aecidium is seen to consist of a cup- 
like casing of cells, peridium , with thickened and striated 
outer walls and orange contents, enclosing vertical series of 
aecidiospores developed in regular rows from the hasidia 
below ; as seen in the figure the peridium is simply formed 
by a modified series of cells with similar origin to the aecidio- 
spores — all spring from a radiating series of basidia, which, 
again, are merely branches from the mycelium. At first the 
peridium forms a closed body (see Fig. io to the right below) 
beneath the epidermis ; but as development proceeds the 
epidermis is ruptured — often at a stoma— and the peridium 
separates above. The aecidiospores ripen from above 
downwards, i. e. the older ones are ripe and separate off, 
