BASCOMBE BRITT HIGGINS 
plasm. That no very poisonous toxin or enzyme is given off is indi- 
cated by the fact that cells penetrated by the haustoria appear healthy 
and are able to deposit a cellulose sheath around the haustorium. 
In every case examined acervuli had already formed and had 
broken the epidermis of the leaf, more commonly on the upper surface, 
but often on both surfaces. In every case also, even the very earliest, 
the tissue of the spot containing the fungus had already separated or 
begun to separate from the surrounding tissue. 
Formation of Absciss Layer Around the Spots. — This separation 
is brought about by the enlargement of a layer of cells at some distance 
from the ends of the mycelium (figs. 14, 15, 16). Their enlargement is 
so abrupt and so great that the active cells separate from the adjoining 
inactive cells inside. The enlarged cells have lost their chloroplastids 
and nuclei, and only a thin layer of protoplasm lines the wall. The 
loss of the chloroplastids causes the watery appearance of this ring of 
tissue which was noted by Duggar (8). The tendency for the forma- 
tion of this layer to follow the veinlets of the leaf as mentioned by this 
author has very rarely been noticed. The spots which drop out are 
usually very nearly round; and, even when quite irregular, the margin 
is evenly curved and smooth, not angular as it would be if the separa- 
tion was limited by the veinlets. 
After the separation is completed all around, the spot turns 
yellow, shrivels rapidly and soon drops out. Sometimes (often in P. 
serotina) the tension exerted by the enlarging cells is not sufficient to 
rupture the cuticle, but the cellular tissues are ruptured and the spot 
dries but remains in position. After the separation has occurred a 
layer of cells just outside the enlarged cells shrivel, forming additional 
protection for the healthy tissue. 
In order to obtain the earlier stages of the process, plants in the 
greenhouse were inoculated with fresh conidia from leaves of P. 
virginiana which had been inoculated with a pure culture from the 
same species. The conidia were placed in a drop of water on a glass 
slide and a little powdered chalk added for the purpose of marking the 
inoculated spots. Small droplets of this water containing spores and 
chalk were placed on the under surface of the leaves of two plants and 
on the ufpper surface of two, and all four plants covered with bell jars. 
After three days some of the inoculated spots were cut out, killed in 
hot (about 60° C.) chrom-acetic acid, and embedded in paraffin. 
This was repeated every 24 hours until acervuli appeared above the 
