6 THOMAS: INFECTION OF APIUM GRAVEOLENS 
layers with fragments of leaf cells mingled with the mycelium of 
the fungus making up the spot. Pycnidia begin to form before 
the tissue breaks down, in fact it is not uncommon, as has been 
noted, to see, with the hand lens, mature pycnidia with ostioles on 
tissue which is still green and in which no shrinkage can be detected. 
The pycnidium originates usually if not always in an intercellular _ 
space, frequently in the substomatal cavity and as it increases in 
size the adjacent cells are broken down and successively become 
replaced by the heavy thick-walled hyphal elements. Occa-. 
sionally a portion of a cell may remain intact until its lumen is 
tightly packed with the mycelium. The origin of the pycnidium 
is not necessarily always subepidermal but may be at any point 
in the mesophyll or palisade tissue. On the petiole, however, the 
pycnidia seem to be restricted to the outer layers, none having 
been observed deeper than the third or fourth cell layer. When 
the leaf tissue collapses the pycnidia become more conspicuous, 
projecting above the general level of the spot. From the sharp- 
ness of the margin, which appears macroscopically between the 
spot and the surrounding tissue, it might be concluded that the 
margin of the mycelial growth is coincidént or nearly so with the 
margin of the spot. In sections, however, the mycelium is found 
at a distance of several cell diameters in advance of the breaking 
down of the tissue. It has been noted further that pycnidia are 
visible with low magnification in the green margin surrounding 
the spot. As the spot ages, there is a reduction in the green color 
of the tissue immediately surrounding it. Strangely enough, 
however, when the leaf yellows from age this partially yellowed 
region retains its color longer than any other portion of the leaf. 
Whether or not the green is intensified in this area as it diminishes 
elsewhere, I am not able to say from my observations. However, 
it is plain that in some way the fungus has caused the prolongation 
of the life of the cells in this region beyond that of the cells of the 
remainder of the leaf. This is further evidenced by the fact that 
on petioles which are wilted gradually, the loss of both chlorophyll 
and water is inhibited in the vicinity of infected spots, especially 
the smaller spots (which do not lose any considerable amount of 
water through the dead tissue). This condition was especially 
marked in a plant in the field which had its lower roots cut by 
mice and was wilting slowly. 
