32 BULLETIN OF THE BUSSEY INSTITUTION. 
of the leaf previously described, and pressing against them. These 
threads are cylindrical in shape, branch in all directions, and are 
only rarely divided by cross partitions. They are of a brownish 
color, and filled with a granular mass in which we never find any 
chlorophyl-grains. If we apply iodine and sulphuric acid we shall 
find that the wall of the filament does not turn blue, or, at least, not 
very decidedly so, whereas the walls. of the true leaf cells become a 
bright blue. This blue color is owing to the presence of cellulose, a 
substance allied to starch, and composed of oxygen, hydrogen, and 
carbon. ‘The contents of the leaf cells, known as the protoplasm, the 
outer and thicker layer of which has been called the primordial utriele, 
contain, in addition, nitrogen. ‘The contents of the mycelial threads, 
except that chlorophyl-grains are wanting, are essentially similar to 
those of the leaf cells. 
We need not limit our observations to the brown spots on the leaves. 
If we examine the adjacent green part, or even the stalk, we shall find 
the same filaments running between the cells, the only difference be- 
tween the two cases being that, in the latter, the leaf-cells have a fresher 
look, and the chlorophyl-grains are still green. Sometimes little pro- 
jections are given off by the filaments, which depress the walls of the 
adjacent cells, or even perforate them, making their way into the in- 
terior. Such projections, however, are not common, either in the 
leaf or stalk. These filaments are what is known to botanists as the 
mycelium, or vegetative threads of a fungus, the Peronospora infestans ; 
and, if we examine any potato plant affected by the rot, even before 
any spots have appeared on the leaves, we shall always find these 
threads in the leaves, stem, and, in fact, nearly the whole plant. 
It is a well known fact in vegetable physiology that the assimilation 
of food is done by cells containing chlorophyl; and, since the mycelium 
contains no chlorophyl, the Peronospora must steal its food from the 
already assimilated material in the potato cells. It does this by direct 
absorption. The leaf-cells are capable of doing a certain amount of 
extra work, and can support not only themselves, but a given amount 
of the Peronospora also. Accordingly, we see some leaves green and 
apparently healthy, which, on microscopic examination, are found to 
contain some of the mycelium of the Peronospora. \But there is a 
limit to the capacity of the green cells for work, and, when the para- 
site has grown to such an extent as to demand too much of them, they 
