1918] SINNOTT—FOOD RESERVE 173 
Two ways at once suggest themselves through which ease or 
difficulty of diffusion might affect this enzyme activity. First, 
the water content of the cells may be modified, those to which water 
has easy access having a higher content than those which are more 
isolated or protected. That such a condition actually occurs is 
indicated by the fact that the starch grains in cells near vessels 
usually stain more lightly with iodine than the others, showing that 
they possess a higher proportion of water. Differences in water 
content doubtless affect the whole physiological activity of the cell 
and may well determine the type of enzyme action. As to why, 
on this supposition, there should be such radical seasonal alterations, 
however, is not clear. There are doubtless changes in the water 
content of the tissues after leaf-fall and again at the spring awaken- 
ing, and these changes will of course be felt most by those cells to 
and from which diffusion is easy. In the case of fat, at least, we 
know that abundance of water favors lipolytic action, and paucity 
of water favors the synthesis of fat, facts which probably help to 
determine the increase or decrease of fat with the seasons. 
A second suggestion is that the enzymes themselves are carried 
by the water as it diffuses through the tissues, and thus effect the 
characteristic changes in the cells which they enter. That these 
changes take place in the fall we may perhaps ascribe to the presence 
of enzymes in the sap which is withdrawn into the tissues of the 
stem from the leaves before the latter are shed. The enzymes 
would thus be particularly abundant in the phloem, the ordinary 
channel of conduction from the leaves downward, and they probably 
would occur in the water of the vessels. They would be progres- 
sively less common in those parts of the plant farthest from the 
leaves. This would explain (1) why the changes are most marked 
in the phloem'and adjacent regions and around the vessels; (2) 
why they are more marked in twigs than in branches and trunks; 
and (3) why in roots they are practically absent. 
_ To determine whether or not lipase is actually present in leaves, 
a series of experiments was undertaken. The leaves of a number of 
Species of trees were gathered in the latter part of summer, dried, 
and finely pulverized, and the leaf powder of each was mixed with 
olive oil and bottled up, a number of bottles being made for each 
