182 
MR. H. MARSHALL WARD OH THE STRUCTURE 
leaves were infected—one leaf on each plant—and controlled as described. The date 
of sowing the conidia on the leaf is given in the second column, and the date on 
which the pallid spot was clearly visible in the third column : it should be noted that 
some difficulty occurs in deciding exactly when the pallid spot is visible, a difficulty 
which depends partly on the observer and partly on the hue of the leaf. The yellow 
spot of a Uredinous fungus is much easier to detect than these pallid greenish-white 
sjDots in their younger stages. 
Plant. 
Date of sowing conidia. 
Date on which the 
spot appeared. 
Number of days 
occupied in developing 
spot. 
days. 
A 
May 13 ... 
June 1 .... 
19 
B 
May 13 . 
May 31 ... . 
18 
C 
May 13 . 
June 1 
19 
D 
May 16 ... 
June 2 ... . 
17 
E 
May 16 ... 
Failed 
E 
May 16 ... . 
Failed 
G 
May 24 ... 
June 7 ... . 
14 
H 
May 24 ... 
June 7 ... . 
14 
I 
May 24 ... 
Failed 
K 
June 2 ... . 
June 21 ... 
19 
L 
June 2 .... 
June 10 ... 
17 
M 
June 2 ... . 
June 20 ... 
18 
Hence the periods given in the Table are only approximate—indeed, they could not 
be otherwise so long as we are ignorant of the exact period occupied by the conidium 
in germinating, and by the fungus in making its progress through the tissues. 
And now arises the question—can we throw any light on the problem as to the 
relative ease with which a parasitic mycelium invades its host ? 
So far as it goes, the following evidence seems of some value. I found that those 
plants of Ranunculus Ficaria which grew in the shaded damp ditch were infected 
more easily than plants growing in open drier situations. The differences between 
these two kinds of plants, so far as the leaves are concerned, are chiefly as follows :— 
The more shaded plants have much larger leaves with much longer petioles : the 
laminae are undoubtedly softer in texture, and brighter green in colour, the smaller 
tougher leaves of the plants in the open being of a dark and glossy green, especially 
above. These differences correspond to differences in minute structure : the shaded 
leaves have shorter, looser, palissade cells, and more intercellular spaces between them 
and in the large loose spongy parenchyma. Moreover, the stomata on the upper 
surface appear to be more numerous : on the lower surface the stomata seem to be 
larger, but I cannot say they are more numerous. The stomata have wider 
openings in the damp shaded plants, and the cell walls of all the parts are thinner 
and more watery ; of course it may be assumed that there is more aqueous vapour in 
the intercellular spaces. Taking all these facts into consideration, I see no difficulty 
in explaining the differences in the times occupied in infections; and they also throw 
