EXPOSURE ON LENGTH AND BREADTH OF LEAVES. 379 
exposed to the sun being actually, as well as relatively, broader. 
The last case, Ipomæa Pescapre, is interesting, as the Forms 
1 and 2 were taken from the same plant. Those in Form 1 
were gathered from the upright stems, seldom eighteen inches 
high, springing from the main root, while those in Form 2 
were from the prostrate branches of the same plant, which are 
sometimes seventy feet long. 
TABLE IV. 
Climbing-plants with petiolate leaves. 
| | Length of | Breadth of Leaf- 
| leaf. | leaf. | ratio. 
DE LN 
| Mikania scandens. A.......... kode | 1:175 131 
i Å B 1-029 1217 85 
| Ipomea palmata. A. ......... | 1408 | 1:42 99 | 
| % 5 B cs. | 1462 | 1:486 ‘98 | 
| Ipomea Pescapre (1) ......... | 3:223 | 3:16 1:02 
P. ER ee | 24 | 4 6 
The difference in this last case is not due to exposure, but was 
produced by the greater development of the basal veins of the 
leaf in Form 2. I find Sir John Lubbock has pointed out a 
similar difference in the leaf of the ivy. It seems to me possible 
that this is due to the angle at which the blade is inclined to the 
petiole; and one finds, I think, pretty generally that when there 
is a distinct and abrupt angle between the petiole and midrib, 
the leaf tends to become cordate, peltate, or generally broad 
towards the base. In this case and the ivy the leaves on the 
upright stems, which are narrower, have the petiole and midrib 
almost in a straight line. On this point, however, I have not 
finished my experiments. Such relatively broad leaves are 
common in climbing and prostrate plants; and if this angle of 
petiole and midrib does affect the form of the leaf, it may perhaps 
modify the effect of sheltered conditions. Still, in the three 
cases given the rule is followed. 
The general result of these experiments is confirmed by 
Dufour. His experiments (Ann. Sci. Nat., Bot. ser. 7, t. v 
