Plants in temperate climates tend to have leaves with serrated margins, i.e. they have jagged edges; plants in warmer and more humid climates tend to have what are known in botanical jargon as entire margins, that is, smooth and unserrated. The difference is illustrated in the photographs.More at the link (and source credit for the graph). That article didn't address the question of "why." I found some related discussion in a Smithsonian article about fossilized leaves:
Rather than there being a sharp cut-off between the temperate and tropical styles of leaves, there is a continuous relationship between the climate and the mix of leaf types found in it: that is, as the climate gets a little hotter and wetter, the proportion of entire margins increases a little. This means that looking at a single leaf doesn't tell us that we are looking at a temperate or tropical climate; but
looking at a whole lot of species will allow us to do something a whole lot better than simply dividing climates into tropical or temperate: we can actually estimate the average annual temperature.
We can establish by observation that the ratio of temperate to tropical leaf styles is a surprisingly good indicator of average annual temperature, as illustrated by the graph [right], showing the relationship between floras and temperature in the forests of East Asia.
Scientists are still trying to understand the exact basis for this relationship, but they think it’s because plants in colder climates need to get a jump-start on converting sunlight to energy (photosynthesis) in the spring. Having more teeth enables more water to move out of the leaves, increasing the flow of sap and ramping up photosynthesis. This is important if you need to start photosynthesizing lots of food quickly, say because you only have a brief growing season before the cold comes. If you’re in a warm climate though, jagged edges do more harm than good: losing water can be dangerous to the leaf and to the whole plant, especially when it’s hot. This set of tradeoffs makes one leaf shape more favorable (and thus more predominant) at certain temperatures.