Fig.1 Leaf of tropical areas having drip tip for water run off.
How could the shape of plant leaves indicate temperature, you ask? Surprisingly, they do so very well. There is a general relationship between leaf shape and the climate. In 1978 Jack Wolfe, of the United States Geological Survey, put the relationship on a quantitative footing. Using data for present –day forests in eastern Asia, he showed that there is a remarkable correlation between the mean annual temperature and the shapes of the leaves. The particular features of leaves that seem to be most distinctive in this regard is the nature of the leaf margin. In tropical areas, where temperatures and precipitation are high, leaves tend to be large and have smooth edges, without serrations, and they often have a narrow, elongated tip-sometimes referred to as a drip tip (Fig.1.)- that facilitates water runoff. In contrast, leaves in cooler regions are typically smaller, narrower, and usually have jagged edges. In today’s forests these characteristics are specific to climatic conditions through out the globe.
Warmer leaf temperatures promote both photosynthesis and transpiration; thus, plants in drier climates tend to have smaller leaves to reduce evaporative cooling, while in more humid climates larger leaves are common because the attendant water cost is less critical (Givnish, 1984).
Since plants are stationary they must respond developmentally and ultimately evolutionarily, to their environment. As a result, it's not surprising that leaf morphology (shape) has been shown to be related to climate. For example, some the following correlations have been reported (a) leaf length is directly related to the mean annual temperature (MAT), (b) leaf area is directly correlated to both MAT and mean annual precipitation (MAP); and (c)leaf width is directly correlated with MAP. Thus, leaves are longer and larger in climates with warmer temperatures and higher rainfall.
Another interesting observation that was first reported more than 100 year ago is that woody deciduous plants having leaves with toothed margins (termed serrate) predominate in temperate climates while species with smooth (termed entire) leaf margins predominate in frigid (arctic) and tropical climates.
Reference:
Givnish TJ. 1984. Leaf and canopy adaptations in tropical forests. In: Medina E, Mooney HA, Vasquez-Yanes C, eds. Physiological ecology of plants of the wet tropics. The Hague, the Netherlands: Dr. W. Junk Publishers, 51–84.
Macdougall,J.D. 1996. A short history of planet earth, mountains, mammals, fire, and ice. John Wiley & Sons, Inc. Canada.
http://www.employees.csbsju.edu/ssaupe/biol106/Labs/leaf%20lab.pdf
