Field of Science

Tropical leaf drip tips - yam

A student asked a good question the other day after telling them about past climatic differences. How do we know what the climate was like millions of years ago? One answer too frequently heard is that we can't know because no one was there to observe the climate. In the real world observations can still be made about the past. For example, leaves display different forms as adaptations to different climates. Here's a leaf from a yam (Dioscorea), which is not a sweet potato. The leaf shows a couple of adaptations for a wetter environment. First it's a fairly large leaf, and leaf size is very generally correlated positively with warmer wetter climates. Note the attenuated apical tip, a drip tip, with a drip on the tip, and like the pouring lip of a beaker or pitcher, that tip helps water drip and pour off the leaf more efficiently. The major veins that generally follow the out line of the leaf margin from base to tip form valleys, gutters, again to help water pour off the leaf. So when you find fossils with such features, or with features associated with very seasonal climates, or very dry climates, paleobotanists, infer that they were adapted to a particular climate suited to that leaf type. And if that's not so, then maybe those leaves weren't well designed after all.

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