FYI: Leaf Story

[Michael Corriss <mcorriss@gate.net>]

Ahoy!
(Food for some dinosaurs.)
-mpc

Editors' Choice: Highlights of the recent literature
SCIENCE, Volume 305, Issue 5683,  
dated July 23 2004, is now available at:

         http://www.sciencemag.org/content/vol305/issue5683/twil.shtml


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PALEOBIOLOGY: Leaf Story
---------------------------------------------------------------------------H.
 Jesse Smith

The first tiny megaphyll leaves appeared in vascular land plants 50
million
years before this leaf type became widespread. Why did adoption of such
an 
advantageous photosynthetic structure take so long?

Osborne et al. performed a morphometric analysis of 300 plant fossils
from 
the Devonian and Carboniferous Periods in order to assess trends in leaf 
size and to estimate the rates of convective and evaporative heat loss. 
They used these data to test the theory that high levels of atmospheric
CO2
delayed the increase in leaf size by restricting stomatal development; 
larger leaves, possessing only limited cooling capacity, would have 
intercepted more solar energy and thereby suffered lethal overheating.
As 
CO2 decreased, leaf blades were able to grow in size, and stomatal
number 
rose abruptly. This analysis therefore supports the idea that the
evolution
of leaves was constrained by atmospheric CO2 concentrations. -- HJS

Proc. Natl. Acad. Sci. U.S.A. 101, 10360 (2004).

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