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How Leaf Veins Changed the World

When we think of the dominance of flowering plants on the landscape, we usually invoke the evolution of flowers and seed characteristics such as an endosperm and fruit. However, evolutionary adaptations in the structure of the angiosperm leaf may have been one of the critical factors in the massive diversification that elevated them to their dominant position on the landscape today.

Leaves are the primary organs used in water and gas exchange. They are the centers of photosynthesis, allowing plants to take energy from our closest star and turn it into food. To optimize this system, plants must balance water loss with transpiration in order to maximize their energy gain. This requires a complex plumbing system that can deliver water where it needs to be. It makes sense that plant physiology should maximize vein production, however, there are tradeoffs in doing so. Veins are not only costly to construct, they also displace valuable photosynthetic machinery.

It appears that this is something that flowering plants do quite well. Because leaves fossilize with magnificent detail, researchers are able to look back in time through 400 million years of leaf evolution. What they found is quite incredible. There appears to be a consistent pattern in the vein densities between flowering and non-flowering plants. The densities found in angiosperm leaves both past and present are orders of magnitude higher than all non-flowering plants. These high densities are unique to flowering plants alone.

This innovation in leaf physiology allowed flowering plants to maintain transpiration and carbon assimilation rates that are three and four times higher than those of non-flowering plants. This gives them a competitive edge across a multitude of different environments. The evolution of such dense vein structure also had major ramifications on the environment.

This massive change in transpiration rates among the angiosperm lineage is likely to have completely changed the way water moved through the environment. These effects would be most extreme in tropical regions. Today, transpiration from tropical forests account for 30-50% of precipitation. A lot of this has to do with patterns in the intertropical convergence zone, which ensures that such humid conditions can be maintained. However, in areas outside of this zone such as in the Amazon, a high abundance of flowering plants with their increased rates of transpiration enhances the amount of rainfall and thus forms a sort of positive feedback. Because precipitation is the single greatest factor in maintaining plant diversity in these regions, increases in rainfall due to angiosperm transpiration effectively helps to maintain such diversity. As angiosperms rose to dominance, this effect would have propagated throughout the ecosystems of the world.

Photo Credit: Bourassamr (Wikimedia Commons)

Further Reading:

Cretaceous Seeds Shine Light on the Evolution of Flowering Plants

What you are looking at here are some of the earliest fossil remains of flowering plants. These seeds were preserved in Cretaceous sediments dating back some 125–110 million years ago. Fossil evidence dating to the early days of the angiosperm lineage is scant, which makes these fossils all the more spectacular. Thanks to a large collaborative effort, Dr. Else Marie Friis is shining light on the evolution of seeds.

Finding these fossils is not a matter of seeing them with the naked eye. These seeds are tiny, ranging from half a millimeter up to 2 millimeters in length. They were discovered using an advanced form of X-ray microscopy. The advantage of this technique is not only that it is nondestructive but it also allows researchers to investigate the internal structures of the seeds that would otherwise be impossible to see. Their preservation is mind blowingly delicate, allowing researchers to see minute details of the embryo and even subcellular structures like nuclei. 

Dr. Friis' team was able to look at over 250 fossil seeds from 75 different taxa. They were able to make 3D models of the embryos, allowing for more detailed studies than ever before. For some of the fossils, the detail was such that they were able to match them to extant lineages of flowering plants. For others, this technique is allowing for better reclassification of now extinct species. 

By far the most exciting part about these fossils are what they can tell us about the ecology of early flowering plants. In all instances, the embryos within the seeds were small, immature, and dormant. This suggests that seed dormancy is a fundamental trait of flowering plants. What's more, this lends support to the hypothesis that angiosperms first evolved as opportunistic, early successional colonizers. Seed dormancy allows flowering plants to wait out the bad times until favorable environmental conditions allowed for germination and seedling establishment. 

Photo Credit: Dr. Else Marie Friis

Further Reading:

The Dawn Redwood

The dawn redwood (Metasequoia glyptostroboides) is one of the first trees that I learned to identify as a young child. My grandfather had one growing in his backyard. I always thought it was a strange looking tree but its low slung branches made for some great climbing. I was really into paleontology back then so when he told me this tree was a "living fossil" I loved it even more. It would be many years before I would learn the story behind this interesting conifer.

Along with the coast redwood (Sequoia sempervirens) and giant sequoia (Sequoiadendron giganteum), the dawn redwood makes up the subfamily Sequoioideae. Compared to its cousins, the dawn redwood is the runt, however, with a max height of around 200 feet (60 meters), a mature dawn redwood is still an impressive sight.

Until 1944 the genus Metasequoia was only known from fossil evidence. As with the other redwood species, the dawn redwood once realized quite a wide distribution. It could be found throughout the northern regions of Asia and North America. In fact, the fossilized remains of these trees make up a significant proportion of the fossils found in the Badlands of North Dakota.

Fossil evidence dates from the late Cretaceous into the Miocene. The genus hit its widest distribution during a time when most of the world was warm and tropical. Evidence would suggest that the dawn redwood and its relatives were already deciduous by this time. Why would a tree living in tropical climates drop its leaves? Sun.

Regardless of climate, axial tilt nonetheless made it so that the northern hemisphere did not see much sun during the winter months. It is hypothesized that the genus Metasequoia evolved its deciduous nature to cope with the darkness. Despite its success, fossil evidence of this genus disappears after the Miocene. For this reason, Metasequoia was thought to be an extinct lineage.


All of this changed in 1943 when a Chinese forestry official collected samples from a strange tree growing in Moudao, Hubei. Though the samples were quite peculiar, World War II restricted further investigations. In 1946, two professors looked over the samples and determined them to be quite unique indeed. They realized that these were from a living member of the genus Metasequoia.

Thanks to a collecting trip in 1948, seeds of this species were distributed to arboretums around the world. The dawn redwood would become quite the sensation. Everyone wanted to own this living fossil. Today we now know of a few more populations. However, most of these are quite small, consisting of around 30 trees. The largest population of this species can be found growing in Xiaohe Valley and consists of around 5,000 individuals. Despite its success as a landscape tree, the dawn redwood is still considered endangered in the wild. Demand for seeds has led to very little recruitment in the remaining populations.

Photo Credit: [1] [2] [3]

Further Reading: [1] [2]