And how come the biggest trees in the world are conifers?

Dec 23, 2005 11:24 GMT  ·  By

The conifers, like firs and pines, belong to a class of primitive trees - the flowerless trees. How come they weren't displaced by more evolved trees and they still rein the temperate region forests?

One of the most important problems a tree has to solve is water transportation from the soil to the tip of every branch. The water is also used to transport the needed nutrients. Thus, plants don't have blood, they use water.

For transporting the water the plants have a large number of tiny "pipes", called tracheids that are positioned relatively parallel to one another, each having a number of valves. In case of conifers, these tracheids are a few millimeters long, while the angiosperms' tracheids are a few centimeters long. ("Angiosperms" is the technical term denoting all the flowering plants.) In other words, it would seem that flowering trees (such as oaks) should have a decisive advantage over the conifers because supposedly they should have a better water transport system. However, in many places all over the world it is the conifers that rein the forests. Moreover, the larges trees in the world (sequoias) are conifers.

In today's issue of Science magazine, John Sperry from Utah University publishes a paper that solves this mystery. Sperry and his team have discovered that the conifers' valves have a different structure. Here is an electronic microscope image of how a conifer valve (on the left) looks and a flowering tree valve (on the right).

As you can see, the conifer's valve allows water to pass much easily, while the angiosperm's valve has a homogeneous porous surface less permeable. Sperry has measured that water passes 59 times more easily through a conifer's valve than through a oak's valve. Thus, the conifers and the flowering trees have developed alternative methods for facilitating water transport: the conifers have more evolved valves, while the angiosperms have more evolved tracheids.

This is an interesting fact from one more point of view: it is another proof that flowering trees haven't evolved from conifers, but that they have a common ancestor. The conifers have appeared more than 280 million years ago and they have inherited the primitive tracheids that appeared around 400 million years ago. Their valves appeared around 220 million years ago. On the other hand, the flowering tree have appeared 146 million years ago, but not from conifers, they appeared from other more primitive plants from which they have inherited the primitive, 400 million years old, valves. But eventually their tracheids gradually evolved into longer ones.

Photocredits: Springer Science and Business Media, G.L. Comstock, W.A. Cote and E. Wheeler.

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