A Tangled Tree
Some plants have vascular systems, but no plants have hearts. Let’s start at the beginning.
The first plants on this planet were aquatic and can loosely be called algae. It wasn’t until about 450 million years ago that plants began to inhabit terrestrial space. Why did they start in the water? Because life is easier there. As an aquatic plant, you don’t have to deal with all the trials and tribulations of the full force of gravity nor do you have to have an extensive, water seeking root system because you’re already surrounded by water.
The first land plants can be collectively referred to as the bryophytes. Within the bryophytes, we have things like liverworts, hornworts, and mosses. These plants avoided the gravity issue by being short and stout and only grew in damp places to avoid drying out. They were extremely successful on land, growing in any nook they could find. Incredibly enough, the bryophytes developed the ability to withstand dry conditions by entering a state of dormancy when left high and dry, and then were able to spring back to life as soon as water was reintroduced.
About 420 million years ago, we begin to see plants that have traits that solve the problems of being big and dealing with gravity. The evolution of columnar cells called tracheids that act as conducting vessels for vascular fluid allowed plants to grow bigger and taller than their stubby ancestors. These plants also developed lignin, a small molecule that aids in the toughening of otherwise non-rigid, cellulose plant cell walls. These were the first vascular plants and from them come nearly every plant species in existence today.
Now all this change didn’t happen after one trip around the sun. If you look at the numbers, it took about 30 million years to get from nonvascular to vascular plants, meaning it took 30 million years for tracheids and lignin to arise. That’s 30 million years for one cell type and one very tiny molecule. Geological time is almost unfathomable for organisms like us that expire after only 80 years.
So why do we still have mosses, liverworts, and hornworts today? Because those organisms still have an adaptive role to fill. The eventual appearance of larger plants did not spell the end for our little nonvascular friends because the two groups fill different evolutionary spaces. Mosses don’t require soil and are capable of growing on uninviting surfaces like bare rock, while larger vascular plants require some sort of substrate, whether it be sand or soil. Bigger is not necessarily better, nor is vascularity necessarily better, it’s just different. Evolution is not an active, linear series of progressions from “lower” to “higher” organisms, rather it is a passive, nonlinear process pushed and pulled by a tangle of stochastic events that lead to the development of new organisms. Think of it as a great web or a tree rather than a straight line.
Today we can take a walk and see mosses, grasses, shrubs, and trees all in one glance, without even a second thought. After all the scientific inquiry made across the duration of human existence, we still have only a vague notion of how much biodiversity there is on this planet. It has taken about 4.5 billion years for the Earth to get to this point so it’s no surprise that we’re struggling to wrap our minds around it all.
How lucky are we to engage in that struggle, only if for blip on the timescale of the universe.
Source
Tudge, Colin. The Tree: A Natural History of What Trees Are, How They Live, and Why They Matter. New York: Crown, 2006. Print.
