Some of your cells reach maturity, thicken their cell walls with a brand new chemical called lignin[1] and then die back, creating very fine tubes within you which can carry water by a combination of capillary action and osmosis. They are called “tracheids”. You have to sacrifice some cells, expend some energy making lignin, and be at risk of pumping any nasty chemicals throughout your system, but you will be able to photosynthesise more.
The structures are a huge success! Finally, you have the recipe for massive expansion on land!
This period of development on Earth is known as the carboniferous period. The air is around 55% carbon dioxide. Effectively, you have all the carbon you can fix, you can grow and grow and grow and still photosynthesise because you can provide water to the farthest sections of you.
You expand and expand and become hugely successful. The land of the Earth becomes green for the first time. Jungles abound, spreading almost from pole to pole.
However… (isn’t there always a however..?)
The amount of carbon dioxide in the atmosphere is dropping. All of the plants of this era are changing the climate dramatically. Within (??) million years, the carbon dioxide level has dropped to under 30%, and the least efficient collectors simply become extinct.
You need to become more efficient or it might be game over.
You currently photosynthesise all over, although more towards the tips because they get more light. Each tip is a long thin green point with a single vascule running up the centre of it. It’s where most of your photosynthesis takes place, but it would be better if it were wider, able to collect more light.
One day, a single plant experiences a mutation which causes the vascular system to branch inside the leaf.
Will you take that mutation and have the opportunity to make larger leaves, or stick without it and make more leaves instead?
[1] Lignin, being a brand new, super-tough chemical, has never been seen by bacteria before. For now, when you, or any of your descendents, die, you fall over and … lie there. You don’t decompose because the bacteria simply don’t know how to take you apart!
For millions of years, fallen plants just piled up, higher and higher, growing new plants on top of the old ones, until they were in heaps hundreds of metres thick. Since then, through tectonic activity, they have been compressed, and hardened, and turned into coal and oil deposits - which humans have relatively recently worked out how to extract for the energy they contain.
As a result, the level of carbon dioxide in the atmosphere dropped and dropped, as the carbon was locked away. We now have a new era - the anthropocene - in which humans are making global scale changes to their planet, including raising the carbon content of the atmosphere to a dangerous level. Well, dangerous for humans. Obviously evolution will allow life to continue, even if the humans are extinct.
Sadly, we can’t just stop burning carbon and wait for natural processes to deposit new coal beds - in the meantime bacteria have found a way to break down lignin. We will need to do it for ourselves.
