Vascular Land Plants

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?

Fewer, broader, leaves
Lots of thin leaves