Tree of Plants

Everyone knows what plants are like; they have leaves and roots, flowers and seeds. Or do they? All of these classic features of plants are actually relatively recent developments in plant evolution. Conifers don't have flowers, ferns don't have seeds or flowers and moss doesn't have leaves, roots, seeds or flowers! Leaves, roots, flowers and seeds are all features that evolved as plants adapted, starting at something like seaweed, to life on the land.

This term's issue of Phenotype has a bit of a focus on plants, and my research comic for this issue focuses on how plants evolved and adapted to land. You can download a pdf of this feature here, the full issue for the summer (Trinity) term will be available soon here.

While I was making this I started reconsidering just what the plant life cycle looks like, as a classic school education about how plants reproduce isn't very accurate! The classic teaching is that the pollen produced by a flower is like sperm in mammals (and humans), and the ovum in the flower is like the egg in mammals. In fact pollen and the developing seed are more like small haploid multicellular organisms, gametophytes, that used to be free living. If you go back through evolutionary time towards ferns then the gametophyte is a truly independent multicellular organism. Go back further still and the bryophytes spend most of their time as the gametophyte.

If you imagine the same evolutionary history for humans then it is easy to see how different this life cycle is to animals; if the ancestors of humans had a life cycle similar to ferns then, roughly speaking, ovaries and testicles would be free-living organisms that sprout a full grown human once fertilisation successfully occurs. I can't help but think that would have been a little strange!

Software used:
Autodesk Sketchbook Pro: Drawing the cells.
Inkscape: Page layout.