Photo by Steel Wool licensed under CC BY-NC-ND 2.0
For plants, the journey onto land did not happen over night. It began some 485.4–443.4 million years ago during the Ordovician. The best evidence we have for this comes in the form of fossilized spores. These spores resemble those of modern day liverworts. Under high powered microscopes, one can easily see that they were indeed adapted for life on land. These early plants were a lot like the hornworts, liverworts, and mosses we see today in having no vascular tissues for transporting water, an adaptation that would not come along for another few million years.
Without vascular tissues, plants like liverworts and mosses cannot transport water very far. They instead rely on osmosis and diffusion to get water and nutrients to where they need to be, which severely limits the size of these types of plants to only a few centimeters. This growth pattern carried on well into the Silurian. Until then, the greening of our planet happened in miniature.
Photo by Sabrina Setaro licensed under CC BY 2.0
Around 415 million years ago, however, plants became vascularized. This changed everything. It set the stage for the botanical world we know and love today. Paleobotanists place the fossil remains of these newly evolved vascular plants in the genus Cooksonia. Based on what we would call a plant today, Cooksonia probably pushes the limits. However, in some species the branching structure is full of dark stripes, which have been interpreted as vascular tissues. It still wasn't a very tall plant with the tallest specimen standing only a few centimeters but it was a major step towards a much taller green world.
Cooksonia did not have any leaves that we are aware of but some species certainly had stomata (another major innovation for water regulation in plants). Each branched tip ended in a sporangium or spore-bearing capsule. It has been suggested that Cooksonia may not represent an individual photosynthetic plant but rather a highly adapted sporophyte that may have relied on a gametophyte for photosynthesis. This hypothesis is supported by the diminutive size of many Cooksonia fossils. They simply do not have enough room within their tissues to support photosynthetic machinery. Because of this, some botanists believe that vascularization sprang from a dependent sporophyte that gradually became more and more independent from its gametophyte over time. Until an associated gametophyte fossil is found, we simply don't know.
Photo Credits: Steel Wool (http://bit.ly/1AjLYh8) and Sabrina Setaro (http://bit.ly/16mdyxw)