When you think of the Age of the Dinosaurs, you probably imagine a humid jungle choked with ferns and massive moss-draped trees. Plants have been around for about 475 million years--compare this with 100,000 years for modern humans--and have seen and survived upheavals that eradicated entire species. Plant evolution tells the story of how a kingdom changed to survive.
Moving to Land
Plants began their life in the water in a form a lot like algae or seaweed. In this form, cells were constantly bathed in water rich in nutrients and dissolved gases. When plants first appeared on land, they encountered the new challenge of needing to acquire water and nutrients that were less readily available. According to Stephen P. Broker of Yale University, the challenge of making water and nutrients available to all parts of terrestrial plants drove many of their adaptations.
The first land plants were the bryophytes, the most familiar extant example of which is the mosses. Bryophytes lack roots or vascular systems, meaning that all of their cells need to be in close proximity to a source of water. For this reason, mosses remain small and tend to grow in cool, damp locations. Bryophytes also require water to carry the sperm to the egg during reproduction.
The development of vascular tissue to transport water and nutrients allowed ancient plants to push up from the ground. The first vascular plants were the club mosses, which dominated the landscape 40 million years ago, sometimes growing to the size of trees. Horsetails and ferns followed suit, and these early vascular plants developed root systems that eliminated, somewhat, their reliance on a constant source of water. However, these early plants still reproduced with spores and still relied on water for reproduction.
Spores presented challenges to early plants. Not only did the plants require water to carry the sperm to the egg to produce the spore, but spores were relatively unprotected structures that relied on the wind and good luck to germinate. The first seeds evolved on seed ferns, now extinct, as long as 385 years ago. Early seeds needed the wind, not water, to carry sperm encased in pollen grains to fertilize the egg, which was covered by a protective seed. Seeds not only protected embryonic plants but contained a food source used when seedlings first germinated.
Early seed-bearing plants included ginkgos, conifers and cycads, a type of palm tree that bears its seeds on cones. These plants still relied a good deal on chance, however, with respect to reproduction. Flowering plants further evolved structures that made reproduction deliberate. The earliest flowering plants appeared 132 million years ago and were similar to magnolias found today. Flowers not only attracted pollinators but encased the developing seed inside of an ovary, offering it further protection.