From still unknown flowers, trees, ferns and mosses in the rainforest to the vegetables and grain we eat at home, plants share a deep and in many ways still mysterious history. This history has shaped the evolution of the other kingdoms as well as the physical properties of Earth itself.
Land plants evolved about 500 million years ago from green algae that could survive periods of occasional drying thanks to a layer of durable polymer called sporopollenin. Charophyceae, also evolved from these green algae and are land plant’s closest relative. Charophyceae share many characteristics with land plants, including the storage of starch inside of plastids, phytochrome, and chlorophylls a and b. However, there are five key derived traits that distinguish land plants, also known as embryotophtes, from charophyceans. These are (1) localized regions of cell division at the tips of shoots and roots, (2) a life cycle in which two multicellular body forms alternate, (3) multicellular organs called sporanging that produce walled spores, (4) multicellular organs called gametangia that produce gametes, and (5) a multicellular and dependent embryo.
Scientists recognize two kinds of embryotophtes (land plants) based on the presence or absence of vascular tissue. These are bryophytes (nonvascular plants) and tracheophytes (vascular plants). Bryophytes are divided into three phyla – Hepatophya (liverworts), Anthocerophyta (hornworts), and Btyophyta (mosses). Tracheophytes can be distinguished not only by the presence of lignified tissue but also by a dominant sporophyte stage. Tracheophytes are also divided into two groups: seedless plants and seed producing plants.
The earliest vascular plants lacked seeds and instead reproduced by spores. Seedless plants include four phyla of living vascular plants, (Pterophyta, Psilophyta, Arthrophyta, and Lycophyta) and at least three other phyla known only in fossil form. The seed arose about 360 million years ago. A major genetic adaptation it protects and provides nutrients to the female gametophyte and the sporophyte embryo. Heterspory, endospory, ovules, and pollen also evolved around this time. Today’s living spermatophyte forms five groups: cycads, gingko, conifers, gnetophytes and angiosperms. Traditionally, they are divided into gymnosperms and angiosperms. In gymnosperms, the seed develops on the surface of scale, leaves, or at the end of short stalks and is unenclosed. In Angiosperms, the seed is encased within an ovary.
Angiosperms make up 90% of today’s plant biodiversity and they dominate both the natural and cultivated landscape. Angiosperms are vascular seed plants that produce flowers and fruit. Angiosperms can be divided into eight groups: Amborella, Nymphaeales, Austrobaileyales, Chloranthales, Magnoliids, Monocots, Ceratophyllum, and Eudicots. The relationship between these groups, as well as the time origin of angiosperms, is still a topic of discussion.
To see how molecular biology techniques add to our understanding of plant evolution, read Wickett et al. (2014) Phylotranscriptomic analysis of the origin and early diversification of land plants. To engage your student with key molecular biology techniques that are changing the way we view and use plants try one of these experiments: