2.5.1: Introduction to Early Land Plants

Learning Objectives

• List the shared derived characteristics of land plants.
• Relate these adaptations to the movement from aquatic to terrestrial habitats.
• List the ancestral characteristics that land plants share with green algae.
• Describe the basic life cycle shared by plants.

Land Plants

Land plants are sometimes referred to as “embryophytes” due to the evolution of the embryo, a zygote that is retained and nourished by the female gametophyte as it grows. Embryophytes share many common features, most corresponding to the selective pressures from the initial movement onto land. The embryo is one of these, providing higher likelihood of success for offspring in a new, harsh environment. In addition to the embryo, all plants have the same basic life cycle: alternation of generations (Figure \(\PageIndex{1}\)). Much like the marine algae, the first plants were living in an environment where they needed to increase their chances of reproductive events. Multicellular stages on both sides of the life cycle increases the number of reproductive propagules. All plants are also multicellular, with tissues and multicellular gametangia.

Figure \(\PageIndex{1}\): A generalized life cycle for plants. Alternation of generation involves two multicellular individuals. The top half is haploid and contains the spores, gametophyte, and gametes. The bottom half is diploid and contains the zygote and sporophyte. The multicellular haploid (n) individual is called the gametophyte. The gametophyte produces structures called gametangia where haploid gametes are produced by mitosis. Gametes do not grow, but instead must fuse together to form a diploid (2n) zygote. In plants, the zygote is retained on the megagametophyte as an embryo, where grows by mitosis into a multicellular diploid individual, the sporophyte. The sporophyte makes sporangia in which meiosis takes place, producing haploid spores. These spores are released and can grow by mitosis into haploid gametophytes. Diagram by Peter coxhead / derivative work: Peter coxhead, Public domain, via Wikimedia Commons.

Other adaptations to life on land include the desiccation-resistant compound sporopollenin. This is found in the cell walls of spores of early land plants and in pollen of seed plants, hence the name sporo - pollen - in. It is also found in the cell walls of a few green algae. On the exterior, plants are surrounded by a waxy cuticle that helps protect them from their outer environment.

Much like their green algal predecessors, plants store their carbohydrates as starch inside plastids, plastids with two membranes (the result of primary endosymbiosis), cell walls containing cellulose, and have the pigments chlorophyll a, chlorophyll b, and carotenoids. For this reason, along with genetic sequencing, land plants and green algae are grouped together in the Viridiplantae (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): This figure represents the lineages contained within the Viridiplantae. It was retrieved from an open-access paper analyzing over 1,000 transcriptomes to assess the relationships between these lineages. The original caption follows: "a–e, Green algae. a, Acetabularia sp. (Ulvophyceae). b, Stephanosphaera pluvialis (Chlorophyceae). c, Botryococcus sp. (Trebouxiophyceae). d, Chara sp. (Charophyceae). e, ‘Spirotaenia’ sp. (taxonomy under review) (Zygnematophyceae). f–p, Land plants. f, Notothylas orbicularis (Anthocerotophyta (hornwort)). g, Conocephalum conicum (Marchantiophyta (thalloid liverwort)). h, Sphagnum sp. (Bryophyta (moss)). i, Dendrolycopodium obscurum (Lycopodiophyta (club moss)). j, Equisetum telmateia (Polypodiopsida, Equisetidae (horsetail)). k, Parablechnum schiedeanum (Polypodiopsida, Polypodiidae (leptosporangiate fern)). l, Ginkgo biloba (Ginkgophyta). m, Pseudotsuga menziesii (Pinophyta (conifer)). n, Welwitschia mirabilis (Gnetophyta). o, Bulnesia arborea (Angiospermae, eudicot, rosid). p, Paphiopedilum lowii (Angiospermae, monocot, orchid). a, Photograph reproduced with permission of Thieme Verlag, Stuttgart⁶⁶. b–e, Photographs courtesy of M. Melkonian. f–j, l–n, p, Photographs courtesy of D.W.S. k, Photograph courtesy of R. Moran. o, Photograph courtesy of W. Judd." One Thousand Plant Transcriptomes Initiative., Leebens-Mack, J.H., Barker, M.S. et al. One thousand plant transcriptomes and the phylogenomics of green plants. Nature 574, 679–685 (2019).

The First Land Plants

Bryophytes and seedless vascular plants were the first groups to evolve on land. While the order of events during this period in plant evolution remains somewhat murky, evidence points to bryophytes evolving first, followed by the SVPs.

Bryophytes are the only group of land plants that lacks lignified vascular tissue, though some mosses do contain a form of conducting tissue. They tend to be small of stature and lack true stems, roots, or leaves. Bryophytes are also the only group of land plants to have a gametophyte dominant life cycle.

Seedless vascular plants transition to a sporophyte dominant life cycle with a branching sporophyte. They evolve lignified vascular tissue, developing true roots, stems, and leaves. This tough, lignified tissue allows some species within this group to form trees up to 100 feet tall.

Attribution

Content by Maria Morrow, CC-BY-NC