2.5.4: Chapter Summary

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Melissa Ha, Maria Morrow, & Kammy Algiers
Yuba College, College of the Redwoods, & Ventura College via ASCCC Open Educational Resources Initiative

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The characteristics of early plants can be interpreted best as evolutionary adaptations to life outside of water (a waxy cuticle, stomata, decay- and desiccation-resistant sporopollenin) and a new harsh landscape (multicellular gametangia, matrotrophy, mutualistic symbioses with bacteria and fungi).

As these early plants evolved, so too did their environment. Early bryophytes, in association with their fungal partners and perhaps with the help of other lichens, caused chemical weathering of the rocky substrate due to their metabolic activity. Increased moisture retention would result in more physical weathering as water invaded new cracks in the rock, expanding and traveling deeper over cycles of freezing and thawing, respectively. In addition to the mineral substrate being formed, bryophytes would add organic matter as they lived, grew, reproduced, and died.

This activity formed Earth's first soils, paving the way for plants to evolve true roots that could excavate into the soil substrate for water and mineral nutrients, increasing both physical and chemical weathering. The non-vascular bryophytes remained relatively restricted to remain close to the substrate and water, but seedless vascular plants were less restricted. Water could be pulled from the soil through lignified xylem tissue and transported through stems and out leaves held over 100 feet in the air, exiting out pores regulated by guard cells. An abundance of photosynthates produced by newly branching sporophytes, able to make large leaves thanks to stomata and vascular tissue, can be transported around the plant through specialized phloem tissue and used to fuel faster growth and more complex structures. Relationships with mutualists continued to develop and forests are formed in tropical swamps, creating the world's first coal deposits.

Though they grew tall and adapted to aerial dispersal of spores, early plants continued to be dependent on water for fertilization.

A plant with thin, flattened green stems that form Y-shapes. There are round sporangia on the sides of the stems, which have no leaves. — Figure \(\PageIndex{1}\): Though not discussed in this chapter, you are likely to come across *Psilotum* in your botanical studies. These strange plants belong to the Polypodiopsida and are commonly referred to as whisk ferns. Oddly, they do not produce true leaves. Instead, photosynthesis is performed by the stems. These dichotomously branching stems extend underground, forming dichotomously branching rhizomes with projecting rhizoids. Their subterranean gametophytes parasitize mycorrhizal fungi, which later form a mutualism with the sporophyte generation. Fused sporangia are produced along the stems. Photo by Toshihiro Nagata, CC-BY-NC.

After completing this chapter, you should be able to...

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.
List the shared derived characteristics of bryophytes.
Connect these characteristics to selection pressures these organisms would have faced.
Name the three phyla included in the bryophytes.
Use morphological traits and cellular components to distinguish between hornworts and other bryophytes.
Identify structures and phases in the hornwort life cycle; know their ploidy.
Label a hornwort sporophyte and describe its development.
Use morphological traits and cellular components to distinguish between liverworts and other bryophytes.
Identify structures and phases in the Marchantia life cycle; know their ploidy.
Use morphological traits and cellular components to distinguish between mosses and other bryophytes.
Identify structures and phases in the moss life cycle; know their ploidy.
Label a moss sporophyte and describe its development.
List the shared derived characteristics of seedless vascular plants.
Connect these characteristics to selection pressures these organisms would have faced.
Describe the importance of extinct seedless vascular plants in modern society.
Describe the characteristics of lycophytes.
Differentiate between homosporous and heterosporous strobili.
Differentiate between ferns, horsetails, and lycophytes.
Identify features of vegetative and reproductive shoots of Equisetum.
Identify features and phases of the fern life cycle; know their ploidy.
Label a fern gametophyte and sporophyte.

Attribution

Content by Maria Morrow, CC-BY-NC