6: Seedless Vascular Plants

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As bryophytes began to colonize the terrestrial surface, they produced organic acids during metabolism that aided in the breakdown of the rocky substrate. When they died, their organic matter mixed with the weathered rock, forming the Earth’s earliest soils. Formerly abundant to the first photosynthesizers to become terrestrial, access to sunlight became competitive as bryophytes expanded. This led to selection for individuals that could lift themselves higher and transport water throughout their tissues. Eventually, this selection resulted in the evolution of vascular tissue -- pipes that could bring water up from the ground so that parts of the plant could be raised upward, and those parts raised upward could transport their photosynthates down to the lower parts of the plant. The cells in the xylem (water-transporting vascular tissue) contained lignin, the tough, decay-resistant compound that wood is made out of. This rigid molecule in the vascular tissue allowed for structural support, allowing plants to grow taller -- some over 100 feet! The vascular system also allowed for the specialization of organs: roots for water absorption, leaves for photosynthesis, and stems for structural support.

Seedless vascular plants (SVPs) also began to rely more on the sporophyte stage. The sporophyte became the larger, nutritionally independent stage of the life cycle. Branching sporophytes offered more sites for meiosis to occur, resulting in increased opportunities for variation, which could be interpreted as more options in an increasingly competitive environment. There are approximately 20,000 known extant species, most of which are ferns.

6.1: Lycophytes
Lycophytes are seedless vascular plants with sporophytes that have microphylls and branch dichotomously. Sporangia are produced in strobili. Some are homosporous (e.g. Lycopodium), while others are heterosporous (e.g. Selaginella).
- 6.1.1: Lycopodium
- 6.1.2: Selaginella
6.2: Ferns and Horsetails
Ferns and horsetails are homosporous seedless vascular plants that produce megaphylls. Taxonomy has been in flux for this group, most recently classified as either phylum Monilophyta or class Polypodiopsida.
- 6.2.1: Horsetails
- 6.2.2: Ferns
6.3: Extinct SVPs
During the Carboniferous period, seedless vascular plants dominated the landscape in tree-like forms.