Land Plants Without Vascular Tissues
Bryophytes are small, low‐growing plants of mostly moist environments in the temperate and tropical zones where they grow on the ground and as epiphytes on the trees and undergrowth. In the alpine and boreal zones, bryophytes often are the dominant life form. Some bryophytes are desert dwellers, and a few are aquatic; none are marine.
The bryophytes are of botanical interest because their ancestors apparently were among the first land plants. The existing species today have some green algal features and some vascular plant attributes making them intermediates—more complex than green algae, but not quite vascular plants. Once thought to be monophyletic, the bryophytes are now recognized as having three independent lineages.
Bryophytes are plants because they are photosynthetic with chlorophylls a and b, store starch, are multicellular, develop from embryos, have sporic meiosis—an alternation of generations—and cellulose cell walls. Some mosses have simple water and food conduction‐type cells (but these are not the same as the xylem and phloem tissues of vascular plants). They have no lignified cell walls (like wood) for strength, so the plants remain small. Neither do they have leaves, stems, or roots. They absorb water from their surfaces by capillarity. The “leaves” of leafy liverworts and mosses are undifferentiated tissues and lack stomata, and the moss “stems” lack vascular tissues.
Within the bryophytes, the three lineages show a progressive development of body structure—the liverworts have the most simple, the hornworts intermediate, and the mosses the most specialized thalli. The gametophyte is the dominant, independent, long‐lived generation (the leafy moss plant and the flat, green thalli of the liverworts and hornworts are the gametophytes). The sporophyte is dependent on the gametophyte and is the short‐lived generation (the stalked capsule at the top of the leafy moss plant and the stalks on the liverworts and hornworts are the sporophytes).
They reproduce vegetatively (asexually) from small bits of haploid thallus tissue called gemmae, which are isolated in cups on the thalli and washed out by rain. Their sexual reproduction in which multicellular embryos are produced from fertilized eggs is restricted by the need for external water on the thalli surfaces in which the biflagellate sperms can swim to the egg.
Table outlines some of the major similarities and differences between the three lineages of bryophytes.