Non-vascular plants is a general term for those plants without a vascular system (xylem and phloem). Although non-vascular plants lack these particular tissues, a number of non-vascular plants possess tissues specialized for internal transport of water.
Non-vascular plants have no roots, stems, or leaves, since each of these structures is defined as containing vascular tissue. The lobes (rounded parts) of the liverwort may look like leaves, but they are not true leaves because they have no xylem or phloem. Likewise, mosses and algae have no such tissues.
All plants have a life cycle with an alternation of generations between a diploid sporophyte and a haploid gametophyte, but nonvascular plants include the only plants that have a dominant gametophyte generation. In these plants, the sporophytes grow attached and are dependent on gametophytes for taking in water and other materials. Non-vascular plants grow from spores.
Non-vascular groups
The term non-vascular plant is no longer used in scientific nomenclature. Non-vascular plants include two distantly related groups:
- Bryophytes - the Bryophyta (mosses), the Marchantiophyta (liverworts), and the Anthocerotophyta (hornworts). In these groups, the primary plants are the haploid gametophytes, with the only diploid portion being the attached sporophyte, consisting of a stalk and sporangium. Because these plants lack water-conducting tissues, they can't become as big as most vascular plants.
- Algae - especially the green algae. Recent studies have demonstrated that the algae actually consist of several unrelated groups. It turns out that common features of living in water and photosynthesis were misleading as indicators of close relationship. Only the green algae are still considered relatives of the plants.
Both of these groups are occasionally termed the "lower plants"; the term "lower" refers to these plants' status as the earliest plants to evolve. However, the term "lower" plants is not precise, since it frequently is used to include some vascular plants, the ferns and fern allies.
In the past, the term non-vascular plant included not only all the algae, but also the fungi as well. Today, it is recognized that these groups are not closely related to plants, and have a very different biology.
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[[zh:非維管植物]
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