A typical plant cell has a rigid cell wall, a large central vacuole and structures called plastids, some of which contain special pigments, such as chlorophyll that gives the organism its color, while others serve as storage areas for starch. Animal cells lack these distinctive features but various organisms have them.
All seed plants are made of plant cells. Seed plants, also called spermatophytes, include gymnosperms and angiosperms. Gymnosperms, or organisms that produce naked seeds on cones or modified cones, include such conifers as the pine and hemlock, as well as the ginkgo tree, ephedra bush and palm-like plants called cycads. Angiosperms, or flowering plants that produce their seeds in a protective covering called an ovary, include not only such floral beauties as the rose and lilac but also hardwood trees, grasses, cereal grains and many weeds, such as the thistle and leafy spurge. Not all seed plant cells are identical. For example, mature water-conducting cells called vessels lose their nuclei and cytoplasm so that they form a conduit through which water can freely flow. Moreover, while most seed plants have cells with plastids called chloroplasts, the Indian pipe lacks these structures.
Ferns resemble seed plants in many respects, including the cell types of which they are composed. Like seed plants, fern cells have chloroplasts and cell walls composed of cellulose. However, ferns have a life cycle in which the first generation is an easily recognizable fern that reproduces from spores and the second generation is a very small plant that reproduces sexually. The vegetative cells of the sexual generation differ from normal seed plant vegetative cells in that they are haploid. They have only one set of chromosomes instead of two. Other organisms resemble ferns not only in their life cycles but also in their cellular composition. They are commonly called fern allies and include club mosses, horsetails and whisk ferns.
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Mosses and Liverworts
Mosses and liverworts, also called bryophytes, look like miniature leafy plants, but in reality they have no true leaves or roots. However, their cells are true plant cells. A larger New Zealand species even has lignin in its cell walls besides the normal cellulose. (Lignin is a tougher material that often occurs in the cell walls of more complex plants.) Like the ferns, one of the bryophyte generations has haploid vegetative cells, but whereas the smaller fern generation is haploid, the haploid bryophyte is the leafy form that is most noticeable to the casual observer.
In older classification systems, the term thallophyte applied to a heterogeneous assemblage of organisms: algae, fungi, blue-green algae and bacteria. In current classifications, these organisms have been separated from the plant kingdom and assigned kingdoms of their own. The green algae, brown algae, red algae and most golden algae are unequivocally composed of plant cells with cellulose cell walls and plastids. Euglena has plastids but no cell wall and, therefore, is not a plant cell. Oomycetes, sometimes considered a fungus, has cellulose cell walls, plastids and a large central vacuole but other fungi have cell walls of chitin. Bacteria and blue-green algae are made of cells that differ radically from the typical plant cell.