The Plant Kingdom (Plantae)(Last modified: 20 June 2007)
Plants provide nourishment for our bodies and souls. With the help of protists and fungi, plants provide the oxygen we breathe and the food that sustains us -- either directly or indirectly, by feeding other animals. Plants provide shade over our heads and cool carpets under our feet while surrounding us with beautiful colors and marking the change of seasons.
Prominent plants give us a handle on ecological communities. Descriptions such as "Redwood-Tanoak Forest" or "Oak Grassland" indicate not only the plants we may find there but the animals, fungi, and climate as well.
Classification of the plant kingdom can be especially confusing to the amateur naturalist. For example, according to modern botany:
- A palm tree has more in common with a blade of grass than with other trees.
- A strawberry plant is more closely related to an apple or apricot tree than to a clover or geranium.
- A Ginko (Maidenhair) tree is so different from other plants that it is in a phylum by itself. But if you have to group it with other plants, it belongs with conifers such as Pine trees.
At least four classification systems are in common use: Plants are classified into 12 phyla or divisions based largely on reproductive characteristics; they are classified by tissue structure into non-vascular (mosses) and vascular plants (all others); by "seed" structure into those that reproduce through naked seeds, covered seeds, or spores; or by stature divided into mosses, ferns, shrubs and vines, trees, and herbs.
All of these higher-level groupings are decidedly lopsided: the vast majority of the 270,000 plant species are flowering herbs. The categories listed below provide slightly better balance: the largest phylum has been split while the other phyla are grouped according to one or more of the methods described above.
- Mosses are non-vascular plants -- they cannot transport fluids through their bodies. Instead, they must rely on surrounding moisture to do this job for them.
Though small in stature, mosses are very important members of our ecosystem. They lay the foundations for other plant growth, prevent erosion, and contribute to the lush green appearance of many forested areas.
The 24,000 bryophyte species, sometimes grouped into a single phylum are now grouped in three phyla: Mosses (Bryophyta), Liverworts (Hepatophyta) and Hornworts (Anthoceraphyta). They reproduce by spores, never have flowers, and can be found growing on the ground, on rocks, and on other plants.
- Ferns and allies have a vascular system to transport fluids through their bodies but like the mosses, they reproduce from spores rather than seeds. The main phylum, the Ferns (Filicinophyta = Pteridophyta) includes around 12,000 species.
Three other phyla are included as fern allies: the Horsetails (Sphenophyta = Equisetophyta, 40 species; right, accompanied by an orchid), Club mosses (Lycopodophyta, 1,000 species), and Whisk ferns (Psilophyta, 3 species)
The gymnosperms add the next level of complexity to plant evolution: they reproduce from seeds instead of spores. The seeds, however, are "naked" (Greek: gummnos) -- not covered by an ovary. Usually, the seed is produced inside a cone-like structure such as a pine cone hence the name "conifer." Some conifers, such as the Yew and Ginko, produce their seeds inside a berry-like structure.
Conifers are fairly easy to identify: In addition to the aforementioned cones, these trees and shrubs typically have needle-like, scale-like or awl-like leaves. And they never have flowers.
Approximately 600 species are counted as conifers including the pines, firs, spruces, cedars, junipers, and yew. Species within the conifer ranks give us pine nuts -- pesto's magic ingredient -- as well as juniper berries for gin.
Conifer allies include three small phyla containing fewer than 200 species all together: Ginko (Ginkophyta) with a single species: the Maidenhair Tree (Ginko biloba); palm-like Cycads (Cycadophyta) ; and herb-like cone-bearing plants (Gnetophyta) such as Ephedra.
- Angiosperms add the final improvement to plant reproduction: they grow their seeds inside an ovary (Greek: angeion = vessel) which is, itself, embedded in a flower. After it is fertilized, the flower falls away and the ovary swells to become a fruit.
Angiosperms in the class Dicotyledoneae grow two seed-leaves (cotyledons). In addition, foliage leaves typically have a single, branching, main vein originating at the base of the leaf blade, or three or more main veins that diverge from the base.
The vast majority of plants are Dicots. Most trees, shrubs, vines, and flowers belong to this group of around 200,000 species. Most fruits, vegetables and legumes come from this class.
- Monocots start with one seed-leaf. The main veins of their foliage leaves are usually unbranched and nearly parallel to each other. Around 30,000 plants are classified as monocots including many of the prettiest members of kingdom Plantae: orchids, lilies, irises, palms and even the Bird-of-Paradise plant. The grasses which carpet our lawns and meadows are also monocots.
Monocots provide us with our primary sources of nutrition, supplying us and the animals we eat with grains such as wheat, oats, and corn, as well as fruits such as dates and bananas.