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Land plants (embryophytes)
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Plants are one of the five kingdoms used to classify all life. The plant kingdom, known as Plantae is divided into progressively smaller subgroups, usually based on their family tree. The primary division is between vascular and non-vascular plants - with the vascular plants being of most interest to gardeners.
Non-vascular versus vascular
Vascular plants - including both those that flower and those that don't - have great diversity, since their adaptable root and shoot systems have given them the ability to grow in many habitats. Some, like ferns, reproduce by means of spores, which is how non-vascular plants reproduce as well, but the vast majority (over 250,000 species) reproduce by seed. Non-vascular plants are primitive types such as liverworts and mosses, which lack conductive tissue to circulate water and nutrients. This limits them to moist environments. Widespread in the wild, but of limited value in gardens due to their smaller size and somewhat dull appearance.
Plants that bear seed
Vascular plants that do bear seed are divided into gymnosperms ("naked seed") and angiosperms (meaning "covered seed"). Gymnosperms produce seed which are only partly enclosed by tissues from the parent plant. Conifers form the largest gymnosperm order, with about 650 species; they bear seed on the scales of the cones they produce. Many conifers, like pines (Pinus) and cypresses (Chamaecyparis, Cupressus) are very important horticulturally. Other gymnosperms include cycads and ginkgos. Angiosperms, also called flowering plants, differ from gymnosperms in producing their reproductive organs in flowers, with the seeds fully covered in an ovary.
Lifespan
Flowering plants are divided by life span into the following categories: annuals, biennials and perennials. Annuals live for one season of growth. Biennials live for two seasons, usually using the first to produce foliage and gather food reserves, and the second season usually comprises flowering and fruiting before dying. Perennials live for several seasons, usually flowering each year once established. Some perennials have been known to live thousands of years.
Plant classification and nomenclature
This is how plant classification is broken out, descending from the more general order name to the much more specific cultivars.
- Order - Group of families sharing set of basic characteristics. Names end in -ales.
- Family - Group of genera sharing set of basic characteristics. Names end in -aceae. Limits of families are unclear and often controversial.
- Genus (plural is genera) - Group of plants sharing wide range of characteristics. Names printed in italics with an initial capital letter. Hybrid genera are indicated with an x before the genus.
- Species - Group of plants which can breed together to produce similar offspring. Species are given a two part name, printed in italics: the first part with an initial capital letter is for the genus name (abbreviated) and the second part is the species specific name.
- Subspecies - Naturally occurring and distinct variant of a species, often isolated from others from the species. The abbreviation "subsp." is followed by the name of the subspecies in italics.
- Varietas (variety) and forma (form) - minor subdivision of a species, with a very slight difference from the rest of the species. Shown by "var." or "f." followed by the variety or form name in italics.
- Cultivars - Selected or raised artificially, these are distinct variants of species, subsp., var., f., or hybrids. The cultivar name follows the full scientific name in single quotes, without italics, eg. Calluna vulgaris 'Firefly', though if the parentage is unclear, the cultivar name may simply follow the genera name, eg. Rosa 'Goldfinch'.
- Varietas (variety) and forma (form) - minor subdivision of a species, with a very slight difference from the rest of the species. Shown by "var." or "f." followed by the variety or form name in italics.
- Subspecies - Naturally occurring and distinct variant of a species, often isolated from others from the species. The abbreviation "subsp." is followed by the name of the subspecies in italics.
- Species - Group of plants which can breed together to produce similar offspring. Species are given a two part name, printed in italics: the first part with an initial capital letter is for the genus name (abbreviated) and the second part is the species specific name.
- Genus (plural is genera) - Group of plants sharing wide range of characteristics. Names printed in italics with an initial capital letter. Hybrid genera are indicated with an x before the genus.
- Family - Group of genera sharing set of basic characteristics. Names end in -aceae. Limits of families are unclear and often controversial.
Species, hybrids and cultivars
In nature, species are usually pretty uniform in habit, foliage, flowers and fruit. Variation is part of evolutionary processes, and these differences are assigned the subdivisions of subspecies, varietas and forma for classification. A sub-species is like a mini-species, which has distinct morphological or genetic differences and sometimes distinct distribution geographically. A varietas is a wild variety, with differences from the species being less clear-cut. Forma is used for variations in color, and for similar minor differences. These differences remain more or less uniform in the wild, but under cultivation can lead to hybrids where the distinctions blur.
These hybrids of different species can be named by gardeners, and propagated to maintain them. They can happen by accident, when growing the species close to each other, or on purpose, by hand pollinating from one species to the other. The offspring will share characteristics of both parents. A hybrid has a multiplication sign × in the name, indicating it is a cross between to species. So the Camellia × williamsii (C. japonica × C. saluenensis) is a cross between the two species in parenthesis, which was named williamsii, most likely after the guy who hybridized the two species. Seedlings from such a cross - if the cross is fertile - may produce a fair amount of variation. Some of these might get selected and given cultivar names, eg. C. × williamsii 'Mary Christian'. Sometimes, the parentage of a cultivar is unknown, so it does not even get a species name attached to it, simply the cultivar name, such as C. 'Leonard Messel'. Hybrids also occur naturally in the wild.
Plants don't always have to be from the same genera to cross them. Some genera are closely related, enough so that they can be hybridized. One example is the crossing that has been done between the orchids Brassavola and Cattleya to produce the intergeneric hybrid named × Brassocattleya. A "×" at the beginning of a name indicates such a cross between two genera. This name is applied to all crosses between the two genera, no matter who makes them, though individual cultivars may be selected, propagated and named. Grafts between genera are rarely possible, but there are a few known cases. One is known as + Laburnocytisus adamii, with the "+" indicating a graft.
A cultivar (cultivated variety) is any hybridized or selected plant which has clearly distinct, uniform and stable characteristics, and which can be propagated in any way, whether it be seeds or cloning. Characteristics may have to be maintained by removing those offspring which are not "true", meaning they vary from the characteristics of the cultivar.
Sports are mutations that result in plants that differ from their parents. A mutation which can be propagated can be named like any other cultivar and maintained. This is how many variegated cultivars occur. Not all sports are stable, meaning that they can revert back to the characteristics of their parents.
Monocots and dicots
All plants that flower are classified as either monocotyledons (monocots) or dicotyledons (dicots). Monocots have one seed leaf (cotyledon) when they sprout, and all their leaves have veins running parallel to their length, slender, non-woody stems (palms are the exception), and their flower parts are arranged in threes. Their modified sepals look like petals. Dicots have 2 seed leaves when they sprout, their foliage has a network of veins, they have thick or woody stems, and flower parts (enclosed in leaf-like sepals) are arranged in multiples of 4, 5, 7 or more.