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Important structures in plant development are
buds In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of a stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be spec ...
,
shoot In botany, a plant shoot consists of any plant stem together with its appendages, leaves and lateral buds, flowering stems, and flower buds. The new growth from seed germination that grows upward is a shoot where leaves will develop. In the sp ...
s,
root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...
s,
leaves A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, st ...
, and
flower A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechanis ...
s;
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...
s produce these tissues and structures throughout their life from
meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
s located at the tips of organs, or between mature tissues. Thus, a living plant always has embryonic tissues. By contrast, an animal
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
will very early produce all of the body parts that it will ever have in its life. When the animal is born (or hatches from its egg), it has all its body parts and from that point will only grow larger and more mature. However, both plants and animals pass through a phylotypic stage that evolved independently and that causes a developmental constraint limiting morphological diversification. According to plant physiologist A. Carl Leopold, the properties of organization seen in a plant are emergent properties which are more than the sum of the individual parts. "The assembly of these tissues and functions into an integrated multicellular organism yields not only the characteristics of the separate parts and processes but also quite a new set of characteristics which would not have been predictable on the basis of examination of the separate parts."


Growth

A
vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...
begins from a single celled
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...
, formed by
fertilisation Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...
of an egg cell by a sperm cell. From that point, it begins to divide to form a plant
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
through the process of embryogenesis. As this happens, the resulting cells will organize so that one end becomes the first root while the other end forms the tip of the shoot. In
seed A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiosper ...
plants, the embryo will develop one or more "seed leaves" (
cotyledon A cotyledon (; ; ; , gen. (), ) is a significant part of the embryo within the seed of a plant, and is defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germinating seed." The numb ...
s). By the end of embryogenesis, the young plant will have all the parts necessary to begin in its life. Once the embryo germinates from its seed or parent plant, it begins to produce additional organs (leaves, stems, and roots) through the process of organogenesis. New roots grow from root
meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
s located at the tip of the root, and new stems and leaves grow from shoot
meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
s located at the tip of the shoot. Branching occurs when small clumps of cells left behind by the meristem, and which have not yet undergone
cellular differentiation Cellular differentiation is the process in which a stem cell alters from one type to a differentiated one. Usually, the cell changes to a more specialized type. Differentiation happens multiple times during the development of a multicellular ...
to form a specialized tissue, begin to grow as the tip of a new root or shoot. Growth from any such meristem at the tip of a root or shoot is termed
primary growth In botany, secondary growth is the growth that results from cell division in the cambia or lateral meristems and that causes the stems and roots to thicken, while primary growth is growth that occurs as a result of cell division at the tips of ...
and results in the lengthening of that root or shoot. Secondary growth results in widening of a root or shoot from divisions of cells in a cambium. In addition to growth by
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
division, a plant may grow through cell elongation. This occurs when individual cells or groups of cells grow longer. Not all plant cells grow to the same length. When cells on one side of a stem grow longer and faster than cells on the other side, the stem bends to the side of the slower growing cells as a result. This directional growth can occur via a plant's response to a particular stimulus, such as light (
phototropism Phototropism is the growth of an organism in response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light contain a hor ...
), gravity ( gravitropism), water, ( hydrotropism), and physical contact ( thigmotropism). Plant growth and development are mediated by specific
plant hormone Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pat ...
s and plant growth regulators (PGRs) (Ross et al. 1983).Ross, S.D.; Pharis, R.P.; Binder, W.D. 1983. Growth regulators and conifers: their physiology and potential uses in forestry. p. 35–78 ''in'' Nickell, L.G. (Ed.), Plant growth regulating chemicals. Vol. 2, CRC Press, Boca Raton FL. Endogenous hormone levels are influenced by plant age, cold hardiness, dormancy, and other metabolic conditions; photoperiod, drought, temperature, and other external environmental conditions; and exogenous sources of PGRs, e.g., externally applied and of rhizospheric origin.


Morphological variation during growth

Plants exhibit natural variation in their form and structure. While all organisms vary from individual to individual, plants exhibit an additional type of variation. Within a single individual, parts are repeated which may differ in form and structure from other similar parts. This variation is most easily seen in the leaves of a plant, though other organs such as stems and flowers may show similar variation. There are three primary causes of this variation: positional effects, environmental effects, and juvenility. There is variation among the parts of a mature plant resulting from the relative position where the organ is produced. For example, along a new branch the leaves may vary in a consistent pattern along the branch. The form of leaves produced near the base of the branch differs from leaves produced at the tip of the plant, and this difference is consistent from branch to branch on a given plant and in a given species. The way in which new structures mature as they are produced may be affected by the point in the plants life when they begin to develop, as well as by the environment to which the structures are exposed. Temperature has a multiplicity of effects on plants depending on a variety of factors, including the size and condition of the plant and the temperature and duration of exposure. The smaller and more
succulent In botany, succulent plants, also known as succulents, are plants with parts that are thickened, fleshy, and engorged, usually to retain water in arid climates or soil conditions. The word ''succulent'' comes from the Latin word ''sucus'', meani ...
the plant, the greater the susceptibility to damage or death from temperatures that are too high or too low. Temperature affects the rate of biochemical and physiological processes, rates generally (within limits) increasing with temperature. Juvenility or heteroblasty is when the organs and tissues produced by a young plant, such as a
seedling A seedling is a young sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle (embryonic root), the hypocotyl (emb ...
, are often different from those that are produced by the same plant when it is older. For example, young trees will produce longer, leaner branches that grow upwards more than the branches they will produce as a fully grown tree. In addition, leaves produced during early growth tend to be larger, thinner, and more irregular than leaves on the adult plant. Specimens of juvenile plants may look so completely different from adult plants of the same species that egg-laying insects do not recognize the plant as food for their young. The transition from early to late growth forms is referred to as ' vegetative phase change', but there is some disagreement about terminology.


Adventitious structures

Plant structures, including, roots, buds, and shoots, that develop in unusual locations are called ''adventitious''. Such structures are common in vascular plants. Adventitious roots and buds usually develop near the existing vascular tissues so that they can connect to the
xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
and
phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is c ...
. However, the exact location varies greatly. In young stems, adventitious roots often form from
parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word � ...
between the
vascular bundle A vascular bundle is a part of the transport system in vascular plants. The transport itself happens in the stem, which exists in two forms: xylem and phloem. Both these tissues are present in a vascular bundle, which in addition will inc ...
s. In stems with secondary growth, adventitious roots often originate in phloem parenchyma near the
vascular cambium The vascular cambium is the main growth tissue in the stems and roots of many plants, specifically in dicots such as buttercups and oak trees, gymnosperms such as pine trees, as well as in certain other vascular plants. It produces secondary xy ...
. In stem cuttings, adventitious roots sometimes also originate in the callus cells that form at the cut surface. Leaf cuttings of the ''
Crassula ''Crassula'' is a genus of succulent plants containing about 200 accepted species, including the popular jade plant (''Crassula ovata''). They are members of the stonecrop family (Crassulaceae) and are native to many parts of the globe, but cu ...
'' form adventitious roots in the epidermis.


Buds and shoots

Adventitious buds develop from places other than a shoot apical meristem, which occurs at the tip of a stem, or on a shoot node, at the leaf axil, the bud being left there during the primary growth. They may develop on roots or leaves, or on shoots as a new growth. Shoot apical meristems produce one or more axillary or lateral buds at each node. When stems produce considerable secondary growth, the axillary buds may be destroyed. Adventitious buds may then develop on stems with secondary growth. Adventitious buds are often formed after the stem is wounded or
pruned Pruning is a horticultural, arboricultural, and silvicultural practice involving the selective removal of certain parts of a plant, such as branches, buds, or roots. The practice entails the ''targeted'' removal of diseased, damaged, dead, ...
. The adventitious buds help to replace lost branches. Adventitious buds and shoots also may develop on mature tree trunks when a shaded trunk is exposed to bright sunlight because surrounding trees are cut down.
Redwood Sequoioideae, popularly known as redwoods, is a subfamily of coniferous trees within the family Cupressaceae. It includes the largest and tallest trees in the world. Description The three redwood subfamily genera are '' Sequoia'' from coasta ...
(''Sequoia sempervirens'') trees often develop many adventitious buds on their lower trunks. If the main trunk dies, a new one often sprouts from one of the adventitious buds. Small pieces of redwood trunk are sold as souvenirs termed redwood burls. They are placed in a pan of water, and the adventitious buds sprout to form shoots. Some plants normally develop adventitious buds on their roots, which can extend quite a distance from the plant. Shoots that develop from adventitious buds on roots are termed suckers. They are a type of natural
vegetative reproduction Vegetative reproduction (also known as vegetative propagation, vegetative multiplication or cloning) is any form of asexual reproduction occurring in plants in which a new plant grows from a fragment or cutting of the parent plant or spe ...
in many
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
, e.g. many grasses, quaking aspen and Canada thistle. The Pando quaking aspen grew from one trunk to 47,000 trunks via adventitious bud formation on a single root system. Some leaves develop adventitious buds, which then form adventitious roots, as part of
vegetative reproduction Vegetative reproduction (also known as vegetative propagation, vegetative multiplication or cloning) is any form of asexual reproduction occurring in plants in which a new plant grows from a fragment or cutting of the parent plant or spe ...
; e.g. piggyback plant (''
Tolmiea menziesii ''Tolmiea menziesii'' () is a species of flowering plant in the family Saxifragaceae. It is known by the common names youth on age, pick-a-back-plant, piggyback plant, and thousand mothers. It is a perennial plant native to the West Coast of No ...
'') and mother-of-thousands ('' Kalanchoe daigremontiana''). The adventitious plantlets then drop off the parent plant and develop as separate clones of the parent. Coppicing is the practice of cutting
tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...
stems to the ground to promote rapid growth of adventitious shoots. It is traditionally used to produce poles, fence material or firewood. It is also practiced for
biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bio ...
crops grown for fuel, such as poplar or willow.


Roots

Adventitious rooting may be a stress-avoidance acclimation for some species, driven by such inputs as hypoxia or nutrient deficiency. Another ecologically important function of adventitious rooting is the vegetative reproduction of tree species such as ''Salix'' and ''Sequoia'' in
riparian A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the terrestrial biomes of the Earth. Plant habitats and communities along the river margins and banks a ...
settings. The ability of plant stems to form adventitious roots is utilised in commercial propagation by
cutting Cutting is the separation or opening of a physical object, into two or more portions, through the application of an acutely directed force. Implements commonly used for wikt:cut, cutting are the knife and saw, or in medicine and science the scal ...
s. Understanding of the physiological mechanisms behind adventitious rooting has allowed some progress to be made in improving the rooting of cuttings by the application of synthetic auxins as rooting powders and by the use of selective basal wounding. Further progress can be made in future years by applying research into other regulatory mechanisms to commercial propagation and by the comparative analysis of molecular and ecophysiological control of adventitious rooting in 'hard to root' vs. 'easy to root' species. Adventitious roots and buds are very important when people propagate plants via cuttings,
layering Layering has evolved as a common means of vegetative propagation of numerous species in natural environments. Layering is also utilized by horticulturists to propagate desirable plants. Natural layering typically occurs when a branch touches ...
,
tissue culture Tissue culture is the growth of tissues or cells in an artificial medium separate from the parent organism. This technique is also called micropropagation. This is typically facilitated via use of a liquid, semi-solid, or solid growth medium, su ...
.
Plant hormones Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pat ...
, termed
auxin Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essenti ...
s, are often applied to stem,
shoot In botany, a plant shoot consists of any plant stem together with its appendages, leaves and lateral buds, flowering stems, and flower buds. The new growth from seed germination that grows upward is a shoot where leaves will develop. In the sp ...
or
leaf A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ...
cuttings to promote adventitious root formation, e.g., African violet and
sedum ''Sedum'' is a large genus of flowering plants in the family Crassulaceae, members of which are commonly known as stonecrops. The genus has been described as containing up to 600 species, subsequently reduced to 400–500. They are leaf succul ...
leaves and shoots of
poinsettia The poinsettia ( or ) (''Euphorbia pulcherrima'') is a commercially important flowering plant species of the diverse spurge family Euphorbiaceae. Indigenous to Mexico and Central America, the poinsettia was first described by Europeans in 183 ...
and
coleus ''Coleus'' is a genus of annual or perennial herbs or shrubs, sometimes succulent, sometimes with a fleshy or tuberous rootstock, found in the Old World tropics and subtropics. The relationship among the genera ''Coleus'', ''Solenostemon'' and ...
. Propagation via root cuttings requires adventitious bud formation, e.g., in
horseradish Horseradish (''Armoracia rusticana'', syn. ''Cochlearia armoracia'') is a perennial plant of the family Brassicaceae (which also includes mustard, wasabi, broccoli, cabbage, and radish). It is a root vegetable, cultivated and used worldwi ...
and
apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ancest ...
. In layering, adventitious roots are formed on aerial stems before the stem section is removed to make a new plant. Large houseplants are often propagated by
air layering Layering has evolved as a common means of vegetative propagation of numerous species in natural environments. Layering is also utilized by horticulturists to propagate desirable plants. Natural layering typically occurs when a branch touches ...
. Adventitious roots and buds must develop in tissue culture propagation of plants.


=Modified forms

= *Tuberous roots lack a definite shape; example:
sweet potato The sweet potato or sweetpotato ('' Ipomoea batatas'') is a dicotyledonous plant that belongs to the bindweed or morning glory family, Convolvulaceae. Its large, starchy, sweet-tasting tuberous roots are used as a root vegetable. The young ...
. *Fasciculated root (tuberous root) occur in clusters at the base of the stem; examples:
asparagus Asparagus, or garden asparagus, folk name sparrow grass, scientific name ''Asparagus officinalis'', is a perennial flowering plant species in the genus '' Asparagus''. Its young shoots are used as a spring vegetable. It was once classified in ...
,
dahlia Dahlia (, ) is a genus of bushy, tuberous, herbaceous perennial plants native to Mexico and Central America. A member of the Asteraceae (former name: Compositae) family of dicotyledonous plants, its garden relatives thus include the sunflower, ...
. *Nodulose roots become swollen near the tips; example:
turmeric Turmeric () is a flowering plant, ''Curcuma longa'' (), of the ginger family, Zingiberaceae, the rhizomes of which are used in cooking. The plant is a perennial, rhizomatous, herbaceous plant native to the Indian subcontinent and Southeast ...
. *Stilt roots arise from the first few nodes of the stem. These penetrate obliquely down into the soil and give support to the plant; examples:
maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American English, North American and Australian English), is a cereal grain first domesticated by indigenous peoples of Mexico, indigenous ...
,
sugarcane Sugarcane or sugar cane is a species of (often hybrid) tall, perennial grass (in the genus '' Saccharum'', tribe Andropogoneae) that is used for sugar production. The plants are 2–6 m (6–20 ft) tall with stout, jointed, fibrous stalk ...
. *Prop roots give mechanical support to aerial branches. The lateral branches grow vertically downward into the soil and act as pillars; example:
banyan A banyan, also spelled "banian", is a fig that develops accessory trunks from adventitious prop roots, allowing the tree to spread outwards indefinitely. This distinguishes banyans from other trees with a strangler habit that begin life as a ...
. *Climbing roots arising from nodes attach themselves to some support and climb over it; example: Epipremnum aureum. *Moniliform or beaded roots the fleshy roots give a beaded appearance, e.g.: bitter gourd, Portulaca.


Leaf development

The genetics behind leaf shape development in ''Arabidopsis thaliana'' has been broken down into three stages: The initiation of the leaf primordium, the establishment of dorsiventrality, and the development of a marginal
meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
. Leaf primordium is initiated by the suppression of the genes and proteins of the class I '' KNOX'' family (such as ''SHOOT APICAL MERISTEMLESS''). These class I KNOX proteins directly suppress
gibberellin Gibberellins (GAs) are plant hormones that regulate various developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence. GAs are one of the longest-known classes of plan ...
biosynthesis in the leaf primodium. Many genetic factors were found to be involved in the suppression of these genes in leaf primordia (such as ''ASYMMETRIC LEAVES1,'' ''BLADE-ON-PETIOLE1'', ''SAWTOOTH1'', etc.). Thus, with this suppression, the levels of gibberellin increase and leaf primorium initiates growth.


Flower development

Flower development is the process by which
angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants ...
produce a pattern of
gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. T ...
in
meristem The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
s that leads to the appearance of an organ oriented towards
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
, the flower. There are three
physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemica ...
developments that must occur in order for this to take place: firstly, the plant must pass from sexual immaturity into a sexually mature state (i.e. a transition towards flowering); secondly, the transformation of the apical meristem’s function from a vegetative meristem into a floral meristem or
inflorescence An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are forme ...
; and finally the growth of the flower’s individual organs. The latter phase has been modelled using the ''ABC model'', which describes the biological basis of the process from the perspective of
molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
and
developmental Development of the human body is the process of growth to maturity. The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through mitosi ...
genetics. An external stimulus is required in order to trigger the differentiation of the meristem into a flower meristem. This stimulus will activate mitotic cell division in the meristem, particularly on its sides where new
primordia A primordium (; plural: primordia; synonym: anlage) in embryology, is an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells. A primordium is the simplest set of cells capable o ...
are formed. This same stimulus will also cause the meristem to follow a
developmental Development of the human body is the process of growth to maturity. The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through mitosi ...
pattern that will lead to the growth of floral meristems as opposed to vegetative meristems. The main difference between these two types of meristem, apart from the obvious disparity between the objective organ, is the verticillate (or whorled)
phyllotaxis In botany, phyllotaxis () or phyllotaxy is the arrangement of leaves on a plant stem. Phyllotactic spirals form a distinctive class of patterns in nature. Leaf arrangement The basic arrangements of leaves on a stem are opposite and alterna ...
, that is, the absence of
stem Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushro ...
elongation among the successive whorls or verticils of the primordium. These verticils follow an acropetal development, giving rise to
sepal A sepal () is a part of the flower of angiosperms (flowering plants). Usually green, sepals typically function as protection for the flower in bud, and often as support for the petals when in bloom., p. 106 The term ''sepalum'' was coine ...
s,
petal Petals are modified leaves that surround the reproductive parts of flowers. They are often brightly colored or unusually shaped to attract pollinators. All of the petals of a flower are collectively known as the ''corolla''. Petals are usuall ...
s,
stamen The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the fila ...
s and
carpel Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) '' pistils' ...
s. Another difference from vegetative axillary meristems is that the floral meristem is «determined», which means that, once differentiated, its cells will no longer divide. The identity of the organs present in the four floral verticils is a consequence of the interaction of at least three types of
gene products A gene product is the biochemical material, either RNA or protein, resulting from expression of a gene. A measurement of the amount of gene product is sometimes used to infer how active a gene is. Abnormal amounts of gene product can be correlate ...
, each with distinct functions. According to the ABC model, functions A and C are required in order to determine the identity of the verticils of the
perianth The perianth (perigonium, perigon or perigone in monocots) is the non-reproductive part of the flower, and structure that forms an envelope surrounding the sexual organs, consisting of the calyx (sepals) and the corolla ( petals) or tepals when ...
and the reproductive verticils, respectively. These functions are exclusive and the absence of one of them means that the other will determine the identity of all the floral verticils. The B function allows the differentiation of petals from sepals in the secondary verticil, as well as the differentiation of the stamen from the carpel on the tertiary verticil.


Floral fragrance

Plants use floral form, flower, and scent to attract different insects for
pollination Pollination is the transfer of pollen from an Stamen, anther of a plant to the stigma (botany), stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by Anemophily, wind. Pollinating agents can ...
. Certain compounds within the emitted scent appeal to particular
pollinator A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains. Insects are the m ...
s. In '' Petunia'' ''hybrid''a, volatile
benzenoids A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. P ...
are produced to give off the floral smell. While components of the benzenoid biosynthetic pathway are known, the enzymes within the pathway, and subsequent regulation of those enzymes, are yet to be discovered.Schuurink, R.C., Haring, M. A., Clark, D. G. (2006) "Regulation of volatile benzenoid biosynthesis in petunia flowers." ''Trends Plant Sci'', 11 (1). doi: 10.1016/j.tplants.2005.09.009 To determine pathway regulation, ''P. hybrida'' Mitchell flowers were used in a petal-specific
microarray A microarray is a multiplex lab-on-a-chip. Its purpose is to simultaneously detect the expression of thousands of genes from a sample (e.g. from a tissue). It is a two-dimensional array on a solid substrate—usually a glass slide or silicon ...
to compare the flowers that were just about to produce the scent, to the ''P. hybrida'' cultivar W138 flowers that produce few volatile benzenoids. cDNAs of genes of both plants were sequenced. The results demonstrated that there is a transcription factor upregulated in the Mitchell flowers, but not in the W138 flowers lacking the floral aroma. This gene was named ODORANT1 (ODO1). To determine expression of ODO1 throughout the day, RNA gel blot analysis was done. The gel showed that ODO1 transcript levels began increasing between 1300 and 1600 h, peaked at 2200 h and were lowest at 1000 h. These ODO1 transcript levels directly correspond to the timeline of volatile benzenoid emission. Additionally, the gel supported the previous finding that W138 non-fragrant flowers have only one-tenth the ODO1 transcript levels of the Mitchell flowers. Thus, the amount of ODO1 made corresponds to the amount of volatile benzenoid emitted, indicating that ODO1 regulates benzenoid biosynthesis. Additional genes contributing to the biosynthesis of major scent compounds are OOMT1 and OOMT2. OOMT1 and OOMT2 help to synthesize orcinol O-methyltransferases (OOMT), which catalyze the last two steps of the DMT pathway, creating 3,5-dimethoxytoluene (DMT). DMT is a scent compound produced by many different roses yet, some rose varieties, like ''Rosa'' ''gallica'' and Damask rose ''Rosa'' ''damascene'', do not emit DMT. It has been suggested that these varieties do not make DMT because they do not have the OOMT genes. However, following an immunolocalization experiment, OOMT was found in the petal epidermis. To study this further, rose petals were subjected to ultracentrifugation. Supernatants and pellets were inspected by
western blot The western blot (sometimes called the protein immunoblot), or western blotting, is a widely used analytical technique in molecular biology and immunogenetics to detect specific proteins in a sample of tissue homogenate or extract. Besides detect ...
. Detection of OOMT protein at 150,000g in the supernatant and the pellet allowed for researchers to conclude that OOMT protein is tightly associated with petal epidermis membranes. Such experiments determined that OOMT genes do exist within ''Rosa gallica'' and Damask rose ''Rosa damascene'' varieties, but the OOMT genes are not expressed in the flower tissues where DMT is made.Scalliet, G., Lionnet, C., Le Bechec, M., Dutron, L., Magnard, J. L., Baudino, S., Bergougnoux, V., Jullien, F., Chambrier, P., Vergne, P., Dumas, C., Cock, J. M., Hugueney, P. (2006). "Role of Petal-Specific Orcinol O-Methyltransferases in the Evolution of Rose Scent." ''Plant Physiol'', 140: 18-29. doi: https://doi.org/10.1104/pp.105.070961


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