How to Build a Plant - Plant
Development and Growth
What is a seed? Seeds and Fruit.

The plant seed proper is derived from a single fertilised ovule within the ovary. In angiosperms (flowering
plants) the ovary wall (pericarp) forms the fruit around the seed. In some cases, however, the ovary wall is
a simple tight-fitting dry skin around the seed, as is found in achenes and grains. An
achene is a type of
indehiscent fruit (a fruit which does not open at maturity to shed the enclosed seeds, but which
permanently retains the seed(s)) enclosing a single seed. Asteraceae (Compositae), for example,
produce achenes. The achenes may collectively form a compound fruit called an
etaerio or head of
achenes as in the buttercup (
Ranunculus), Anemone and Clematis. The type of achene found in many
Asteraceae, such as the sunflower (
Helianthus) is called a cypsela in which part of the fruit wall is actually
formed of receptacular tissue (the receptacle is the swollen end of the flower stalk bearing the floral
parts). In this sense the cypsela could be regarded as a false fruit.

The grain or
caryopsis is similar to the achene, and is sometimes considered a type of achene. However,
in the achene the enclosed seed is attached to the enclosing fruit wall at one point, whereas in the
caryopsis the seed is attached over its whole surface. The caryopsis is found in cereals and grasses.
These types of fruit are often referred to loosely as 'seeds' and indeed it can be difficult to separate the
seed from the enclosing ovary.

Nuts are a type of achene in which the seed is obviously distinct from the fruit, which forms a tough
woody wall. One or more nuts may be enclosed in a cup-like structure, or cupule, which develop from
sepals and so are not part of the fruit proper. Examples include the fruit of many trees, such as oak,
hazel, sweet chestnut, beech and some non-woody plants such as docks and rhubarb. To distinguish a
seed from a fruit, remember the following points:

1. The seed has its own outer skin called the testa. If two layers are present around the embryo then we
have a fruit. If three layers are present then the third may be a cupule or receptacular tissue.

2. The fruit usually has a stalk, derived from the flower stalk (pedicel), whilst the seed has a scar or hilum
where it was joined to the placenta.

3. The fruit often has the remains of one or more style protruding from one end (stylar remains).

Therefore, we can see that the acorn (
Quercus) and nut of the sweet chestnut (Castanea) are fruit - nuts
proper - both have stylar remains from which the pericarp can be peeled away, whilst the conker of the
horse chestnut (
Aesculum) with its prominent hilum is actually a seed.

The
samara is yet another type of achene, but one in which the pericarp is extended into a wing as in ash
(
Fraxinus), elm (Ulmus). This aids wind dispersal as these fruit spin through the air like helicopter rotor
blades, generating lift. Sycamore and maple (
Acer) produce double samaras which often separate after
wind dispersal. The
double samara is an example of a type of fruit called a schizocarp. This is a dry
fruit formed from a syncarpous gynaecium (a female floral structure consisting of more than one carpel
joined together). The separate carpels separate from one-another to form
mericarps - each mericarp
contains one seed.

The
capsule is a type of dehiscent dry fruit formed from a syncarpous gynaecium. Dehiscent fruit split
open to release the enclosed seeds. In capsules, the carpels open by slits, pores or teeth. A familiar
example is the fruit of the horse chestnut (
Aesculus) which splits open to release a conker (seed). The
poppy has a porous capsule which scatters seeds as it blows about on its long stalk in the wind.

The
drupe is a fleshy (succulent) fruit containing one or more seeds, each enclosed in a hard stony layer
which is the inner layer of the pericarp, called the
endocarp. The stone may contain a single seed, as in
the plum (
Prunus) or there may be several distinct portions as in ivy. The stone is enclosed in a fleshy
middle layer of the fruit wall, called the
mesocarp, whilst the outermost layer of the fruit wall forms the
'skin' or
epicarp. The cherry, almond and walnut are examples of drupes. Many small drupes, or druplets
(drupels), may be clustered together on the same receptacle, as in the blackberry and raspberry (Rubus).

Berries are fleshy fruits containing one or more seeds enclosed only by their own hardened seed coats
(testas), e.g. citrus, banana, tomato, grape, squash family, black currant and gosseberry. In
false fruits,
part of the 'fruit' is actually derived from tissues outside the carpel. In the
pome of the apple (Malus) and
pear (
Pyrus) the fruit proper is the seed-containing core whilst the flesh is derived from part of the
receptacle. In the rose, the fruit is called a
hip and consists of a fleshy receptacle flask-like structure
containing achenes.

The seed-pod, characteristic of the
legume family, such as peas, beans, clover, lupins and vetches,
belongs to a type of fruit called a legume. The legume is a dehiscent dry fruit formed from a single carpel
containing one or more seeds. The legume pod splits along both a dorsal and a ventral suture.
Seed Dispersal

Left: the achene of a dandelion (Taraxacum) like that of
many Asteracea has a pappus of hairs which form a
'parachute-like' structure to give the fruit buoyancy, aiding
wind dispersal.

In orchids, the fruit is a capsule which splits along 3 to 6
longitudinal slits, whilst the valves remain joined at both
ends of the fruit. The seeds of orchids are minute, about 1
mm in length and are often called 'dust seeds'. The
Greater Butterfly Orchid (
Platanthera chlorantha) may
produce as many as 25 000 seeds in each capsule. The
testa of orchid seeds consists of a honeycomb of air
spaces formed by the walls of dead cells and is ideally
suited to both wind and water dispersal.

The seed-pods of legumes rely on the movements of the
valves which may split and twist apart explosively,
scattering seeds.


Learn more about seed dispersal in
legumes.
Germination

Seeds may remain dormant for many years in unfavourable conditions, germinating only when favourable
conditions arise. Seeds can typically remain viable for 10 to 50 years, though reports of some seeds
remaining viable for 600 years or more are fairly well-authenticated. Reports of seeds germinating after
thousands of years are not generally verifiable.
Seeds remain viable for so long because their tissues are dry, preventing or slowing metabolic
processes.
Dormancy may not simply be a case of remaining dormant until favourable conditions occur,
rather there may be a true dormant phase requiring a specific stimulus before germination begins. Not all
plant seeds go through dormancy - many cultivated varieties have been selected precisely because they
have no dormant phase and so will germinate as soon as they are planted in suitable conditions. In order
to germinate a seed must first become hydrated by
imbibing water, then enzymes must be activated or
synthesised to increase metabolic activity. The tough seed coat may be impermeable, preventing water
from gaining entry. Seeds typically have a pore through which water can enter, though a plug of material
may block the pore until the seed is shaken about in which case the plug dislodges. Other seeds require
mechanical damage (
scarification) to the seed coat to allow water to enter. Exposure to varying
weather, such as seasonal changes in temperature, abrasion during dispersal, the passage through an
animal's gut, fire or microbial action in the soil may breach the seed coat, allowing imbibition to take
place. Some seeds simply require a minimum period of chilling before dormancy can be broken.

This apparently odd requirement can be explained when it is understood that optimal germination may be
after the ensuing winter has passed, or after fire has burnt away competing vegetation or when dispersal
has had time to carry the seed far from its source. Fruit may contain chemicals that prevent the seeds
germinating until either the fruit has rotted away, or been removed during passage through an animal's
gut. Fruit may also block light needed to signal germination or prevent water from entering the seed by
osmosis.

Seeds requiring light for germination are said to be
photodormant. Phytochrome, a light-sensitive
pigment in seeds (and other plant parts) absorbs red light present in sunlight to stimulate germination
(when in the red-light sensitive form to convert into the far-red sensitive form). Only when the seed has
imbibed enough water can this light-sensitive pigment operate. Temperature also affects the responses
of this pigment to light.

Mobilising Food Reserves

The diagram above illustrates germination in a grass seed, such as barley. The embryo (germ) is
surrounded by food reserves in the cells of the
endosperm which is itself surrounded  by 2 to 4 layers of
living cells called the aleurone layer. Imbibition, the entry of water, the
scutellum (cotyledon or
embryonic leaf) synthesises and secretes
gibberellin (gibberellic acid, GA). The GA stimulates the
aleurone cells to release hydrolytic enzymes, such as alpha-amylase, to digest the food reserves
stored in the endosperm, releasing nutrients which the germinating embryo can utilise. Amylase, in
particular, digests starch into sugars. The scutellum can also provide enzymes for this purpose and may
be more important as a source of enzymes than the aleurone layer in some species, and is more
important in the first two days, before the aleurone layer takes over.

Endosperm is a characteristic food storage tissue in monocotyledon seeds. Endosperm cells are most
often triploid (they have three sets of chromosomes, 3n) since they derive from secondary fertilisation of
one sperm nucleus with two polar nuclei in the central cell of the embryo sac (see flowers for a
description of fertilisation). In some plants the endosperm is pentaploid (5n) as in
Lilium. However, the
number of polar nuclei involved varies with species. Endosperm may be produced by free nuclear
division - the fertilised nucleus divides mitotically (see mitosis) to create a large multinucleate cell, which
may remain thus, or may become partitioned by the later formation of cell walls. Alternatively, the
fertilised cell nucleus may divide normally, producing two walled cells which further divide into more walled
cells. In others the endosperm may develop by a combination of both methods. Carbohydrates, proteins
and lipids are the main food reserves. Starch being dominant in cereals.

Legumes (Fabaceae) such as peas and beans are dicotyledons, so the embryo has two embryonic
leaves or cotyledons which assume the main role of food storage and are swollen with food reserves.
The cotyledons swell with food reserves as the embryo develops at the expense of the endosperm. In
some seeds the embryo is barely developed when the dormant seeds are dispersed, in which case the
seed reserves will be mostly endosperm. In others the embryo is in quite an advanced stage of growth by
the time it becomes dormant, in which case the endosperm may be completely utilised and the cotyledons
gorged with food reserves which ultimately came from the endosperm. Orchids are an exception, their
tiny 'dust seeds' contain embryos at a very early stage of development but there is no endosperm,
instead the seed will rely on symbiotic fungi to supply it with nutrients when it germinates.
Becoming a Seedling

The developed embryo consists of an embryonic root, or radicle, with a protective root sheath
(
coleorhiza) covering the root apical meristem. A section of axis called the hypocotyl joins the radicle to
the node bearing one
cotyledon (monocots) or two cotyledons (dicots). Above the cotyledon-bearing
node is a short section of axis called the
epicotyl which ends in the shoot apex which consists of at least
the
shoot apical meristem, though visible leaf primordia may occur on either side just beneath the
shoot apex, forming a
plumule (embryonic terminal shoot bud).

The radicle enlarges first. In barley (a monocot) the coleorhiza has been shown to have a key role in
regulating seed dormancy. It is the first tissue to imbibe water and is porous to facilitate the uptake of
water. Its cells then elongate and separate, and only now can the growing radicle break free and enter
the soil. The endosperm tissue may perform a similar role in dicots.

As the shoot emerges, germination may be hypogeal or epigeal. In
hypogeal germination, the
cotyledon(s) remain below ground, supplying the germling with their stored food reserves. This happens
particularly in those species in which the cotyledons are laden with food reserves. In this case, the
epicotyl elongates more than the hypocotyl pushing the plumule or shoot apex above ground, whilst
leaving the cotyledons in the soil.

In
epigeal germination, the hypocotyl elongates rapidly, pushing the cotyledon(s) above ground where
they develop chlorophyll and photosnythesise as the first seedling proto-leaves. This is more important if
the cotyledons carry little food reserves, but nutrients are being supplied by the endosperm within the
seed instead. Epigeal germination occurs in sycamore (
Acer pseudoplatanus) for example.

Many orchids and some other plants begin life as a small subterranean tuber- or spinning top-shaped
protocorm: a more-or-less spherical or conical body several mm in diameter which contains a cylinder of
vascular tissue and puts out root-like threads called rhizoids. Rhizoids are projections from single cells
and so are comparable to root hairs. Their function is to facilitate infection by
mycorrhizal fungi. These
fungi supply the protocorm with nutrients and some orchids will remain below ground for a year or more,
relying on food supplied by the fungus and not photosynthesising until they reach the surface. Some
orchid seeds require infection by the fungal partner before germination, whilst some will germinate first,
but only in the presence of a suitable fungus. Later, the protocorm puts out a number of roots which host
the fungus, with the rest of the protocorm eventually becoming free of 'infection'. A growing tip of the
protocorm, which may be set inside a depression, will produce the first leaf primordia. The protocorm
develops into a rhizome, or a tuber, depending on the species.

Phenology

Phenology is the study of timing of developmental events in an organism's life-cycle. For example,
consider the life-cycle of the
Lizard Orchid, Himantoglossum hircinum in northern temperate climes.
Seeds germinate in either the first or second autumn (depending on sufficient rainfall in the growing
season) to produce a protocorm. In the first spring, the protocorm develops an axillary bud (in the axil of
a scale leaf) at its tip which gives rise to a tuber. The protocorm disappears in summer. In the second
autumn, a short mycotrophic (literally: 'fungus-eating') rhizome (underground stem) develops from the
tuber tip and puts out a mycotrophic root from a scale leaf axil (adventitious root). The second tuber
develops and attains full size in summer.

After about three years the tuber gives rise to young seedlings. This exhausts the tuber, which is
replaced by another which expands with food reserves as photosynthesis proceeds. After a few years a
two-leaf plant will be produced, then a few years later a three-leafed plant, both of which have only a low
probability of flowering. After a number of years the mature plant with 4 or more leaves is produced.

One or occasionally two tubers survive each summer underground and put up new leaves and roots
each autumn. The exhausted tuber(s) will whither to be replaced by a new one which expands with food
reserves as the leaves photosynthesise. In late May (March to July) the leaves and roots die back and
the plant flowers over a 2-3 week period in June and July. The lowermost flowers on the spike develop
and open first and it takes about two weeks for all the flowers to open. New plants are occasionally
produced asexually by the development of new tubers on the roots. New roots and leaves are produced
each year.

Pods/capsules are produced in July, maturing in 4-6 weeks. Each capsule produces up to 2000 minute
seeds (each 'dust seed' is a little above 0.01 micro grams (about one hundredth of a millionth of a gram)
in mass). The embryo of orchids is minute and typically undifferentiated.

Analysis of such life-cycle events can tell us a great deal about a plant's biology. It can tell us about a
plant's requirements, for example how it responds in dry years or cold years and how it depends on a
fungal partner entirely for early growth, as in the Lizard Orchid.

Growth of Plant Organs

Roots and shoots elongate by producing new cells in specialised regions in the tip/apex called apical
meristems
. The primary root of the seedling and any side-branches that grow from it all have apical
meristems. The tip of a growing root is covered by a mass of cells called the
root cap (calyptra) which
protects the growing root tip as it pushes through the soil and secretes a slime of mucilage forming a
mucigel over its outer surface. As well as protecting the root tip and easing its passage through the soil
the mucilage is likely to have functions in interactions with micro-organisms and soil minerals. As cells get
sloughed off the root cap they are replaced by cell division within the root cap, such that the oldest cells
are outermost (i.e. tipmost). See also
tree roots.

Inside the root tip, just behind the root cap, is the root
apical meristem (see diagram below). Here cell
division (mitosis) produces new cells which form columns or files passing towards the base of the root as
it grows (the newest cells are at the root tip, the oldest at its base). Behind the meristem is a region of
cell elongation, where cell division has more-or-less ceased but the root still elongates as the newly
formed cells elongate. further back towards the base, this zone of cell elongation is followed by a zone of
cell differentiation, where vascular cells (phloem sieve cells and xylem vessel elements) are forming as
the elongated cells mature, and where some of the epidermal cells put out root hairs. Each root hair is an
extension of a single epidermal cell.
Secondary growth in stems

Development of flowers

Perennation

Orchids may be perennial, but without producing woody parts above ground. Their curious habit reflects
the fact that temperate orchids are generally thought to have descended from their tropical cousins which
are epiphytes, with persistent stems which grow attached to the host tree and store nutrients and later
adapted to life in soil as terrestrial plants.  Monocarpic plants flower only once and the die, and most are
annuals. Many annuals germinate in spring, grow in summer and autumn and die before winter,
overwintering as seeds. Others may germinate in the autumn and overwinter as seedlings beneath the
snow. Biennials, such as carrot (Daucus carota), usually complete their life-cycle over a two-year period.
Biennials typically germinate in spring and spends the summer as a rosette of leaves that die back in late
autumn. The plant then overwinters as a root and a compressed apical meristem surrounded by
protective dead leaves. The meristem and protective leaves constitute a perennating bud. In the second
summer the apical meristem bolts into a flowering stalk. Some monocarpic plants are perennial, living for
several years before flowering once and dying, for example many bamboos may live for more than 50
years, then flower once and die.  Polycarpic plants flower more than once and are perennials. Woody
perennials only use some of their axillary buds (buds developing in the axils of leaves) for flower
formation, keeping the terminal buds for vegetative growth the following year. Secondary growth from
secondary
lateral meristems produces layers of new xylem (wood) and phloem each year, contributing
to the increasing girth of the stem and roots.

Herbaceous perennials, e.g. field bindweed (Convolvulus arvens) and many grasses are non-woody
and die back each year, except for one or more perennating buds close to the soil. Some form
underground storage organs to store nutrients for growth to resume the next growing season, such as:
bulbs, corms, tubers or rhizomes.


Growth Form

Plants may exhibit determinstic or non-deterministic growth and sympodial or monopodial growth forms.
See
plant architecture for an explanation of these growth forms.


Bibliography

Orchid protocorms: https://ia601601.us.archive.org/32/items/jstor-2468794/2468794.pdf
Lizard Orchid: http://onlinelibrary.wiley.com/doi/10.1046/j.0022-0477.2001.00640.x/pdf


Article updated:  20th Feb 2016
Lotus, Trefoil
Above: the flowers and seed pods (fruit) of the trefoil, Lotus, a member of the pea-family.
Riit tip meristem
Notice that there is a region of cells in the root tip which rarely undergo cell division, called the
quiescent centre. If the meristem or root cap are damaged then the quiescent centre can activate and
replace these parts by cell division.

Roots still need to increase in girth far behind the zone of elongation, as the roots mature. In
gymnosperms (e.g. conifers) and dicotyledonous plants, a
vascular cambium: a cylinder of cells
located between the phloem and xylem, undergoes cell division in the periclinal plane (
periclinal
divisions
: parallel to the surface of the root, that is perpendicular to a radius) to add more layers of
cells to the inside (new xylem cells) and outside (new phloem cells). The cambium increases in
circumference by cell divisions in the anticlinal plane (
anticlinal divisions: perpendicular to the surface
of the root or parallel to a radius) adding more cells to the cambium cylinder. The roots of
monocotyledons do not undergo secondary growth but thicken slightly as the cells expand.

A second cylinder of cambium develops later in the
pericycle. The pericycle is the outermost layer of
cells of the vascular cylinder and encloses the vascular tissue. It is surrounded by the endodermis,
cortex and epidermis of the younger absorptive parts of the root system (see
water transport in plants).
This becomes the
cork cambium (phellogen) in which periclinal divisions add new cork (phellem) cells
to the outside and some phelloderm (secondary cortex) inside. The endodermis, cortex and epidermis
rupture as the root expands and are sloughed off, leaving the cork cambium outermost as the root bark.
The cork cell walls are waterproofed by deposition of suberin.

Shoots develop in a similar way: a
shoot apical meristem (SAM) elongates the shoot (main stem or
branch). In addition more basal meristems, called
intercalary meristems (so-called because they are
inserted between or intercalated between oldernon-dividing cells) account for elongation in many
monocots, such as grasses. These occur in the nodes and at the base of leaves. The apical meristem
will form periodic outgrowths, protuberances of cells called leaf primordia, produced by periclinal
divisions. The primordia become leaves after further cell expansion accompanied by anticlinal divisions
at the surface of the primordium to increase its perimeter.

Leaf primordia are produced in a regular pattern according to the phyllotaxis (characteristic arrangement
of leaves) of the plant species. Explaining the mechanism which determines where and when a leaf
primordium forms is an ongoing and fascinating area of research.
shoot apical meristem
Above: the shoot apical meristem (SAM) can be divided into various regions according to the determined
fate of the cells within each region. It can be divided into outer layers of cells (tunica) and inner layers of
cells (corpus). CZ: central zone of small undifferentiated and slowly dividing cells. PZ: peripheral zone of
rapidly dividing cells which will later differentiate and mature into various tissues; CZ cells left behind as
the SAM grows. OZ: organ zone, cells here give rise to leaf primordia; these are older PZ cells. CM:
central meristem, gives rise to the stem interior, including vascular tissues.

The projections in the OZ are the leaf primordia. These are formed from anticlinal divisions followed by
periclinal divisions in the second or third cell layer. The first (superficial-most) layer of cells undergo
anticlinal divisions as the organ enlarges, maintaining a single-celled covering layer or epidermis. Mitosis
throughout the body of the primordium gives rise to elongation of the developing leaf. Cells at the margin
of the tip (distal margin) of the primordium divide so as to broaden the developing leaf and determine
leaf shape. Cells at the base of the primordium give rise to the petiole (leaf stalk). In the developing
leaf-blade the plate meristems bring about elongation of the blade or wing by anticlinal divisions whilst
marginal meristems (one on each side of the primordium) determine the number of
mesophyll cell layers
in the leaf.

In monocots, such as grasses,leaf veins are initiated at the apex and their development progresses
towards the base (basipetal development), whereas in dicots the veins are initiated from base to the
apex (acropetal; development).
Plant stem development
Above: development of a non-woody flowering-plant stem.

Tb: terminal bud, housing the apical meristem (Am) which gives rise to leaf primordia (Lp) which will later
develop into leaves and the various immature tissues: protoderm (Pr) develops into the covering epidermis
(Ep); the ground meristem (Gm) tissue develops into the outer cortex (Co) and the inner pith (Pi) which
may or may not be hollow; the procambium develops into the vascular bundles, consisting of inner
water-conducting xylem (Xy) and outer food-conducting phloem (Ph). In the mature stem, new xylem and
phloem may continue to be produced from the vascular cambium (Ca)