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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 fertilized 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.
Above: the flowers and seed pods (fruit) of the trefoil, Lotus, a member of the pea-family.
Seed
Dispersal
Above:
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 favorable 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 synthesized 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.
Mobilizing
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) synthesizes 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 utilize. 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 fertilization
of one sperm nucleus with two polar nuclei in the central cell of
the embryo sac (see flowers for a description of fertilization). 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 fertilized
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 fertilized
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 utilized 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 photosynthesizing 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 photosynthesize. 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.
Notice
that there is a region of cells in the root tip which rarely undergo
cell division, called the quiescent
center.
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.
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 or protuberances 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).
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).
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. Distinctions are
not clear-cut, with many 'biennials' capable of living more than two
years under conditions under which growth is slow, but when they do
flower then monocarpic plants will afterwards 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.
Secondary
growth in stems
Development
of flowers
Vernation
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, 1st Jan 2024