and Growth of the Seedling
dormancy is Broken
Organization and the Plant
and Vascular Tissue
Growth in the Root
in the Young Root Tip
and Growth in the Seedling
and Secondary Growth in the Stem
- a multicellular embryo with
meristematic tissues for the first leaves and root already
- oils for high energy food storage and
to help prevent decay
- a seed coat to prevent desiccation and
provide mechanical protection
- carbohydrates in cotyledons which when
broken down will supply the energy needed for rapid uptake of water
Angiosperm seeds are an effective
evolutionary adaptation allowing plants to weather rough conditions. If
they germinate when the conditions are not right the species could
An Australian medical journal
reported that a woman had suffered from persistent sore throat
until doctors discovered a pear or apple seed had germinated in
her throat and put down roots.
While it was reporting this
peculiar plant performance, the journal went on to mention that
doctors in south Africa had found a little boy with a
chrysanthemum seed growing out of one eye and a British woman
who had a tomato plant growing out of her upper gum. It seems a
seed had gotten caught under her dentures and found the
temperature and moisture conditions ideal. (Gardens for All)
Dormancy may be broken when conditions
favor plant growth. All seeds require:
- adequate supply of water
- favorable temperatures
- presence of oxygen
Depending on the species some special
conditions for seed germination are:
- freezing to help open the seed coat
- heat or smoke from fire
- abrasive action of sand or grit to
wear down the seed coat
- hydrolyzing action of animal digestive
dormancy is Broken
After the uptake of water the plant
hormone gibberellic acid is released. It acts on the aleurone layer
found just below the seed coat to stimulated the enzyme alpha-amylase.
The amylase mobilizes the starchy reserves of the endosperm. The
resulting sugars quickly supply the meristematic tissues the energy they
need to begin cell division.
In all seedlings a root develops first
and grows down into the soil. Then the shoot must emerge from the embryo
and search out light above the soil and leaf litter. When stems and
leaves emerge above ground chloroplasts in them rapidly mature turning
the plant green. Although there are differences from plant to plant the
basic pattern is the same and is illustrated below.
The epicotyl which emerges from a
seedling is an example of primary growth.
Organization and the Plant
All plants have a special tissue that
continues to divide called apical meristem which is generally
found at the tip of roots or shoots. The apical meristem in turn gives
rise to just 3 types of primary tissue which can divide and
differentiate into all the other plant tissues. These primary meristems
- Ground Meristem
Given the right environment each plant
cell can develop into a new plant -- the ultimate in cloning. Such
cells and tissues are totipotent.
matures to form epidermis. This flattened irregular layer covers
roots, shoots, and leaves. Some epidermal cells specialize to become:
- guard cells
(always in pairs) to help in gas exchange, generally found in the
lower epidermis of leaves.
- root hairs
which emerge from the epidermal cells just above the root tip help
increase the absorptive surface area of the root.
Many epidermal cells produce a waxy
water proofing, the cuticle of a leaf for instance.
- leaf epidermis produces a wax
- stem and root epidermis make subrin
Where secondary growth occurs the
epidermis within becomes a special layer of cells called periderm.
Periderm is responsible for the production of a waterproofing
material called subrin.
meristem differentiates into 3 important tissues:
cells are generally spherical or slightly irregular in shape with
thin walls. Parenchyma forms a versatile tissue which can be used
as storage in the cortex region of roots or it may be loaded with
plastids such as chloroplasts in the spongy or palisade layers of
leaves. Wounds are filled with parenchyma cells.
with its thickened but flexible walls is primarily used for
support without preventing growth. Collenchyma cells lack the
hardening agent called lignin. They remain alive at maturity,
their thick walls create a supportive cylinder just inside the
epidermis of young stems or within vascular tissue of leaf veins.
has the ability to very the shape and thickness of its cell walls.
The mature cells are dead and can form either strong supporting
fibers or extremely hard stone cells (sclereids) common in seed
coats and nut shells. These cells contain lignin.
Primary Growth in
gives rise to the following vascular tissue:
- primary xylem
- primary phloem
- vascular cambium (important in
xylem has the dual roles of support and water transport,
although the latter is by far the most important. When mature the
lifeless tracheids and vessel elements form hollow, continuous,
conduits from root to leaf
plants contain tracheids, long slender cells with tapered
ends. Pits along the sides and at the ends of these cells line up
permitting water to flow from cell to cell. The pits of pit pairs
consist of thin primary cell walls only.
have vessel elements. These shorter, wider, cells align end
to hollow end to form uninterrupted pipelines. The cell wall at
the end of vessel elements is often missing, and pits dot their
sides for lateral movement of water.
phloem is living tissue which is involved in the active
transport and bulk movement of sugars and other nutrients. It is a
complex tissue that has two major components:
- sieve elements and
- companion cells
The backbone of
phloem function is carried out by sieve tube members which when
arranged end to end form sieve elements. Because the end walls of
sieve tubes have pores materials dissolved in the cytoplasm can stream
from cell to cell. Sieve tube members, while alive at maturity, lack a
nucleus so their energy and metabolic needs are supplied by companion
cells which lie adjacent to them. Thus companion cells also play a
role in conduction of food.
Differentiation in the Young Root Tip
The root tip to be successful must
accomplish 3 tasks:
- growth (root apical meristem)
- penetration of the soil (root cap and
region of elongation)
- absorption of water and minerals (root
hairs in region of maturation and mycorrhiza)
Primary growth involves the ability of a
cell with a potentially rigid cell wall to:
- divide -- root apical meristem
- elongate -- see article about
Roots can penetrate soil without
destroying the delicate meristem cells because they create a
protective cap. The turgor pressure helps elongate cells above the
root tip which forces the tip deeper into the soil.
As the epidermal cells of the root tip
mature many will produce hair-like extensions. These root hairs
intermingle with soil particles and perhaps the hypha of fungi to
create a large absorptive surface area.
- The Stele (and cross section of
a young root)
- The epidermis is a single layer of
cells which first produce root hairs and when mature the wax
- Cortex cells are thin walled and
generally used for storage
- The endodermis produces the
water tight casparian strip
- The stele is composed of
conductive tissues such as xylem and phloem and gives the root
support. The pericycle lines the outer edge of the stele
and can produce lateral roots
Notice which cells of the leaf have
chloroplasts and which do not.
- Adventitious -- roots emerge from
- Aerial roots are generally
adventitious and found above ground (Corn)
- Prop roots are usually aerial
roots which help support the trunk (Mangroves)
- Tap roots are a dominant main root
which can grow deep into the soil.
- Diffuse root systems as in grasses
have numerous branches producing a shallow mat.
Secondary Growth in the Stem
The primary growth in the stems of monocots
and dicots differ in the arrangement of vascular bundles. In monocots
the vascular bundles have a random appearance scattered throughout the
stem, but in dicots these bundles are located in a circle near the outer
edge just below the epidermis.
In all vascular bundles the xylem is
located toward the center of the stem and the phloem faces the
Secondary growth results in an increase
in the plant stem's girth or thickness. This occurs mostly in dicot
Perennials also exhibit open growth and
an indeterminate life span. It is possible for these plants to live for
a very long time, perhaps as long as 10,000 years, constantly growing in