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 Left illustration shows a cross section of a woody stem undergoing primary growth. At the core of the stem is pith. Toward the outside are egg-shaped vascular bundles. Xylem is located toward the inside of the vascular bundle, and phloem is in the middle. Sclerenchyma cap the outside of the bundle. Right illustration shows a cross section of a woody stem undergoing secondary growth. As in primary growth, the core of the stem is pith. Outside the pith is a ring of secondary xylem. Rounded bundles of primary xylem tissue project from this ring into the pith. Outside the secondary xylem is a ring of secondary phloem tissue. The vascular cambium separates the xylem from the phloem. Outside the secondary phloem is the cortex layer. Bundles of primary phloem project outward from the secondary phloem into the cortex. A cork ring surrounds the cortex. The cork is separated from the cortex by a thin cork cambium. The bark of the tree extends from the vascular cambium to the epidermis.
In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis.

Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.

Primary growth

Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight (shoots).

The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.

Watch this BBC Nature video showing how time-lapse photography captures plant growth at high speed.

Secondary growth

The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium (see [link] ). The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside, and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.

In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm    . The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels , which allow the interior cells to exchange gases with the outside atmosphere ( [link] ). This supplies oxygen to the living and metabolically active cells of the cortex, xylem and phloem.

Questions & Answers

what is matter
Thomas Reply
it is any thing that has weight and occupies space
Anye
describe photosynthesis
Mavis Reply
What is equilibrium
Mavis
What is equilibrium
Mavis
like corporal intern balance right?
FRANCISCA
on my own understanding is just a balanced state
Stanley
what is a chromosome?
Wise Reply
Are thread-like structures located inside the nucleus of animal and plant cells.
Canab
thx
Wise
what are the difference between Biotic community and Ecological nitche.
Ganiyat Reply
what is the celll
KAMOLIKA Reply
A cell is the simplest bit of living matter that exist independently
Ganiyat
cell is the basic unit of life
Shadrack
what is ecdysis
Shadrack
what is genetics
Sebastian Reply
The cell is the simplest bit of living matter that can exist independently.
Ganiyat
what happenes when the cell of an organism Is removed?
Isaac Reply
The cell will not function properly
Eunice
what is cell
Maarig Reply
cell is stractural and functional unit of our human body.
Rohini
The study of cells are referred to as?
Kenneth Reply
what is active transport
johnny Reply
is the movement of molecules through a semi permeable membrane with the use of energy
Kenneth
is the movement of substances across a membrane against the concentration gradient by using energy.
Wise
what is living things
Aminu Reply
these are organisms that take in respiratory gases e.g plants and animals
Kenneth
they are organisms that undergoe the various life processes such as growth, respiration, reproduction, excretion etc
Miriam
what is gland
Igwe
an organ synthesizes a substance such as hormones or breast milk
Brenden
Why do plants contain oxygen
Alfonso Reply
what is biology?
Tshepi Reply
study of life.
Kiran
what is peristalsis
Tshepi
peristalsis is the contraction and relaxation of the oesophagus to allow food to pass through
Nana
what is a cell?
Kevin
cell is the basic structural and smallest unit of life
Onyinyechi
They can be Prokaryotic and Eucaryotic
FRANCISCA
is a branch of science that deals with the study of living organisms
Kenneth
Biology is a brunch of science that deals with
Mavis
do you tell me way of studing to be succesfull ?
subhan Reply
for studying to be successful ,do study smart not study hard I mean,make effective notes be concentrated on every topic and try to understood not follow the way of rattafication
Rohini
ok
Yula
What's the difference between studying hard and studying smart
Emmanuel
how is formed?
Joanita Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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