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Vascular tissue

The xylem and phloem that make up the vascular tissue of the stem are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. When the stem is viewed in cross section, the vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, the individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue ( [link] ).

 Part A is cross section of a dicot stem. In the center of the stem is ground tissue. Symmetrically arranged near the outside of the stem are egg-shaped vascular bundles; the narrow end of the egg points inward. The inner part of the vascular bundle is xylem tissue, and the outer part is sclerenchyma tissue. Sandwiched between the xylem and sclerenchyma is the phloem. Part B is a cross section of a monocot stem. In the monocot stem, the vascular bundles are scattered throughout the ground tissue. The bundles are smaller than in the dicot stem, and distinct layers of xylem, phloem and sclerenchyma cannot be discerned.
In (a) dicot stems, vascular bundles are arranged around the periphery of the ground tissue. The xylem tissue is located toward the interior of the vascular bundle, and phloem is located toward the exterior. Sclerenchyma fibers cap the vascular bundles. In (b) monocot stems, vascular bundles composed of xylem and phloem tissues are scattered throughout the ground tissue.

Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements. The latter two types conduct water and are dead at maturity. Tracheids are xylem cells with thick secondary cell walls that are lignified. Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent. Vessel elements are xylem cells with thinner walls; they are shorter than tracheids. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant.

Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. A series of sieve-tube cells (also called sieve-tube elements) are arranged end to end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles.

Ground tissue

Ground tissue is mostly made up of parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith    , while the layer of tissue between the vascular tissue and the epidermis is known as the cortex    .

Growth in stems

Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth    , and is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant, and is caused by cell division in the lateral meristem. [link] shows the areas of primary and secondary growth in a plant. Herbaceous plants mostly undergo primary growth, with hardly any secondary growth or increase in thickness. Secondary growth or “wood” is noticeable in woody plants; it occurs in some dicots, but occurs very rarely in monocots.

Questions & Answers

what is micro-organism
Jackson Reply
what is the hypothesis
Jackson
hypothesis is a proposed explanation for a phenomenon
Miriam
hypothesis is raw materials
KP
what is biology
KP
what does mean stigma
Amira Reply
what is the full of the MOST dangerous disease in the world where one stops sleeping and just dies :Hint ; FFI
God Reply
fatal familial insomnia which affects the thalamus
Miriam
there are other dangerous diseases like CAD i.e coronary artery disease
Miriam
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Thomas Reply
it is any thing that has weight and occupies space
Anye
matter is any substances that occupies spaces and has mass
Jackson
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
photosynthesis is the process by which plants and other organisms convert light energy to chemical energy
Miriam
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

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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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