Plant tissues: meristems, xylem and phloem
Plant tissues explained: apical, intercalary and lateral meristems, parenchyma, collenchyma, sclerenchyma, xylem, phloem and the three tissue systems.
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A tissue is a group of cells with a common origin that work together to perform a function. Plants are built from relatively few tissue types, but the way they are arranged explains a great deal about crops. It explains how a wheat stem keeps growing after it is cut, why jute and flax make strong fibre, how water rises 100 metres in a tree, and why grafting needs cambium to meet cambium.
Plant tissues fall into two broad groups. Meristematic tissues keep dividing. Permanent tissues have stopped dividing and taken on a specialised form and function.
Meristematic tissues
Meristem cells are small and thin-walled, with dense cytoplasm, a large nucleus, tiny vacuoles and no intercellular spaces. They divide repeatedly, and their daughter cells differentiate into permanent tissues.
Meristems are classified by position:
- Apical meristems sit at the tips of roots and shoots. They cause primary growth, which increases length. The shoot apex also produces leaf and bud primordia; the root apex is protected by a root cap.
- Intercalary meristems sit at the base of internodes or leaf sheaths, mainly in grasses. They let cereals and grasses regrow after grazing or mowing, and help lodged stems bend back upright.
- Lateral meristems run parallel to the long axis. The vascular cambium produces secondary xylem inward and secondary phloem outward. The cork cambium (phellogen) produces bark. Together they cause secondary growth, which increases girth in dicots and gymnosperms.
Meristems can also be classified by origin. Primary meristems come directly from embryonic cells; secondary meristems, such as cork cambium and interfascicular cambium, arise later from permanent cells that start dividing again.
Most monocots lack a vascular cambium, so they do not form true wood. Palms and bamboos thicken in other ways.
Simple permanent tissues
Parenchyma
Parenchyma is the most common tissue. Its cells are living and thin-walled, with intercellular spaces. Parenchyma stores starch, as in potato tubers and cereal endosperm, and carries out photosynthesis as chlorenchyma in leaf mesophyll. It also helps with buoyancy as aerenchyma in rice and aquatic plants, and with wound healing. Parenchyma can resume division, which is why cuttings root and tissue culture works.
Collenchyma
Collenchyma cells are living and elongated, with cell walls thickened unevenly at the corners with cellulose and pectin. It gives flexible support to young stems and leaf stalks, and is found just under the epidermis in dicots. Celery strings are collenchyma.
Sclerenchyma
Sclerenchyma cells are dead at maturity, with thick lignified walls. There are two forms:
- Fibres are long and tapering. They give tensile strength and are the source of jute, flax, hemp and sisal. (Cotton is not a fibre in this sense: it is a seed hair.)
- Sclereids (stone cells) are short and irregular. They form nut shells and the gritty texture of pear flesh.
Complex permanent tissues
Xylem
Xylem conducts water and minerals upward from the roots and gives mechanical support. It has four elements:
| Element | Living or dead | Role |
|---|---|---|
| Tracheids | Dead | Water conduction through pits; the main conducting cell of gymnosperms |
| Vessels | Dead | Wide tubes formed from vessel elements joined end to end; the main conductors in angiosperms |
| Xylem parenchyma | Living | Storage and lateral transport |
| Xylem fibres | Dead | Support |
Protoxylem forms first and has narrow cells; metaxylem forms later and has wider cells. In stems the protoxylem lies toward the centre (endarch); in roots it lies toward the outside (exarch). This is a classic exam distinction.
Phloem
Phloem carries sugars and other organic solutes from sources such as leaves to sinks such as roots, fruits and growing tips. Its four elements are:
- sieve tube elements, living but without a nucleus, joined end to end through sieve plates;
- companion cells, living and nucleated, which keep the sieve tubes working;
- phloem parenchyma;
- phloem fibres (bast fibres).
Gymnosperms have sieve cells and albuminous cells instead of sieve tubes and companion cells. Phloem transport is explained by Münch's pressure-flow hypothesis.
Why ringing a tree kills it slowly
If a ring of bark is removed from a tree trunk, the phloem is cut but the xylem is left intact. Water still moves up, so the leaves stay green for a while. Sugars, however, can no longer reach the roots, which slowly starve and die, and the tree dies with them. Fruit growers use partial or temporary girdling of branches for the opposite effect: more sugar stays above the cut, which increases fruit set and size in grapes and some fruit trees.
The three tissue systems
Sachs grouped tissues into three systems that run through the whole plant:
- Dermal system: the epidermis, with cuticle, stomata and hairs (trichomes), and later the periderm (bark) in woody plants.
- Ground system: everything between the epidermis and vascular tissue, including cortex, pith, mesophyll and endodermis.
- Vascular system: xylem and phloem arranged in vascular bundles, which may be conjoint, radial or concentric. A conjoint bundle with cambium between xylem and phloem is open, as in dicot stems. One without cambium is closed, as in monocot stems.
Key points
- Meristems are dividing tissues: apical (length), intercalary (grasses) and lateral (girth).
- Parenchyma is living and stores food; collenchyma is living with thickened corners and gives flexible support; sclerenchyma is dead and lignified (fibres and sclereids).
- Xylem has tracheids, vessels, parenchyma and fibres, and carries water up. Phloem has sieve tubes, companion cells, parenchyma and fibres, and carries food.
- Only xylem parenchyma is living in xylem; only phloem fibres are dead in phloem.
- Stems are endarch; roots are exarch.
- The three tissue systems are dermal, ground and vascular.
What the exams ask
- Classify meristems by position and give the function of each.
- Distinguish parenchyma, collenchyma and sclerenchyma.
- Name the elements of xylem and phloem and which are living.
- Define endarch and exarch xylem, and open and closed vascular bundles.
- Explain why girdling kills a tree.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do grasses regrow after mowing while most herbs do not?
Grasses have intercalary meristems at the base of leaves and internodes, and their shoot apex stays low near the ground. Cutting removes old tissue above the meristems, which keep producing new growth. Most dicot herbs keep their growing points at the shoot tips, which mowing removes.
Is wood xylem or phloem?
Wood is secondary xylem made by the vascular cambium, laid down year after year as growth rings. The bark outside the cambium contains the secondary phloem and the cork.
Why do sieve tubes lack a nucleus?
Losing the nucleus and most organelles leaves an open channel for sap to flow with little resistance. The neighbouring companion cell supplies the proteins and energy the sieve tube needs, through many plasmodesmata.
Sources
- Esau, K. Anatomy of Seed Plants, 2nd edition. Wiley, 1977.
- Evert, R. F. Esau's Plant Anatomy: Meristems, Cells, and Tissues of the Plant Body, 3rd edition. Wiley, 2006.
- Raven, P. H., Evert, R. F. and Eichhorn, S. E. Biology of Plants, 8th edition. W. H. Freeman, 2013.
- Taiz, L., Zeiger, E., Møller, I. M. and Murphy, A. Plant Physiology and Development, 6th edition. Sinauer, 2015.
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