Plant Biology: Chapter 2 — Plant cells and tissues

🔬 Key Illustrated Scientific Figure

Full-Screen Zoom

Click to enlarge on this page

2.1 Walls and the symplast #

2.1.1 Layers and pits #

Cellulose, hemicelluloses, pectins, lignin, and the rosettes that spin microfibrils are Cell Biology. Anatomy needs the order of layers.

The middle lamella is the shared, pectin-rich, calcium-gelled cement. Its breakdown softens some ripe fruits. The primary wall is laid down during growth; microfibrils, often aligned by cortical microtubules, bias the direction of stretch. The secondary wall is deposited inside it after growth stops. In tracheary elements and fibres it is thick and usually lignified, in layers S1, S2, and S3, of which S2 is thickest. Lignin begins in the compound middle lamella and continues into the secondary wall. It resists compression and digestion. Monomer biosynthesis is Biochemistry.

A pit is a gap in the secondary wall. Bordered pits of conifer tracheids have a torus in a porous margo, which can seal the aperture of an embolised cell. How an embolism spreads is Plant Physiology.

2.1.2 Plasmodesmata #

Plasmodesmata are membrane-lined channels through the wall, with a desmotubule of endoplasmic reticulum. The membrane structure is Cell Biology. The channels cluster in primary pit fields. A simple one often has a size-exclusion limit around 1 kilodalton, and that limit is regulated. Companion cells and sieve-tube elements share specialised branched connections.

The symplast is the continuum of cytoplasms joined by plasmodesmata. The apoplast is walls and water-filled spaces outside the plasma membrane. The two words map where a molecule can move without crossing a membrane. Plant Physiology uses the same maps for water and for phloem loading.

2.2 Ground tissues #

Parenchyma is alive and thin-walled: chlorenchyma, starch storage, secretion, and cells that can divide again in a wound. Aerenchyma is the same tissue opened into large air spaces. Collenchyma is alive, with uneven, usually unlignified primary walls, and supports organs still elongating; celery strings are the example. Sclerenchyma has a thick, usually lignified secondary wall and is often dead when functional. Fibres are long (flax and hemp are phloem fibres). Sclereids are short; pear stone cells are the example. Sclerenchyma supports organs that have finished growing.

An idioblast differs sharply from its neighbours: a tannin cell, an oil cell, or a cell of calcium oxalate raphides. Sachs’s three systems are the map underneath every organ: dermal, ground (cortex, pith, mesophyll), and vascular. Root, stem, and leaf rearrange those three. They are not unrelated objects.

2.3 Vascular tissues #

2.3.1 Xylem #

Tracheary elements are dead, lignified, and hollow. Tracheids, present in all vascular plants, pass water only through pits. Most gymnosperms, lycophytes, and ferns have no other conducting cell. Vessel elements, typical of angiosperms, are wider, stacked as a vessel, and open through scalariform or simple perforation plates. Vessels are not a clean synapomorphy. Amborella and the Winteraceae lack them, and vessel-like cells evolved separately in gnetophytes and in some ferns (Chapter 8).

Protoxylem has annular or helical thickenings and matures while the organ is still elongating. Metaxylem is pitted and matures later. Stems are usually endarch (protoxylem inward); roots are usually exarch (protoxylem outward). Living xylem parenchyma stores starch and may block a retired vessel with a tylosis. Fibres supply strength that wide vessels lack.

2.3.2 Phloem #

Sieve elements conduct sugar from source to sink. A young leaf is a sink and a mature leaf a source, so direction is not fixed as root-to-shoot. Pressure flow is Plant Physiology. At maturity the element keeps a plasma membrane but no working nucleus, and a nucleate neighbour maintains it.

Angiosperm sieve-tube elements meet at sieve plates. P-protein in many eudicots can plug a damaged plate. The companion cell is the sister cell of its sieve-tube element. Transfer cells and intermediary cells are loading variants; the routes are Plant Physiology. Other vascular plants, including gymnosperms, have sieve cells with lateral sieve areas, maintained by albuminous cells (Strasburger cells) that are not their sisters. Same job, different origin.

Protophloem is often crushed as the organ elongates. Secondary phloem does not accumulate: older sieve elements collapse, and only a thin outer layer conducts (Chapter 3).

2.4 Dermal tissue #

The epidermis is usually one layer, from the protoderm. Aerial walls carry a cuticle of cutin and waxes, a secreted layer, not itself a wall. Root epidermis has little cuticle and produces root hairs, single-cell extensions with no meristem and no stele. A hair is not a lateral root.

Stomata are pores between two guard cells: kidney-shaped in most eudicots, dumbbell-shaped and flanked by subsidiary cells in grasses. The guard-cell mother cell is set aside in the protoderm, often by an unequal division. Many broadleaves are hypostomatous. Grasses often have stomata on both faces, and floating leaves often have them above. Density is commonly tens to a few hundred per square millimetre. Uneven walls turn a turgor change into an opening. The ion fluxes are Plant Physiology.

Trichomes are epidermal appendages. They may reflect radiation, slow the air at the surface, hook insects, or hold oils. A nettle sting is a silicified hair that snaps and releases irritants, including histamine: a defence, not a medicine. Where secondary growth splits the epidermis, periderm replaces it: phellem (cork) outside, phellogen (cork cambium), and phelloderm inside. Cork cells are dead, suberised, and usually air-filled. Lenticels keep a gas path after the stomata have gone.

2.5 Meristems #

2.5.1 Apical and intercalary #

An apical meristem keeps dividing and leaves derivatives to differentiate. In many angiosperm shoot apices a tunica of one or more layers divides anticlinally over a corpus that divides in several planes. The outer layer, L1, produces the epidermis. A central zone divides slowly, a peripheral zone initiates leaves and procambium, and a rib zone contributes pith. The WUSCHEL–CLAVATA circuit that maintains that centre in Arabidopsis is Developmental Biology. Phyllotaxy is alternate, opposite, or whorled. Many spirals place successive leaves near 137.5° apart, a packing result, not a universal constant.

The root apex adds the body proximally and the root cap distally. Cap statocytes contain amyloplasts that settle; how that becomes curvature is Plant Physiology. A quiescent centre of slowly dividing cells lies among the initials. Surgery and later cell ablation show that it can restore damaged initials (Clowes described the region). Closed meristems, as in Arabidopsis, have discrete initials; open meristems share them between cap and cortex.

An intercalary meristem is a band of division between tissues already differentiated. Grasses have them at the bases of internodes and of the leaf, which is why mowing does not stop growth. That is primary growth at several sites, not secondary growth.

2.5.2 Lateral meristems #

The vascular cambium has fusiform initials, which produce the axial system (tracheids, vessels, fibres, sieve elements, axial parenchyma), and ray initials, which produce the rays. Periclinal divisions add secondary xylem inward and secondary phloem outward. More xylem is made than phloem, and the xylem is not crushed, so wood accumulates. The cork cambium arises first in the cortex and later in the secondary phloem. Each new phellogen is a separate meristem.

Primary growth makes length. Secondary growth makes girth. A typical grass or lily has no vascular cambium. Palms thicken from a primary thickening meristem while young and do not lay down annual rings. Dracaena and Yucca have a secondary thickening meristem that inserts whole bundles into parenchyma, rather than a cylinder of oak-like wood. A typical cereal stem makes no wood.

2.6 Key terms, summary, and self-check #

Key terms. Middle lamella; secondary wall; torus; symplast; apoplast; collenchyma; sclerenchyma; tracheid; vessel element; endarch; exarch; sieve-tube element; companion cell; albuminous cell; guard cell; periderm; quiescent centre; fusiform initial; ray initial.

Summary. Walls are layered, pits connect tracheary elements, and plasmodesmata define a symplast against an apoplast. Parenchyma does the living ground-tissue work, collenchyma supports organs still stretching, and sclerenchyma supports organs that are finished. Xylem conducts through dead tracheids in all vascular plants and, in most angiosperms, through vessels; phloem conducts through living enucleate sieve elements maintained by companion cells or, in gymnosperms, by albuminous cells of a different origin. Epidermis, stomata, and trichomes are the primary skin, replaced by periderm at secondary growth. Apical and intercalary meristems make length; the two cambia make girth and make more wood than bark.

Self-check.

1. Where is the secondary wall relative to the primary wall, and which sub-layer usually dominates?

2. What is alive in a mature vessel element, and what does the companion cell still provide?

3. How does the origin of a companion cell differ from that of an albuminous cell?

4. Why does repeated mowing not remove every meristem of a grass?

5. What do fusiform initials produce that ray initials do not?

—

—

—