Biology · Plant Structure, Transport & Nutrition

Plant cells, tissues, roots, stems and leaves

Imagine following one parcel of water from a thin root into a leaf. Its route passes through several kinds of tissue; it does not simply enter a hollow plant. At the same time, carbon entering the leaf can later reach that root in organic molecules. A useful anatomical description must explain both connections.

Cells, Membranes & Organisation introduces selective membranes, osmosis and connections between cells. Energy, Enzymes & Metabolism introduces respiration, carbon fixation and ATP coupling. Their necessary ideas are restated here. Plant Diversity & Evolution provides the wider evolutionary context; on a first reading, remember that vascular tissues belong to particular plant lineages. These lessons use selected vascular land plants rather than a universal plant body. Chemistry's concentration and energy chapters and Physics's fluid dynamics chapter offer optional return paths.

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By the end, you can

Trace a material through organs and tissues while preserving the cellular boundaries that regulate its passage.

An organ contains several cooperating tissues

A tissue is a connected group of cells with related structural and functional roles. An organ, such as a root, stem or leaf, contains multiple tissues. Dermal tissue forms protective and exchange surfaces. Ground tissue contributes to tasks such as storage, support and photosynthesis. Vascular tissue provides long-distance routes. These are tissue systems, not three separate organs.

Living plant cells have plasma membranes that regulate exchange. Their walls support the cell and connect to a surrounding wall network, but a porous wall is not equivalent to a selectively permeable membrane. A large vacuole can contribute to storage and pressure. Plastids perform different tasks: a chloroplast supports photosynthesis, whereas many root cells have other plastids rather than green chloroplasts.

Cell function also changes with development. A mature water-conducting tracheid or vessel element has lost its living contents. Living cells remain within xylem tissue, so saying that every xylem cell is dead would erase part of the tissue. In an angiosperm, mature phloem sieve-tube elements are living but lack a nucleus; associated companion cells provide metabolic support. Other vascular lineages have different conducting arrangements.

A root has an outside and an organised interior

In the selected young root section, an epidermis faces the surroundings. Root hairs are extensions of individual epidermal cells, increasing their contact surface. Farther inward lies cortex tissue, then an endodermal layer surrounding the central region containing the vascular tissues. The drawing separates xylem and phloem positions without treating their arrangement as a template for every root.

Water and ions must pass from the external environment toward that interior. A route through cell walls differs from a route through living cytoplasm. The endodermis helps regulate entry through its barriers and membrane transport. Later, we will trace the crossing explicitly; the important first step is recognising that the root surface and a xylem conduit are different locations.

Roots also store organic material and consume energy in respiration. Their usual lack of a large photosynthetic surface does not make them inactive. Carbon delivered from source tissues supports membrane transport, maintenance and new growth. Roots can occur above ground, and stems can occur underground: position alone does not determine an organ's identity.

A selected root from surface to vascular interior

The drawing distinguishes tissue positions in one selected arrangement. Root anatomy varies with lineage and developmental region.

Read the diagram in words

From outside inward, the selected section has epidermis at radii 4.5–5, cortex at 2.5–4.5, endodermis at 2.2–2.5 and vascular interior below 2.2. Central xylem forms a cross-like union; four phloem circles occupy spaces between its arms. An epidermal extension projects outward at the right.

Drawing units
Coordinate origin	Drawing x coordinate	Drawing y coordinate
Center of the selected root section	0	0

A selected root from surface to vascular interior
Description	Inner radius	Outer radius
Epidermis	4.5	5
Cortex	2.5	4.5
Endodermis	2.2	2.5
Central vascular region	0	2.2

Selected xylem positions
Displayed structure	Drawing x coordinate	Drawing y coordinate	Width	Height
1	-1.5	-0.35	3	0.7
2	-0.35	-1.5	0.7	3

Phloem positions
Displayed structure	Center x coordinate	Center y coordinate	Radius
1	-0.95	-0.95	0.25
2	0.95	-0.95	0.25
3	0.95	0.95	0.25
4	-0.95	0.95	0.25

Extension of an epidermal cell
From	To
(5, 0)	(6.5, 0)

A selected root from surface to vascular interior
Process	From	To
Contact with the surroundings	(6.8, 1)	(5, 1)
Routes across cortex toward the endodermis	(4.3, 1)	(2.7, 1)

Phloem positions

Selected root section

Surroundings

Drawing units; omitted detail is not absence of tissue

Original selected young vascular-root arrangement, with a central cross-like xylem pattern. Not measured anatomy or a universal root plan; pericycle and several cell types omitted.

A leaf connects gas spaces, cells and veins

The selected leaf has epidermal surfaces enclosing photosynthetic mesophyll. Spaces between some mesophyll cells connect to a stomatal pore. Two guard cells regulate the opening. Carbon dioxide can move through that gas route and then dissolve and enter cells; water vapour can leave along the same general route.

A vein contains vascular tissues. Xylem supplies water to the leaf, and phloem connects it to other parts of the plant. A mature leaf may export carbon while a young developing leaf imports it. The vein is therefore more than a structural support, and the gas space is more than an empty gap.

Leaf thickness, internal arrangement and stomatal positions differ among species and environments. Stems can photosynthesise, some plants obtain organic carbon from partners, and nonvascular plants have other transport constraints. A functional model becomes stronger when it names the organism and developmental region it represents.

A connected gas route inside a leaf

This selected section preserves a gas pathway without implying that every mesophyll cell has its own stomatal pore.

Read the diagram in words

The upper epidermis is above columnar mesophyll cells. Irregular spongy cells leave connected spaces below. A vein at (7, 4.5) contains separate xylem and phloem positions. The lower epidermis has a pore between guard cells; a route from outside passes through the pore into an air space, with a separate step toward a living cell.

Drawing units
Axis	Minimum	Maximum
Drawing x coordinate	0	12
Drawing y coordinate	0	8

Upper epidermis
Displayed structure	Drawing x coordinate	Drawing y coordinate	Width	Height
Upper epidermis · 1	0	0	12	1

Palisade mesophyll cell
Displayed structure	Drawing x coordinate	Drawing y coordinate	Width	Height
Palisade mesophyll cell · 1	0.30000000000000004	1.3	1.4	2.2
Palisade mesophyll cell · 2	2.3	1.3	1.4	2.2
Palisade mesophyll cell · 3	4.3	1.3	1.4	2.2
Palisade mesophyll cell · 4	6.3	1.3	1.4	2.2
Palisade mesophyll cell · 5	8.3	1.3	1.4	2.2
Palisade mesophyll cell · 6	10.3	1.3	1.4	2.2

Spongy mesophyll
Displayed structure	Center x coordinate	Center y coordinate	Horizontal radius	Vertical radius
1	1.5	4.5	1	0.7
2	4	5	1	0.7
3	10	4.5	1	0.7
4	9.5	6.2	1	0.6

Selected leaf vein
Displayed structure	Center x coordinate	Center y coordinate	Horizontal radius	Vertical radius
Selected leaf vein · 1	7	4.5	1.4	1.1
Xylem · 2	7	4.1	0.35	0.35
Phloem · 3	7	4.9	0.3	0.3

Lower epidermis
Displayed structure	Drawing x coordinate	Drawing y coordinate	Width	Height
1	0	7	4.7	1
2	7.3	7	4.7	1

Guard cells
Displayed structure	Center x coordinate	Center y coordinate	Horizontal radius	Vertical radius
1	5.2	7.5	0.5	0.5
2	6.8	7.5	0.5	0.5

Stomatal pore
Axis	Minimum	Maximum
Drawing x coordinate	5.7	6.3
Drawing y coordinate	7	8

Connected air-space route; gases can move in either direction
Position along the displayed route	Drawing x coordinate	Drawing y coordinate
1	6	8.6
2	6	7.5
3	6	6.2
4	5.5	5.9
5	5.5	4.4

A connected gas route inside a leaf
Process	From	To
Entry into living tissue requires an additional crossing	(5.5, 4.4)	(5.5, 3.2)

Spongy mesophyll

Internal air space

Guard cells

Lower epidermis

Drawing units; no measured dimensions

Original selected dorsiventral leaf arrangement, simplified local section. Not every leaf has these layers or stomatal positions; the vein sheath and many cell types are omitted.

Worked example

Follow two routes through one plant

In our selected vascular plant, follow water from the root surface through the root interior, into xylem, along the stem and into a leaf. At the leaf, some water evaporates from moist internal surfaces and leaves as vapour through stomata. This sequence names tissues and a phase change; it does not predict a rate.

Now follow carbon. Carbon dioxide enters a photosynthetic leaf, and fixation incorporates its carbon into organic molecules. Some of that carbon becomes transported sugar, passes into phloem and reaches a growing root. The root can respire part of it and retain part in biomass. The carbon route is distinct from the water route even though the systems exchange water and influence each other.

An underground swollen organ with nodes and buds can be a modified stem, as in a potato tuber. An organ's developmental structure is more informative than whether it is hidden in soil. Roots, stems and leaves also contain more than one tissue, so identifying an organ does not yet identify the transport route.

Check your understanding

A root cell has a wall but no chloroplast. Does either fact show that the cell is dead?
Show reasoning
No. A wall is compatible with a living cell, and many living root cells have nonphotosynthetic plastids. Evidence about membranes, metabolism and cellular contents is needed; green colour is not a test of life.
Why is 'xylem is dead and phloem is alive' an incomplete comparison?
Show reasoning
It contrasts conducting elements too broadly with entire tissues. Mature tracheids and vessel elements are dead, but xylem also contains living cells. Angiosperm sieve-tube elements remain living and depend on companion cells; other vascular plants have different phloem cell arrangements.
Would a drawing with every leaf cell touching a stomatal pore explain gas exchange well?
Show reasoning
No. It would remove the connected internal air-space route and the distinction between a gas pathway and membrane crossing into cells. A useful selected leaf section preserves mesophyll cells, air spaces, epidermis, guard cells and a pore without implying that every cell has its own pore.

What to carry forward

Identify the organ, tissue, cell state and crossing route together; a label alone does not explain transport.

References

Resources supporting this lesson’s explanations and examples.