Water and mineral ions travel up a plant in xylem made of dead, empty, lignified tubes. Sugars travel in phloem, built from living sieve tube elements that cannot survive without their companion cells.
In a herbaceous dicotyledonous plant the xylem carries water and mineral ions up from the roots, and the phloem carries assimilates such as sucrose and amino acids from sources to sinks. Their positions differ: in the root the xylem is in the centre, often X or star shaped, with phloem between its arms; in the stem the vascular bundles form a ring near the outside, with xylem on the inner side and phloem on the outer side of each bundle; in the leaf midrib and veins the xylem is on the upper side and the phloem on the lower side. Xylem vessel elements are dead, empty and joined end to end, with lignified walls and pits. Sieve tube elements are living but have no nucleus and very little cytoplasm, with sieve plates between them; each is kept alive by a companion cell with a nucleus, many mitochondria and many plasmodesmata.
The lab opens on a stem section. Click the xylem (blue) and phloem (orange), then switch to the root and the leaf. In each organ, note whether the xylem is in the middle, in a ring or along one side.
| Organ | Xylem | Phloem | Other tissues to show on a plan |
|---|---|---|---|
| root | in the centre, an X or star shape | in groups between the arms of the xylem | epidermis (root hairs), wide cortex, endodermis, pericycle |
| stem | inner side of each vascular bundle | outer side of each vascular bundle | epidermis, cortex, ring of bundles with cambium between xylem and phloem, sclerenchyma caps, central pith |
| leaf | upper side of the midrib and each vein | lower side of the midrib and each vein | upper and lower epidermis, palisade and spongy mesophyll, midrib |
The central position of xylem in a root suits an organ that is pulled and twisted by the soil; the ring of bundles near the edge of a stem helps it resist bending.
A plan diagram shows the outlines of tissues only: no individual cells, no shading, clear continuous lines, proportions that match the specimen and label lines drawn with a ruler. Drawing cells in a plan diagram loses marks.
Click each feature of the cut-away cells: the xylem vessel elements on the left, then the sieve tube elements and the companion cell. Read why each feature matters before you try the Stage 1 questions.
Xylem vessel elements are dead, so they cannot actively transport anything; water moves up them passively. Sieve tube elements are alive even though they lack a nucleus. Mixing these up is a common error.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the lab's root section, where is the xylem?
In the stem section, what lies between the xylem and the phloem of each vascular bundle?
In the transport cells tab, why do the xylem vessel elements have an empty lumen?
Which feature of the companion cell supplies ATP for loading sucrose into the sieve tube?
You may also be asked to draw and label the individual cells from slides, photomicrographs or electron micrographs. Here the rules change.
| Cell | What to draw | Labels to give |
|---|---|---|
| xylem vessel element (longitudinal section) | a long tube, open at both ends, with thickened bands of wall | lumen, lignified thickening, pit, cellulose wall |
| sieve tube element | a tube with a perforated end wall, thin cytoplasm lining the wall | sieve plate, sieve pore, cytoplasm, cellulose wall |
| companion cell | a narrower cell beside the sieve tube element | nucleus, dense cytoplasm, mitochondria, plasmodesmata |
In a transverse section of a vascular bundle, xylem vessels look like wide, empty, thick-walled circles; sieve tubes are smaller and thin-walled, each with a small companion cell squeezed beside it.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
A transverse section of a leaf midrib is examined. Which arrangement would be seen?
Which feature of a xylem vessel element prevents it collapsing when the water inside is under tension?
Which statement about sieve tube elements is correct?
A student draws a plan diagram of a stem section and draws each parenchyma cell of the cortex. What should the student have done instead?
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