Tissue Systems and the Anatomy of Root, Stem and Leaf: Quick Revision

A brief note on Tissue Systems and the Anatomy of Root, Stem and Leaf. Read it before a test. Try each question at the end before you open its answer.

The three tissue systems

• Epidermal: the epidermis with a cuticle (absent in roots), stomata with guard cells and subsidiary cells, root hairs (unicellular) and trichomes (multicellular). Guard cells are bean-shaped in dicots and dumb-bell shaped in grasses.
• Ground: everything except the epidermis and the bundles. Cortex, pericycle, medullary rays, pith, and mesophyll in a leaf.
• Vascular: xylem and phloem. Open bundles have cambium (dicot stems). Closed bundles do not (monocot stems). Conjoint in stems and leaves, radial in roots. Endarch xylem in stems, exarch in roots.

Roots

• Layers: epidermis with root hairs, cortex, endodermis with Casparian strips (suberin), pericycle, radial bundles, pith.
• Dicot root: two to four xylem strands (tetrarch when four), small pith, can grow thicker.
• Monocot root: more than six strands (polyarch), large pith, no cambium.

Stems

• Dicot stem: cuticle and trichomes, collenchymatous hypodermis, cortex, starch sheath, pericycle patches of sclerenchyma, medullary rays, a ring of open, conjoint, endarch bundles, a large pith.
• Monocot stem: sclerenchymatous hypodermis, ground tissue with no pith, scattered closed bundles with sheaths, smaller bundles near the edge, a water-containing cavity in each bundle.

Leaves

• Dicot leaf (dorsiventral): more stomata on the lower face. Palisade parenchyma above, spongy parenchyma below. Net venation. In a vein, xylem faces up and phloem faces down.
• Monocot leaf (isobilateral): stomata on both faces, an undivided mesophyll, parallel venation, bulliform cells that help the blade to curl in dry weather.

A worked example

On a leaf, the lower surface has $140$ stomata on each square millimetre, and the upper surface has $20$. How many stomata are on a patch of $30$ square millimetres, counting both surfaces?

1. Both surfaces. $140 + 20 = 160$ stomata on each square millimetre.
2. The patch. $160 \times 30 = 4800$ stomata.
3. The guard cells. Each stoma has two, so $2 \times 4800 = 9600$ guard cells.

Practice with new numbers

A program to try

The program names a stem from the arrangement of its bundles and the middle of the section.

Mistakes to avoid

• The stoma is the pore, not the guard cells.
• Roots have no cuticle, and they have radial bundles.
• Open bundles have cambium. They belong to a dicot stem.
• A root has exarch xylem, a stem has endarch xylem.
• A grass leaf has no palisade layer. Its mesophyll is undivided.

Check yourself

1. Name the three tissue systems.

2. What are Casparian strips, and what do they do?

3. How can you tell a monocot stem from a dicot stem in a section?

4. Where do the palisade cells lie in a dicot leaf, and why there?

5. What do bulliform cells do?