The Leaf

The upper epidermis

the upper protective layer of the leaf and it has no chloroplasts. so is not directly involved with photosynthesis. It is covered with a waxy cuticle to reduce water loss. The waxy cuticle is thickest in the leaves of those plants best adapted to reduce water loss, but being transparent, it does not significantly impede the passage of light.

The palisade mesophyll

is the primary photosynthesising region in the leaf and to maximise the harvesting of light energy, it occurs immediately beneath the upper epidermis - the surface of the leaf that receives most light. The palisade cells are arranged regularly in order to pack in as many as possible. Each cell has many chloroplasts to maximise photosynthesis. The large vacuole in palisade cells helps the light to pass down through the layers with minimum shading.

benefits of having palisade cells perpendicular to the upper epidermis?

As well as the benefits of maximising the incident light on each cell, there are other advantages with this arrangement. For example, sugars moving to the phloem have to pass through fewer cells, as do the gases diffusing to and from the intercellular air spaces.

The large surface area

most leaves this a further adaptation for photosynthesis, both through providing more palisade cells in close proximity to the upper leaf surface and also to facilitate gas exchange with no cell being too far from a gas exchange surface. However, the leaf surface area is a compromise between maximising photosynthesis and restricting water loss.

The spongy mesophyll

loosely and irregularly arranged, leading to the formation of intercellular air spaces that encourages the diffusion of gases for gas exchange. While they do photosynthesise, they have fewer chloroplasts than palisade cells, so have a less significant photosynthetic role than the palisade cells. important in gas exchange and the carbon dioxide and oxygen required by the palisade cells diffuses through the spongy mesophyll cells on their way to and from the palisade layer. The loose and irregular arrangement of these cells gives a large gaseous exchange surface in the leaf.

The vascular tissue

often evident as the leaf veins that branch from the leaf midrib. Xylem vessels transport water and minerals from the roots to the leaves, while phloem sieve tubes transport sugars produced during photosynthesis to other parts of the plant for storage or energy.

lower epidermis

covered with a waxy cuticle but it is usually much thinner than the cuticle on the upper epidermis, as less transpiration tends to occur through the lower epidermis.

Stomata

little pores and their opening and closing is controlled by the presence of guard cells. Guard cells are unusual for epidermal cells in that they do possess chloroplasts. When turgid, the guard cells expand and open the pore of the stoma, thereby facilitating gas exchange. When less turgid, the pore closes and reduces water loss

when does the stomata open and close and why?

In most plants the stomata are closed at night to reduce water loss. While evaporation losses are usually greater during the day when it is warmer (compared to night), the stomata need to remain open to allow the gases involved in photosynthesis and respiration to enter and leave. During darkness, leaf metabolic activity is much reduced, with only respiration taking place and there is usually enough oxygen within the leaf to allow respiration to take place even with the stomata closed.