Plant growth - Principles of Horticulture - page 115

Upper epidermis a single layer
of cells without chloroplasts to
allow light penetration

Spongy
mesophyll
layer

Palisade mesophyll cells
with many chloroplasts
main site of photosynthesis

Palisade mesophyll
layer

Cuticle

Upper epidermis

Waxy cuticle on
surface to reduce
water loss

Spongy mesophyll
cells

Stoma in lower
epidermis for
gas exchange

Lower
epidermis

Air spaces for
gas exchange

Midrib (main vein) delivers water
and nutrients in xylem and takes
away sugars in phloem

Xylem

Collenchyma tissue

Stomata

Supporting
tissues

Phloem

Sclerenchyma tissue

Figure 9.5 Cross-section of a Ligustrum leaf showing its structure as an effi cient photosynthesizing organ

though the other factors may be plentiful. It would be
wasteful, therefore, to increase the carbon dioxide
concentration, light or temperature artificially, if the
other factors were not proportionally increased.

and can move to the top and bottom of the cells
depending on light levels. The spongy mesophyll ,
below the palisade mesophyll, has a loose structure
with many air spaces which allow for the two-way
diffusion of gases. The carbon dioxide from the air
is able to reach the palisade mesophyll and oxygen,
the waste product from photosynthesis, leaves
the leaf. The numerous stomata on the lower leaf
surface (positioned here to reduce water loss) are
the openings to the outside through which this gas
movement occurs. Many small vascular bundles
( veins ) within the leaf structure contain the xylem
vessels that provide the water and minerals for the
photosynthesis reaction and phloem sieve tubes, for
the removal of sugar to other plant parts.
The arrangement of leaves on the plant and the angle
at which they are held maximizes light interception,
as does the large surface area and thin structure of
the leaf. A newly expanded leaf is most efficient in
the absorption of light, but this ability reduces with
age, so leaves may be constantly shed and replaced
within a plant's life cycle, all at once in the case of

The law of limiting factors states that the factor
in least supply will limit the rate of a process,
e.g. photosynthesis.

9

Leaf structure and photosynthesis

Figure 9.5 shows the structure of the leaf and its
relevance to the process of photosynthesis.
The leaf is the main organ for photosynthesis in the
plant, and its cells are organized in a way that provides
maximum efficiency. The upper epidermis is a thin
transparent layer, without chloroplasts, permitting
transmission of light into the lower leaf tissues. The
cylindrical palisade mesophyll cells are packed
together, pointing downwards, under the upper
epidermis. The many chloroplasts within these cells
absorb light to carry out the photosynthesis process