Leaves are specialized plant organs for photosynthesis. They consist of a petiole, lamina, epidermis, and mesophyll tissue. The epidermis is the outer layer that covers and protects the inner tissues. The mesophyll contains two types of tissues - palisade mesophyll with densely packed cells for photosynthesis, and spongy mesophyll with loosely packed cells that provide a surface for gas exchange. Transpiration is the process where water evaporates from the leaf surface and is an important part of photosynthesis, but the rate of transpiration is affected by environmental factors like light, temperature, humidity, and wind.

1. Leaf structure
By Martin Samaj

2. Leaf basics
• Above ground plant organ
specialized for
photosynthesis
• Leaves are the site where
transpiration and guttation
takes place
• Leaves can store food and
water, in other plants they
can serve different
purposes

3. Leaf anatomy
• Leaf of an angiosperm consists of:
 Petiole (leaf stem)
 Lamina (leaf blade)
 Stipules
• Leaf consists of the following tissues:
 Epidermis
 Mesophyll
 Arrangement of veins

4. Leaf anatomy

5. Epidermis
• Outer layer of cells covering the leaf,
covered by a thick waxy cuticle
• It isolates the plant’s inner parts from the
outside
• Serves several functions: regulation of
gas exchange, secretion of metabolic
compounds, prevents water loss from
the upper surface

6. Mesophyll
• Palisade mesophyll consists of densely
packed cylindrical cells with many
chloroplasts
• Palisade mesophyll is the main
photosynthetic tissue and is positioned
where the light intensity is the highest
• Spongy mesophyll consists of loosely
packed cells with few chloroplast
• This tissue provides the main gas
exchange surface

7. Leaves and transpiration
• Photosynthesis depends on gas exchange
over a moist surface.
• Spongy mesophyll cells provide this
surface
• Water often evaporates from the surface
and is lost and this process is called
transpiration
• Transpiration is the loss of water vapor
from the leaves and stems of plants

8. Factors affecting transpiration
• The rate of water loss through
transpiration depends on internal and
external conditions
• Abiotic factors that have effect on the rate
of transpiration
• There are 4 main factors: Light,
Temperature, Humidity and Wind

9. The 4 abiotic factors
• Light - gurad cells close the stomata in the night = greater
rate of transpiration
• Temperature - as the temperature rises the rate of
transpiration increases
• Humidity - The lower the humidity outside the leaf the faster
the rate of transpiration
• Wind - Wind blows the saturated air away and so increases
the rate of transpiration