Page 96 - Biology Form Two
P. 96
Biology for Secondary Schools
(iii) Leaf cuticle: A thick cuticle Environmental factors
resists water loss by transpiration, (i) Temperature: Transpiration rate
whereas a thin cuticle makes water increases as temperature rises.
loss by transpiration easier. Higher temperatures cause the
stomata to open hence increasing
FOR ONLINE READING ONLY
(iv) Number of stomata: The more the rate of water loss to the
stomata a leaf has, the higher atmosphere. Lower temperatures
the rate of transpiration. This is cause the stomata to close or
because during respiration water reduce the size of stomata pores,
evaporates from the plant through hence, preventing or slowing down
the stomata.
the transpiration process.
(v) Position of stomata: Stomata on the (ii) Relative humidity: As the relative
upper surface of the leaf lose water humidity of the surrounding
more easily than those on the lower air rises, the transpiration rate
surface. If a plant has leaves with decreases. It is easier for water to
more stomata on the upper surface, evaporate into dry air than into air
the rate of transpiration is higher saturated with moisture.
compared to a plant that has leaves
with more stomata on the lower (iii) Wind and air movement:
surface. Leaves with stomata on Increased movement of air
both lower and upper surfaces have around a plant results in a higher
the highest rate of transpiration. transpiration rate. Wind moves the
air around and replaces the more
(vi) Size of substomatal air spaces: saturated air close to the leaf with
Larger air spaces allow a higher drier air.
rate of transpiration because the
leaves can hold more water vapour. (iv) Availability of soil moisture:
Smaller substomatal air spaces When soil moisture is low, plants
slow down the rate of transpiration. begin to senesce (age prematurely)
(vii) Sunken stomata: Sunken stomata resulting in leaf loss and reduced
occur in pits. They are not exposed transpiration. In addition, less
to moving air in order to slow water is absorbed by the roots
down the transpiration rate. when the soil is dry.
(viii) Epidermal hairs: Epidermal hairs (v) Light: Brighter sunlight increases
trap water vapour on the surface the rate of photosynthesis in the
of the leaves, thus decreasing the guard cells, causing them to become
rate of transpiration. turgid and open the stomata. Higher
90 Student’s Book - Form Two
05/10/2024 15:36:35
BIOLOGY FORM 2 NEW.indd 90
BIOLOGY FORM 2 NEW.indd 90 05/10/2024 15:36:35
