Stomatal apparatus
The stomata are tiny pores present in the epidermal
surface of leaves, young stems and in certain fruits (e.g.,
banana, Citrus, cucumber, etc.).
This pore is surrounded by kidney shaped or bean shaped
epidermal cells called guard cells. In monocots, the guard
cells generally are dumb-bell shaped. These cells are living,
having nucleus, chloroplasts and cytoplasm. The walls of
guard cells are thickened on inner side. They have one or
two pairs of wall extensions or ledges to prevent entry of
water drops into stomata.
The outer walls of guard cells are thin and more elastic. These
guard cells are surrounded by some specialized epidermal
cells called subsidiary cells or accessory cells.
Stomata are meant for gaseous exchange during photosynthesis and respiration, as well as they are the main source
of transpiration.
When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge
out and force the inner walls into a crescent shape. The opening of the stomata is also aided due to the orientation of the
microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially rather than longitudinally,
making it easier for the stomata to open. When the guard cells lose turgor, due to water loss (or water stress) the elastic
inner walls regain their original shape, the guard cells become flaccid and the stomata close.
Number and distribution of stomata on leaf
On the basis of number and distribution of stomata, leaves are categorized into following types:
- Apple type (mulberry type): Stomata are present only on the lower surface of leaf (hypostomatic leaf),
e.g., apple. - Potato type : Stomata are present on both surfaces of leaf but more on the lower surface, e.g., potato, pea,
tomato and many other dicot plants. - Oat type: The number of stomata is equal on both surfaces of leaf (amphistomatic leaves) e.g., oat and
many other monocots. - Water lily type: The stomata are present only on the upper surface of leaf, e.g., water lily and most floating
plants (Nelumbo, Nymphaea, Victoria, Eurale, etc.). - Potamogeton type : Stomata are either absent or vestigeal (astomatic leaves), e.g., Potamogeton, Hydrilla,
Najas.
period of stomatal opening
On the basis of periods of opening and closing, stomata are of following types:
- Alfalfa type: Stomata remain open throughout the day, remain closed at night, e.g., beans, pea.
- Barley type : Stomata open only for a few hours in day and remain closed in rest period as in cereals.
- Potato type: Stomata remain open throughout the day and night closing only under the period of water stress, when
rate of transpiration exceeds the rate of absorption. - Equisetum type: Stomata seldom close, usually remain open throughout, as in emergent hydrophytes.
- Succulent xerophytes like Opuntia or Bryophyllum type: Stomata remain closed in day hours to discourage
transpiration but open at night for gaseous exchange and CO 2 fixation.