In this article we will discuss about Stomata and Trichomes founds in the leaves of plants.
Stomata (sing.-stoma) are very minute openings found in the epidermal layer of leaves, stem and other aerial parts of the plant. Each stoma remains surrounded by two kidney-shaped or bean-shaped cells called guard cells. Chloroplasts are always present in the guard cells. The inner wall of each guard cell is thicker while its outer wall is thinner. The epidermal cells bordering the guard cells are called subsidiary cells.
In the absence of light, i.e. at nights, the stomata remain closed. On the other hand in the presence of light, i.e. during daytime the stomata remain open. The closing and opening of stomata are due to the movement of guard cells.
Usually in monocotyledonous leaves, the stomata are arranged in parallel rows whereas in dicotyledonous leaves the stomata remain scattered.
In dicotyledons following four main types (Fig. 178) of stomata are found:
1. Anomocytic or Type-A:
(i) No subsidiary cells are present.
(ii) Stoma remains surrounded by several irregularly arranged ordinary epidermal cells.
(iii) This type of stomata are also called Ranunculaceous type or irregular-celled type.
This type of stomata are found commonly in several dicotyledons, including Tridax, Citrullus, Ranunculus etc.
2. Anisocytic or Type-B:
(i) Stoma remains surrounded by three subsidiary cells (Fig. 178).
(ii) Of the three subsidiary cells, one is distinctly smaller than the other two.
(iii) This type of stomata is also called unequal-celled type or cruciferous type.
This type of stomata are seen commonly in Brassica, Sedum etc.
3. Paracytic or Type-C:
(i) One or more subsidiary cells are present.
(ii) Subsidiary cells flank the stoma parallel with the long axis of the guard cells (Fig. 178).
(iii) This type is also called parallel-celled type or Rubiaceous type.
This type is commonly seen in members of Rubiaceae, Vigna, etc.
4. Diacytic or Type-D:
(i) Stoma remains surrounded by a pair of subsidiary cells (Fig. 178).
(ii) Common walls of subsidiary cells remain at right angles to the long axis of the guard cells.
(iii) This type is also called cross-celled type or caryophyllaceous type.
This type is commonly seen in Ocimum, Caryophyllaceae (e.g., Dianthus), etc.
Some of the epidermal cells of axis, and other plant parts grow out in the form of trichomes or hair.
They show following characteristics:
1. These are the elongations or extensions of epidermal cells.
2. They are found singly or in groups.
3. Trichomes may be unicellular or multicellular. Morphologically, root hair are typical hair.
4. They are highly variable in shape varying from small protuberances of epidermal cells to very complex, multicellular, branched or stellate structures.
5. Hair may consist of either living or dead cells.
6. Hair with living cells contain very little cytoplas in their cells.
7. They persist throughout the life of a plant part or may fall very soon.
8. Innumerable types of the hair are known, of which some are uniseriate hair, stinging hair, mucilage hair, etc.
9. Functions of trichomes include:
(i) control of transpiration rate,
(ii) reduction of heating effect of sunlight, and
(iii) protection of body.