Plant Tissues and Organs

Plants are composed of three main tissue systems: the dermal tissue system, the ground tissue system, and the vascular tissue system. The dermal tissue forms the outer protective covering. The ground tissue makes up the bulk of the plant, involved in photosynthesis, storage, and support. The vascular tissue is responsible for long-distance transport of water, minerals, and sugars.

Roots anchor the plant, absorb water and dissolved minerals from the soil, and often store food. Key structures include the root cap (protects the growing tip), meristematic zone (cell division), zone of elongation (cells grow longer), and zone of differentiation (cells specialise). Internally, a dicot root typically shows an epidermis, cortex (for storage), endodermis (regulates water movement), and a central vascular cylinder (stele) containing xylem and phloem.

Stems provide support for leaves and flowers, transport water and nutrients between roots and leaves, and can store food. A typical dicot stem cross-section reveals an epidermis, cortex, vascular bundles arranged in a ring (containing xylem and phloem, often with cambium), and a central pith (for storage). Monocot stems have scattered vascular bundles.

Leaves are the primary sites of photosynthesis. Their structure is adapted for this function: a waxy cuticle reduces water loss; the upper and lower epidermis provide protection; palisade mesophyll cells (densely packed, rich in chloroplasts) are the main site of photosynthesis; spongy mesophyll cells (loosely packed with air spaces) facilitate gas exchange; vascular bundles (veins) transport water and sugars; and stomata (pores, flanked by guard cells) regulate gas exchange and transpiration.

Xylem is a vascular tissue responsible for the unidirectional transport of water and dissolved minerals from the roots to the rest of the plant. It also provides structural support. Xylem is composed of several cell types: tracheids and vessel elements (dead, hollow cells forming continuous tubes), xylem parenchyma (living cells for storage), and xylem fibres (for support).

Phloem is a vascular tissue responsible for the bidirectional transport (translocation) of sugars (produced during photosynthesis) from leaves to other parts of the plant where they are needed for growth or storage. Phloem is composed of sieve tube elements (living cells, but lacking a nucleus, forming tubes), companion cells (living cells that support sieve tube elements), phloem parenchyma (for storage), and phloem fibres (for support).

Stomata are small pores, typically found on the lower epidermis of leaves, that regulate the exchange of gases (carbon dioxide, oxygen, water vapour) between the plant and the atmosphere. Each stoma is flanked by two guard cells. The opening and closing of stomata are controlled by changes in the turgor pressure of the guard cells, which is influenced by factors like light intensity, carbon dioxide concentration, and water availability.

Transpiration is the loss of water vapour from the aerial parts of a plant, primarily through the stomata. This process creates a 'transpirational pull' or 'cohesion-tension' force that draws water up from the roots through the xylem. Factors affecting the rate of transpiration include temperature (higher temperature increases rate), humidity (lower humidity increases rate), wind (increases rate by removing humid air), and light intensity (increases rate by opening stomata).