Govt Exams
The dorsiventral structure of dicot leaves is primarily established by differential light exposure (adaxial surface receives more direct light, promoting compact palisade development) combined with auxin gradients that influence cell differentiation patterns. The abaxial surface, receiving diffuse light and lower auxin concentration, develops the loosely packed spongy mesophyll with intercellular spaces for gas exchange.
Sieve plate perforations with reduced cell wall material allow efficient translocation of assimilates between sieve tube elements with minimal resistance.
Xylem fibers, which constitute a significant portion of secondary xylem, have thick lignified walls that provide tensile strength to the wood.
Vessel elements in angiosperms have perforations (lacking end walls) allowing efficient water transport, while tracheids with bordered pits provide both transport and mechanical support.
Xerophytes like Nerium have reduced intercellular spaces (not large) to minimize water loss. They have sunken stomata, thick cuticle, and multiple palisade layers.
As the root grows and matures, protophloem sieve tubes collapse and are obliterated, replaced functionally by metaphloem which lies closer to the periphery.
Bulliform cells are large, colorless epidermal cells that absorb water and swell, causing the leaf to unfold; they lose water and collapse during drought, causing leaf rolling to reduce transpiration.
Root hairs develop from the root hair zone of epidermis, not from pericycle. The pericycle produces lateral roots, vascular cambium, and in some species, cork cambium.
According to the histogen theory, the calyptrogen (root cap mother region) produces the root cap, while dermatogen produces epidermis, periblem produces cortex, and plerome produces stele.
The stele consists of pericycle, vascular tissues, and endodermis. The cortex lies outside the endodermis and is not part of the stele.