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.
The scattered distribution of vascular bundles in monocots (polystelic arrangement) means that damage to one bundle doesn't severely compromise the entire plant's transport system, unlike the concentric arrangement in dicots where the vascular cylinder is a continuous ring. This provides functional redundancy and allows monocots like grasses to survive grazing and mechanical damage.
Lenticels are porous structures in the periderm (cork) that allow gaseous exchange (respiration) to occur in woody stems where stomata are absent.
Root hairs are unicellular extensions of root epidermis that greatly increase absorptive surface area for water and mineral uptake.
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.
Mesophytes have higher stomatal density as water is readily available, while xerophytes have lower stomatal density as an adaptation to reduce water loss.
The pericycle is the outermost layer of the stele (inside endodermis) and gives rise to lateral roots and the vascular cambium during secondary growth.
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.