Entrance Exams
Govt. Exams
Protoxylem forms first during primary development with narrow vessels and is often crushed, while metaxylem forms later with wider vessels and persists.
Companion cells are closely associated with sieve tube elements and help in loading and unloading of sucrose and control of sieve tube functioning.
Periderm (cork) is produced by cork cambium and replaces the epidermis in older roots and stems, providing protection.
Vascular cambium produces secondary xylem inward and secondary phloem outward, causing increase in diameter (secondary growth) of dicot stems.
Monocot stems have scattered vascular bundles distributed throughout the ground tissue (atactostele), unlike dicots where they form a ring.
Casparian strips are band-like thickening of suberin and lignin on radial and transverse walls of endodermis that regulate water and mineral movement into the stele.
Secondary xylem in dicots shows growth rings (annual rings) due to variations in cell size and wall thickness between spring wood and autumn wood formed in different seasons.
Phloem fibers (bast fibers) provide mechanical support to vascular bundles and stems while permitting the translocation function of sieve tubes and companion cells.
Palisade mesophyll consists of tightly packed columnar cells with numerous chloroplasts oriented toward light, maximizing photosynthetic efficiency.
Stomatal pits create a humid chamber that reduces the vapor pressure gradient between intercellular spaces and external environment, thereby minimizing transpirational water loss.