Roots, together with stems and leaves, constitute the major organs of vascular plants. The main functions of the root system are to anchor and support the plant; to seek out, absorb and transport water and ions from the soil; and to transport and store the products of photosynthesis from the shoot system. Each root can be arbitrarily subdivided into several zones each with different functions: the apical zone of cell division, the source of all cells in the root; zone of cell elongation; zone of cell differentiation; zone of cell maturation. A root system is achieved by branchng of the primary root to produce one or more orders of lateral roots. Root hairs are epidermal enations that are functional in water and ion uptake and are an entry site for nitrogen‐fixing bacteria.

Key Concepts:

• Roots, stems and leaves are the major organs of vascular plants.
  
• Total extent of the root system usually greatly exceeds that of the shoot system.
  
• The root apical meristem contains self‐perpetuating cells, now called stem cells, whose mitotic products differentiate into the cell types of the root.
  
• Root tissues are arranged in concentric cylinders from the exterior inward are epidermis, cortex, endodermis, pericycle and vascular tissue consisting of alternating of bands of phloem and xylem.
  
• Lateral roots are initiated in acropetal sequence from the pericycle tissue.
  
• Adventitious roots are initiated in response to wounding or other environmental stimuli and disrupt the sequence of lateral root initiation.
  
• Tropistic responses of roots to environmental stimuli such as gravitropism, phototropism, thigmotropism and hydrotropism are regulated by internally produced hormones such as auxin, ethylene and abscisic acid.
  
• Root–microbial interactions result in formation of nitrogen‐fixing root nodules or mycorrhizal associations that are important in phosphorous uptake.