Soil Biology and EcologyUnit 2.3 | 15
Students’ Lecture Outline
b) Root form
i. Fibrous roots
• Most monocots (e.g., grasses)
• Primary root replaced by series of adventitious roots
ii. Tap roots
• Most monocots and gymnosperms
• Tap root persists and forms many lateral branches
• Generally deeper than fibrous roots
iii. Root depth
• Species specific, influenced by environmentalconditions
c) Root structure
i. Root cap
• Live cells produced by meristem
• Protects root, like a bud scale
• Constantly replaced (5–6 day turn over)
• Responds to gravity
ii. Meristematic zone: 2 mm zone where most cell division happens
iii. zone of elongation: Rapid growth, cells from meristem
iv. Mucilage
• Covers root from tip to beginning of root hair zone
• Source is part microbial, part root cap
• Possible functions: Nutrient uptake, protection, prevent drying, fill spaces between
root and soil, food for microbes
v. Root hair (differentiation) zone
• Root hairs have life span of days to weeks
• Do not become large structural roots; play a nutrient absorption role
• Are an outgrowth of epidermal cell
• Rye plants can produce over 100 million per day
• None in redwood trees, firs, some pines
• Food sources that support rhizosphere microbes, contribute significant amounts
of soil organic matter
vi. Lateral roots
• Originate from the vascular bundle inside cortex
• Cortex and epidermis are ruptured by new lateral root
• Bacteria colonize these emergence sites
d) Root nutrition
i. Maximum nutrient uptake occurs behind meristem (elongation and root hair zones)
ii. water and nutrients are withdrawn from narrow band around roots
iii. Replenished from surrounding soil by mass flow (the movement of nutrients with the
overall flow of water to plant roots)
iv. If mass flow is slow, depletion zone created, resulting in lack of some nutrients
e) If uptake is slow certain ions may accumulate—all ions in solution move towards root
during mass flow; those not used by plant will accumulate around root