צמיחה ראשונית ומשנית בשורשים ונצרים

The shoot apical meristem produces three primary meristems: the protoderm, ground meristem, and procambium. The protoderm generates the epidermis, a dermal tissue. The ground meristem forms ground tissues including pith and cortex. The procambium produces vascular tissue: primary phloem and primary xylem. These same primary meristems and tissues are produced in roots, though root ground tissue lacks pith.

Shoot primary growth occurs in the shoot apical meristem, where undifferentiated cells divide and differentiate into primary meristems. Root primary growth occurs across three overlapping zones: the zone of cell division containing the root apical meristem, the zone of elongation where cells lengthen, and the zone of differentiation where cells specialize. This zonal organization allows roots to extend and differentiate simultaneously.

Apical dominance is the inhibition of axillary buds located near active apical meristems by hormonal signals. This prevents lateral shoots from growing while the main stem extends. Damage or removal of the apical meristem disrupts apical dominance, allowing axillary buds to develop into branches. This principle is also how pruning encourages bushier plant growth.

Lateral shoots originate from axillary bud meristems on the plant surface and grow outward. Lateral roots, by contrast, originate in the pericycle located near the center of the root and disrupt outer tissues as they emerge. This internal origin of lateral roots reflects the different structural organization of roots compared to shoots.

During secondary growth, the vascular cambium creates secondary xylem, or wood, and secondary phloem, thickening the plant. The cork cambium originates from the pericycle in roots and the cortex in shoots, producing cork cells and phelloderm to form the periderm. Together, these lateral meristems enable woody plants to increase in diameter and develop protective bark.

Although root and shoot apical meristems produce identical primary meristems that generate the same tissue types, specific tissues differ between them. Root epidermis contains root hairs accounting for most surface area, while shoot epidermis does not. Root ground tissue consists mainly of cortex and lacks pith, unlike shoot ground tissue in eudicots. The shoot apical meristem also contains leaf primordia that form leaves.

Primary and secondary growth can occur simultaneously in a plant. Primary growth occurs in newer plant regions, lengthening roots and shoots through cell division and differentiation. Secondary growth transpires in regions that have completed primary growth, thickening roots and shoots through lateral meristem activity. This allows plants to continuously extend while older tissues strengthen.