Plant growth is initiated at regions called meristems – undifferentiated cells capable of indeterminate divisions
- Meristems are equivalent to embyronic stem cells in animals, but are retained throughout the adult life of the plant
- This allows plants to regrow structures and even reproduce asexually (vegetative propagation)
All the differentiated tissues in a plant are derived from meristems – either apical or lateral meristems
- Apical meristems give rise to the primary tissues needed to increase a plant’s length and grow new leaves and fruits
- Lateral meristems give rise to the secondary tissues needed to support an increase in the plant’s width (e.g. bark)
Meristem Tissue Differentiation
Secondary (Lateral) Growth
The thickening of a plant’s stem (secondary growth) is controlled by the cambium (where lateral meristems are found)
- Vascular cambium cells give rise to secondary xylem and phloem, which facilitate water and nutrient transport in the plant
- Relative to the cambium, secondary xylem cells are formed internally and secondary phloem cells are formed externally
- Cork cambium cells produce a progressively thickening layer of cork, which contributes to the bark of a plant
- These cells add girth to the plant stem, resulting in the thickening of the trunk (lateral growth)
Representation of Secondary Growth
Growth Rings
The rate of secondary growth in a plant will change throughout the year according to the seasons
- Growth rates will slow in winter when there is less light available for photosynthesis
This results in discrete growth rings occurring within the plant stem, which are visible when the trunk is cut in cross-section
- Each ring typically marks the passage of one year in the life of the tree
- Growth rings can be counted to estimate the age of the plant (dendrochronology)
Cross-Section of a Plant Stem Showing Lateral Growth
