Unlike animals, in which cell division happens throughout the body, growth in plants is restricted to regions called meristems. Meristem cells are small, thin-walled and undifferentiated, with large nuclei, dense cytoplasm and no large vacuole, and they divide repeatedly by mitosis.Apical meristems at the tips of shoots and roots produce primary growth: an increase in length. Lateral meristems, such as the vascular cambium in the stems of woody plants, produce secondary growth: an increase in width (girth), adding new xylem and phloem each year. In grasses, meristems at the base of the leaves (intercalary meristems) allow leaves to regrow after grazing or mowing.Behind a root tip, three zones can be seen:
• zone of elongation: cells take up water by osmosis into their vacuoles, which enlarge, and the cell walls loosen so the cells expand lengthways, pushing the tip through the soil;
• zone of differentiation: cells specialise, for example into xylem, phloem and root hair cells.Most of the increase in length comes from cell elongation, not from division itself. A plant cell can increase its length many times over as its vacuole fills. The hormone auxin, made in shoot tips, stimulates this elongation in shoots.Because meristems stay active throughout a plant's life, many plants show unlimited growth and can replace parts that are lost.
• zone of elongation: cells take up water by osmosis into their vacuoles, which enlarge, and the cell walls loosen so the cells expand lengthways, pushing the tip through the soil;
• zone of differentiation: cells specialise, for example into xylem, phloem and root hair cells.Most of the increase in length comes from cell elongation, not from division itself. A plant cell can increase its length many times over as its vacuole fills. The hormone auxin, made in shoot tips, stimulates this elongation in shoots.Because meristems stay active throughout a plant's life, many plants show unlimited growth and can replace parts that are lost.