Plant growth occurs when one cell divides into two, differentiating into stems, leaves, flowers and roots. Although you observe plant growth as a tree growing taller or flowers unfolding from their buds, each cell is its own center of activity, minutely contributing to the growth and life cycle of the plant.
Plant cells contain several features that distinguish them from other cells. Plant cells contain a cell wall, a rigid membrane that provides shape for the cell and controls the movement of substances in and out of the cell. Plant cells also contain chloroplasts for manufacturing sugar from sunlight and a vacuole for storing compounds. All cells have a nucleus, which contains chromosomes holding the plant's genetic information. During the plant life cycle, genetic material and many of these structures must be duplicated and divided between two cells.
According to retired biology professor John W. Kimball, before a cell can undertake division, it must duplicate its genetic material so that each cell from the resulting division receives a full set of genes. As genetic material duplicates, the cell also checks it for errors. When the genes are successfully duplicated, the cell is ready to begin dividing.
Once the cell has accumulated enough genetic material to supply two cells, as well as chloroplasts and other cellular structures for both cells, it undergoes a process called mitosis, during which a single cell divides into two. During the first phase, a band of fibers crosses the cell, marking the location of the further division. As division progresses, the chromosomes in the nucleus are pulled apart, and each cell receives a full set of genetic material. As division concludes, new nuclei form around the chromosomes, and cell walls develop, dividing the two cells.
Differentiation refers to the process by which cells acquire the ability to carry out a certain function. In plants, differentiation turns cells into flower buds, roots and leaves. One such role plant cells undertake is reproduction, becoming sperm cells or egg cells through a process called meiosis. Similar to mitosis, during meiosis, one cell divides into two. However, cells end up with only half of the normal number of chromosomes. When egg and sperm cells fuse, the two halves form a full set of genes.
When plant cells begin to age, they enter a stage called senescence, during which the structures inside the cell become less numerous and begin breaking down. Eventually, senescence leads to death of the plant cell, an event called apoptosis. Larry D. Noodén, in his book "Plant Cell Death Processes," credits cellular death with allowing for differentiation and protection from pathogens that, otherwise, could harm plants. Because plant cells are bound with a rigid cell wall, they are not reabsorbed after death, which occurs in animal cells. Perhaps the best example of apoptosis in plants occurs in the xylem, vessels that conduct water from the roots to the tips of the leaves. As the plant matures, xylem cells undergo apoptosis, and the dead cells form rigid, woody tubes.