Growth, increase in the size of an organism, reflecting an increase in the number of its cells, an increase in its protoplasmic material, or both. Cell number and protoplasmic content do not always increase together. Cell division can occur without any increase in protoplasm, thus giving a larger number of smaller cells. Alternatively, protoplasm can be synthesized with no cell division so that the cells become larger. Any increase in the protoplasm requires the synthesis of cell components (such as nuclei, mitochondria, thousands of enzymes) and cell membrane. These, in turn, require the synthesis of macromolecules such as proteins, nucleic acids, and polysaccharides from amino acids, sugars, and fatty acids. These subunits must be synthesized from still simpler substances.
One of the major differences between plants and animals is that the final shape and size of an animal can be predicted within fine limits, whereas it is much more difficult to say just how tall a plant will be or how many branches it will have. Growth in plants is, however, better understood than animal growth.