Although all parenchyma cells in a plant have the potential for growth and production of new cells, unless a plant has been wounded, cell production normally occurs in meristem-atic tissue, which consists of parenchyma cells that remain capable of dividing and producing daughter cells throughout the life of the plant. Because of these meristems, vegetative growth in plants is indeterminate, which means that the plant body is not fixed in its development but is potentially capable of continuous growth. There are two basic types of meristems, apical meristems and lateral ones, and one more specialized type, an intercalary meristem. Apical meristems are responsible for growth in length (height), or primary growth, and lateral meristems or cambia (sing. cambium) for growth in diameter (width) (see section "Secondary Development").
As the name implies, apical meristems occur at the apex of every stem and twig and the apex of every root in a plant. The cells that make up the apical meristem undergo repeated divisions to produce daughter cells; a short distance back from the growing tip, the daughter cells begin to differentiate into xylem, phloem, sclerenchyma, etc. In stems, the apical meristem produces cells and tissues that form the stem, leaves, and axillary buds, or lateral branches. In roots, the apical meristem produces only the cells that make up the root; laterals are produced further back from the meristem itself. Vascular tissue first appears as procambial strands, which will develop into mature xylem and phloem. The first xylem to mature is termed protoxylem and, as noted earlier, usually consists of extensible tracheary elements with helical or annular secondary wall thickenings (FIG. 7.14). Although protoxylem cells are usually smaller in diameter than metaxylem cells, the nature of the wall thickenings is a more exacting way to distinguish these cells in fossil plants. Further back from the meristem, after the stem has for the most part ceased to elongate, the metaxylem matures. It generally consists of non-extensible tracheary elements, such as pitted or scalariform (FIG. 7.7) tracheids. Both proto- and metaxylem are primary xylem, that is, they are produced by the apical meristem. Similar terms are used for phloem cells, that is, protophloem and metaphloem.
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