Stem growth

Growth of stems is initiated in the apical, or terminal, bud at the end of the stem (the apex). Deep inside the apical bud is a tiny mass of small, delicate jelly-like cells, each with a conspicuous nucleus but no cell vacuole. This mass is the apical meristem (see Figure 6.6). Here, cells divide frequently to produce four kinds of meristematic tissues. The first, at the very tip, continues as meristem cells. The second (protoderm) near the outside develops into the epidermis. The third (procambium) becomes the vascular bundles. The fourth (ground meristem) turns into the parenchyma, collenchyma, and sclerenchyma tissues of the cortex and pith. In addition to its role in tissue formation, the apical meristem also gives rise to small leaves (bud scales) that collectively protect the meristem. These scales and the meristem together form the bud. It should be noted that any damage to the sensitive meristem region by aphids, fungi, bacteria or herbicides would result in distorted growth. A fairly common example of such a distortion is fasciation, a condition that resembles a number of stems fused together. Buds located lower down the stem in the angle of the leaf (the axil) are called axillary buds; they contain a lateral meristem and often give rise to side branches.
In some plant families, e.g. the Graminae, the meristem remains at the base of the leaves, which are therefore protected against some herbicides, e.g. 2,4-D. This also means that grasses re-grow from their base after animals have grazed them. The new blades of grass grow from meristems between the old leaf and the stem. This means grasses can be mown which enables us to create lawns. The process of cutting back the grass also leads to it sending up several shoots instead of just one. This process of tillering helps thicken up the turf sward to make it such a useful surface for sport, as well as decoration. Mowing kills the dicotyledonous plants that have their stems cut off at the base and lose their meristems. However, many species are successful lawn weeds by growing in prostrate form; the foreshortened stem (very short internodes) creates a rosette of leaves that helps to conserve water, shades out surrounding plants and the growing point stays below the cutting height of the mower.

Elongation of the plant stem takes place in two stages. Firstly, cell division, described above, contributes a little. The second phase is cell expansion, which occurs at the base of the meristem. Here, the tiny unspecialized mer-istem cells begin to take in water and nutrients to form a cell vacuole. As a result, each cell elongates, and the stem rapidly grows. In the expansion zone, other developments begin to occur.

Cell differentiation
Most importantly, the cells begin to create their cell walls, and the connections between cells (plasmodesmata). The exact shape and chemical composition of the wall is different for each type of tissue cell, since it has a particular function to perform as described earlier; sclerenchyma and collenchyma cells have walls thickened with lignin and cellulose while xylem and phloem vessels have developed walls and structures for transport. Leaf tissues similarly develop from parenchyma cells and form specialized tissues to carry out the process of photosynthesis.