The shoot grows in length and gives raise to the lateral organs (branches, leaves, flowers, fruits and seeds). At the same time, it also grows in thickness. Getting thicker may be accomplished with primary and secondary growth. Stems undergoing primary growth are those where the increase in length and thickness depend on the apical shoot meristm and intercalar meristem activities. Stems showing secondary growth increase in diameter by the activity of the vascular cambium and cork cambium. All plant species perform primary growth. Most monocots only show primary growth, whereas many dicots and all gymnosperms have primary and secondary growth.
The activity of the apical shoot meristem stops not far away from the shoot tip, and the growth in thickness is by increasing the cell size. Palms and other monocots may have very thick stems by adding new vascular bundles in distant regions from the stem apical tip, and by increasing the number of parenchyma cells. This type of growing is referred as anomalous growth. It is a consequence of the secondary grow meristem, which is found more superficial than the vascular bundles. It gives parenchyma cells outward and parenchyma cells and vascular bundles inward. It is actually a non well-delimited region found in the periphery of the stem containing strands of provascular tissue that gives rise to vascular bundles and leaf rays.
Normally, a stem undergoing primary growth shows the following tissues:
Epidermis. It is usually a single cell thick layer covering the stem. Epidermal cells show cutin and suberin in the free cell wall. It is common to have stomata and trichomes, although in lower number than in the leaves.
Cortex. It is usally a thick layer of parenchyma cells found below the epidermis. Cells can do photosynthesis or store substances. Normally, the outer region of the cortex contains support tissue like sclerenchyma or collenchyma, as in monocots. Occasionally, sclereids, glandular cells, and laticifers can be found in the cortex of some species.
In the cortex, the closest region to the epidermis is known as hypodermis, that usually has supporting functions. The parenchyma region near the vascular bundles sometimes forms a layer known as endodermis, that shows different features compared with other regions of the cortex. In some non-angiosperm species, a Casparian stripe is also observed in the endodermis. Endodermis cells has been thought to be involved in the positive gravitropism of the stem thanks to the amyloplasts they content. That is why this layer of cells is also called layer of starch. They have also been related with phototropism, the stimulation of the vascular cambium activity and the elongation of the vascular cells. If a well-developed endodermis is present, there use to be an inner pericycle. Endodermis and pericycle are frequent in aquatic plants.
Vascular bundles. Shoots with primary growth have vascular bundles showing different organizations. The primary vascular tissues, primary xylem and primary phloem, form collateral bundles where the primary phloem is inner and the primary xylem localizes more externally. In a typical monocot plant, vascular bundles are scattered through the parenchyma tissue, organization known as atactosele. In typical dicots and gymnosperms, vascular bundles are lined up in a circle, leaving interfascicular parenchyma between each other. This vascular organization is known as eustele. Only some dicots and conifers form a continuous circle (that is like a cylinder) of vascular bundles during primary growth. In these species, xylem is inward and phloem is outward.
The strength of the monocot stems relies on the sclerenchyma fibers found in the vascular bundles and near the epidermis.
The organizations of stem and root vascular bundles are different (Figure 1). In the root, xylem and phloem alternates, and metaphloem is inner to protophloem. In the shoot, phloem is outer to xylem, and protoxylem is inner to metaxylem. That is, phloem and xylem change positions, and xylem shows a twist. However, root and shoot vascular bundles have to be connected. It happens at the transition region between the root and the shoot.
In primary shoots, and those starting secondary growth, there are vascular bundles that are directed to the leaves, entering through the petiole. They are known as foliar traces (Figure 2) or leaf traces. In this way, a continuity is established between the central shoot vascular bundles and those irrigating the leaves, so that leaves are able to get water and minerals, and evacuate photosynthesized products. Foliar traces are made up of protophloem and protoxylem. One leaf may be connected to one or several leaf traces. For example, the leaves of many dicots are connected through 3 to 5 foliar traces.
In species with siphonostele, ectophloic and amphiphloic, in ferns (see figure) , the vascular cylinder is interrupted above the leaf trace. This gap is known as leaf gap. At this point, the parenchyma of cortex and medulla are continuous. If many leaves are in the same shoot, there are many leaf gaps and then the vascular organization is known as dictiostele.
In eusteles (seed plants under primary growth) there is no leaf gaps because the vascular bundles branch a leaf trace, and the main vascular bundle is not interrupted. Leaf traces that end up in the same leaf may come from the same vascular bundle (they are known as open systems, typically in helical arranged leaves). In other species, several leaf traces from different vascular bundles join to form the final leaf trace that enters the leaf.
In atactostele of monocot plants, leaves are usually fed by many leaf traces. Leaf traces branche off from vascular bundles, and only a few species show interrupted vascular bundles. The vascular bundles in monocots do not run completely parallel along the stem, but they bend toward the surface and then get inner again, in a helical way. They send the leaf trace when they are closer to the stem surface .
In plants showing secondary growth, secondary vascular tissue is absent in places where there was a leaf trace. These interruptions are known as leaf gaps. After some time of growing, they are not visible, or can be only seen in the inner part of the stem. Once secondary growth is started, no new leaf traces will be formed because no leaves are generated in secondary stems.
Medullary region. It is the inner part of the stem. It can be empty (without tissues) or made up of parenchyma tissue.
Tasaka M, Kato T, Fukaki H. 1999. The endodermis and shoot gravitropism. Trends in plant sciences. 4: 103-107