Anatomy of a stem
Summary
Eudicot stems are formed of concentric layers of tissue, with a spongy pith in the centre, used for transport and storage of nutrients. Outside of this lies the vascular tissue, through which water, nutrients and carbohydrates travel throughout the plant. In herbaceous plants, these are arranged like beads on a ring; in woody plants, in a continuous circle. Next is the cortex, which plays a similar role to the pith, followed by the epidermis, the “skin” of the plant. In woody plants, the epidermis is replaced by the periderm as the stem matures; this consists of an external, waterproof layer known as cork, and an inner layer, the phelloderm or secondary cortex. Pores called lenticels take the place of stomata in the periderm, allowing gaseous exchange to take place. Stems grow from meristematic tissue in their apices and their leaf axils; growth from the latter is suppressed whilst the apex is intact, a phenomenon known as apical dominance.
Stems are one of the main large scale structures of a plant (the others being leaves, roots and flowers - see this article). To understand the critical processes that keep plants alive (photosynthesis, respiration, and transpiration), and some of the potential problems plants can face in their environment, we need to understand the anatomy of these structures in a little more detail than we have so far. This is the second of four articles in which we will dig just deep enough into these each of them.
To avoid too much in the way of repetition, we will assume familiarity with some of the material we have already covered in Getting Dirty. In particular, this article covers the main parts of a stem, such as nodes, internodes, the apex, axils, and buds; this article covers the different types of tissues within a plant (including its stem); and this article covers the detailed anatomy of a leaf.
Before we get started, there is one final point to take into consideration. The stems of eudicot and monocot plants (see this article) are fundamentally different in their structure. Most of our discussion will focus on the anatomy of eudicot stems, though we will highlight the most important differences in monocot stems along the way.
Illustrative cross-sections of eudicot and monocot stems
(not to scale)
Image attribution: Duncan Knowles (Public domain)
Cross-section of a stem
If we cut across a (eudicot) stem and examine it under a microscope, we will see tissues arranged in concentric circles. The tissues that are present and in what form depend on whether the plant is herbaceous or woody, and in the case of woody plants, the maturity of the stem.
In a herbaceous plant stem, the centre is filled by a spongy tissue known as “pith” or “medulla” (as some plants grow, this tissue can decay, resulting in hollow stems; such plants are often fast growing, such as Leycesteria formosa, Himalayan honeysuckle). The pith is formed of loosely packed parenchyma cells, and serves to transport nutrients and carbohydrates throughout the plant These travel long distances through the xylem and phloem, but must travel short distances by moving from cell to cell (the “symplast” pathway), and through the spaces between cells (the “apoplast” pathway). The pith also stores nutrients and carbohydrates, thereby buffering their supply for when they are needed by the plant.
Outside the pith lie the vascular tissues, the xylem, the “vascular cambium” (undifferentiated meristematic cells that differentiate into new xylem and phloem cells as the plant grows), then the phloem. These are arranged in “vascular bundles”, which are themselves arranged like beads on a ring around the stem, held within a layer of ground tissue (parenchyma cells). The role of xylem and phloem is discussed in more detail in this article.
Next comes a layer of ground tissue, the “cortex”, followed finally by the epidermis. Like the pith, the primary function of the cortex is to transport and store carbohydrates and nutrients. In some plants, the cortex also contains chloroplasts, allowing a limited amount of photosynthesis to take place in the stem. The epidermis of a stem, much like that of a leaf (this article), consists of a waxy cuticle covering dermal cells, including trichomes and stomata; the latter being essential to allow the stem to exchange gases and water vapour with its environment, and thus to allow respiration to take place.
In a woody plant stem, the vascular bundles are so close together that they form a continuous ring around the stem. As the vascular cambium produces new (“secondary”) xylem cells, the older (“primary”) xylem cells are pushed inwards, compressing the pith and thickening the stem. Similarly, the vascular cambium produces new secondary phloem cells, which push the older, primary phloem cells outwards, thickening the stem.
As a woody plant stem matures, xylem cells become an increasingly large portion of the stem, compressing the pith to a tiny central core. These xylem are the “wood” of the tree. The innermost xylem become compressed and no longer transport water through the plant, and are known as the “heartwood”. Those that are still active are the “sapwood”. In spring time, this thickening takes place more quickly than later in the year. As a result, wood resulting from early season growth (“earlywood” or “springwood”) is usually lighter in colour and thicker in cross-section than wood resulting from late season growth (“latewood”). This is the reason for the formation of tree rings; and for why you can tell the age of a tree by counting its rings.
At the same time, the cell walls of the outermost phloem cells, in reaction to mechanical stresses (air movement causing the stems to bend, and the relentless pressure of new phloem cells developing) begin to thicken. Over time, these cell walls account for the majority of each such phloem cell, and the cell dies. These rigid cells form the inner bark or “bast” of the tree. Some herbaceous plants (such as Linum usitatissimum, flax) also form bast, and several such species are of significant commercial importance.
The outermost cells of a woody plant also undergo transformation as its stems mature. A thin layer of meristematic tissue, the “cork cambium” forms between the epidermis and the cortex. As with the vascular cambium, the cork cambium produces new cells both inwardly and outwardly. Towards the outside of the plant, the cork cambium produces “cork” (also known as “phellem” or “suber”) cells. Towards the inside, “phelloderm” (or “secondary cortex”) cells are produced, which carry out a similar function to the cortex. These three layers (cork, cork cambium and phelloderm), are together known as the “periderm” (or outer bark) of the stem, and replace the epidermis. The periderm, the cortex, and the bast together form the bark of a woody plant.
The cork cells of a woody plant produce a substance called “suberin”, which renders it largely impervious to moisture and gases. This protects the stem from drying out, and offers resistance to other threats, including attack from pathogens, herbivory and in some cases, protection from fire. It also, however, prevents gaseous exchange between the stem and its environment. To address this, pores called “lenticels” form in the cork. Beneath these, the cork cambium produces spongy tissue with large spaces between cells, restoring gaseous exchange.
As a woody plant ages, cells in the thickening cork layer become separated from the inner tissues of the stem. As a result, they lose access to water and nutrients, and as a result, the cells die. These cells form the rough outer layer of bark that we find on the outside of many trees.
Monocot stems
The cross-section of a monocot stem is very different to that of a eudicot stem. Ground tissue within the stem is not arranged into separate cortex or pith layers. Instead, spongy parenchyma cells form around vascular bundles, which are scattered throughout the stem. The majority of the vascular bundles are small and found towards the outside edge of the stem, with smaller numbers of larger vascular bundles being found towards the centre. More compact parenchyma cells form the rest of the ground tissue, becoming increasingly smaller and more densely packed towards the outer edge of the stem.
The epidermis of a monocot stem is formed of a single layer of compactly arranged parenchyma cells, covered by a thick cuticle. Monocot stems do not have trichomes.
Longitudinal section of a stem
New and old growth do not exist in isolation; new growth becomes old growth over time, with young stems becoming mature stems. If we only consider cross-sections of stems, it is hard to see how this transition occurs.
Illustrative longitudinal section of a woody plant stem
(not to scale)
Image attribution: Duncan Knowles (Public domain)
Growth of plant stems occurs almost entirely in areas with meristematic cells. These are undifferentiated cells that are “totipotent”; they have the ability to both divide to reproduce, and to differentiate into specific cell types. Meristematic cells in stems are found in shoot tips, at the axils where leaves join stems (both known as “apical meristems”), and in the vascular and cork cambium (known as “lateral meristems”). Growth from shoot tips is known as “apical” growth (growth from the apex of the stem), that from the axils as “axillary” growth (or informally as “branching” of the stem).
Apical and axillary growth are both considered to be “primary” growth, as they result in the plant as a whole becoming larger. Growth from the vascular and cork cambium results in thickening of the stems (see above) and is considered to be “secondary” growth. Tissues in the apex of a stem produce a chemical (“auxin”) that suppresses axillary growth (to a greater or lesser extent depending on the specific species), resulting in buds forming in the leaf axils but not producing shoots, a phenomenon known as “apical dominance”. Pruning the apex of stems eliminates the production of auxin, allowing axillary growth to proceed; this is why pruning resulting in bushier plants. The new branched shoots themselves have apices, which dominate growth in their axils.
A third type of growth, “adventititious” growth can occur in some plants, and emerges from parenchyma cells close to vascular tissues, or from calluses that form around wounds to the plant. Adventitious growth is the mechanism by which cuttings form roots, so is important in the propagation of plants.
At the apex of the stem, apical meristem tissue specialises into “primary meristem” cells, the precursors to the various parts of the plant (including not only stems, but leaves and flowers). Within the stem, there are three types of primary meristem: the “protoderm”, which will specialise further into dermal tissue (the epidermis and periderm); the “procambium”, which will specialise into vascular tissue (the xylem and phloem); and the “ground meristem”, which will specialise further into the ground tissues of the stem (the pith and cortex).
As the stem grows, new primary meristem cells form, and the older such cells divide and specialise, becoming the tissues seen in the cross-section of the stem. Further division and specialisation in the lateral meristems results in thickening of the stem, together with the development of wood, bast and bark as described above.
In addition to the stem primary meristems, “leaf primordia” (the precursors of leaves) form at the apical meristem, in a pattern and at a timing determined by the specific genetics of the plant. Meristem tissue forms at the base of the leaf primordia, which remains as the stem grows further beyond each such point; this is the axillary meristem, and it is for this reason that growth occurs in the leaf axils.