Conceptual Overview

The primary growth of stems is derived from the shoot apical meristem. In many cases, particularly grasses, there is an intercalary meristem which is a localized meristematic region at the base of an internode in a stem, or at a leaf base. There may also be a primary thickening meristem (PTM) derived from the apical meristem that results in a primary increase in the thickness of the shoot axis in some monocots, or increases the diameter of the stem prior to the completion of elongation (as in palms). Alternatively, there is a secondary thickening meristem (STM) which produces diffuse secondary growth ù not a true bifacial xylem/phloem cambium. The STM may be continuous with the PTM and results from division and/or enlargement of parenchyma with additional vascular bundles added. In some monocots that have a PTM (e.g. Lilium, Yucca, Dracaena) the stem diameter is increased after stem elongation.

Vascular bundles are said to be collateral if the phloem is on the outer side of the xylem. Such a bundle may also be designated as "open"or "closed" depending on the arrangement of fibers. If there is a cap of fibers adjacent to the vascular tissues, but it does not completely envelop the vascular bundle, it is considered to be "open." In that case, the cambium may later develop between the vascular bundles as interfascicular cambium. This is common in most families of dicotyledons and in gymnosperms. If fibers and other thick-walled cells derived from the pericycle form an encircling ring around the vascular bundle, it is then designated as "closed." This is common in Ranunculus and in some composites as well as monocots. However, in some dicots only, the primary phloem may be found on both the inner and outer sides of the xylem, and this arrangement is called bicollateral. In this case, one area of phloem is adjacent to the cortex, and the other is adjacent to the pith. Phloem centripetal to the xylem is called internal. That centrifugal to the xylem is called external. If the phloem surrounds the xylem, the bundles are said to be amphicribral, and if the xylem surrounds the phloem, the bundles are designated as amphivasal.

Unlike roots which, at their apex, produce no lateral organs and have a rootcap, shoot tips produce lateral organs (leaves, branches and floral structures) in an exogenous manner. The types of tissues produced in primary stems include: epidermis, cortex, phloem, xylem and pith. Except for the epidermis, the function of these tissues is basically the same as the corresponding ones from primary roots.

In monocotyledons, the presence of a cambium has been reported in a few families, but there are no more than a few cells formed from it. In some grasses, the pressure of the enlarging cells of the phloem and xylem toward the center of the bundle compresses the cells there into cambium-like shapes. However, secondary growth may occur in perennial stems of monocotyledons resulting in an increase in diameter by means other than due to a fascicular cambium. A new meristem may differentiate in the pericycle or the inner cortex, resulting in new cortical tissue. New procambium strands differentiate in this new cortex, and new xylem and phloem differentiate as in the primary tissues of the vascular cylinder. A phellogen may differentiate in the new parenchyma, and a new periderm becomes the outer tissue of the stem. Such secondary growth has been observed in Agave, Yucca, Aloe, Cordyline, Dracaena, Allium obliquum, Narcissus and Hyacinthus.

In palm trees, the PTM is mostly independent from the stem tip meristems, and originates by periclinal divisions that occur under the bases of young leaf primordia. As the diameter of the palm stem increases, new vascular bundles differentiate from the new cortical cells. Thus, while palm stems are not all primary tissues as often stated, they do not possess woody secondary growth as do dicots.

Secondary growth in dicots contributes secondary xylem and secondary phloem from a vascular cambium. A consideration of secondary phloem is covered in the unit on "Vascular Tissues: Phloem," and secondary xylem is the topic of the unit on "Wood: Taxonomic Variation."