Stems and Root transport systems  - an overview

                       Stem and root transport systems  - an overview                       




     A brief overview of  stem structure    



 Because of their association with leaves, stems are much more complex structurally than are the corresponding roots of the same plant. Unlike roots, shoots are composed of nodes and internodes .One or more than one leaf may be attached to nodes. The stem assumes many forms and sizes and may be of variable length as well. Nodal anatomy is more complex than that of the internodal regions, as here, one or more strands of vascular tissue diverge from the axis of the stem, extending and connecting to the vascular system within the leaf. The point of emergence from the stem, results in the formation of one or more leaf gaps. Stems have several important functions. Firstly, support. They have to support the total aerial mass of the plant ensuring exposure to sunlight and ensure that the plant remains competitive within its ecological niche.


The vascular bundles can take up a variety of arrangements. In dicotyledons they usually occupy one ring, just to the inner side of the cortex. This is described as being eustelic.  In monocotyledons they may form one ring, or may appear to be scattered in several to many rings, or lie without apparent order in the central ground tissue. This is described as being atactostelic. The possession of several rings of vascular bundles is not the prerogative of monocotyledons. Several families of climbers amongst the dicotyledons have several rings of vascular tissue. When vascular bundles are not scattered, the centre of the young stem is usually parenchymatous, and may become lignified in mature stems.


    stelar type    



More information on the stelar types that you will encounter in these exercises, can be accessed by following this link.


Fig 1. Some mechanical systems in dicotyledons. A schematic plant with position of sections indicated. Liquid pressure occurs in turgid cells through the plant. Collenchyma is often conspicuous in actively extending regions and petioles. Sclerenchyma fibres are most abundant in parts that have ceased main extension growth. Xylem elements with thick walls have some mechanical function in young plants and give a great deal of support in most secondarily thickened plants.


The cross-section of a primary stem may have a more or less angular to circular outline. However, it can take on one of a wide range of forms, some of which assist in the identification of a family, as in Labiateae, where the section is square or may help to distinguish genera for example, in the sedges,  Carex species have stems with a triangular section. Often the outline is modified near to nodes or in regions of leaf insertion. Sometimes a wing or ridge of tissue in line with a petiole may continue down the internode as in, for example, Lathyrus. In general, the outline of the section taken in the middle of an internode would be described for comparative purposes.


Many stems have all or most of the following tissues, working from the outside inwards: epidermis, hypodermis, cortex (with both collenchyma and chlorenchyma, or either), an endodermoid layer (or a well-defined starch sheath), vascular bundles in one or more rings, or apparently scattered, and a central ground tissue or pith. Sometimes a pericycle can be distinguished, but this is normally regarded as part of the phloem. A true endodermis with Casparian strips is rarely present.


Fig. 2. Vascular bundle types from stems. A, Cucurbita pepo, diagram of bicollateral bundle, x 15. B, Piper nigrum,  diagram of collateral bundle; cambium remains fascicular, x 15. C, Chondropetalum marlothii, detailed drawing of collateral bundle, lacking cambium, x 110. D, Juncus acutus detailed drawing of amphivasal bundle, x 220. c, cambium; scl, sclerenchyma.