Vascular tissue is made up of different types of plant cells which transport water and organic and inorganic molecules from where they are manufactured or absorbed in the plant, to where they used or stored. These transport tissues are grouped together to form what are known as the vascular bundles. The arrangement of the vascular tissue in a plant stem differs in dicotyledonous and monocotyledonous plants. We will first look at a typical dicot stem and then we will examine the situation in monocots. An understanding of the arrangement of the vascular bundles in a dicot stem is important if you wish to understand how wood is formed in trees.
A cross-section through a dicotyledonous stem showing the arrangement of the vascular bundles
The xylem differentiates from the part of the vascular bundle nearest the centre of the stem and then progressively towards the cambium in the middle of the bundle. Protoxylem is the xylem which differentiates first and may consist of a different combination of xylem cells when compared to the metaxylem which differentiates later and lies closer to the cambium. The xylem is responsible for transporting water and dissolved nutrients.
The phloem starts differentiating on the side of the vascular bundle orientated towards the outside of the stem and then progressively towards the middle, towards the cambium in the middle of the bundle. As in the xylem, the phloem which first differentiated is known as the protophloem and the metaphloem is the later phloem. The protophloem is often associated with schlerenchyma fibres - the phloem fibres. The phloem transports organic substances through the stem.
Between the xylem and the phloem lies an important layer of cells, the cambium, which are still able to divide i.e. they are meristematic. The cambium which lies inside the vascular bundles is called the fascicular cambium. The fascicular cambium consists of vascular bundles which are united by sections of parenchyma cells. These parenchyma cells also become meristematic, and so form the interfascicular cambium. The fascicular and interfascicular cambium form a continuous cylindrical layer inside the stem. The cells divide to form new xylem cells towards the inside of the stem and new phloem toward the outside of the stem by a process called secondary thickening which involves the formation of wood.
|In a typical
dicotyledonous plant stem the vascular
bundles are arranged in a circle in the middle of the stem surrounding a central pith.
To the outside of the vascular bundles is the cortex which is covered by a single
layer of epidermis cells. The xylem of the vascular bundles is orientated towards the
middle of the stem and the phloem towards the outside.
Tissues making up a typical dicotyledonous vascular bundle are described below.
A cross-section through a vascular bundle of a dicotyledonous plant