This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The above descriptions relating to the thickening of stems and the cambium layer apply entirely to Dicotyledons. The structure of a three-year-old stem is represented by fig. 21. Trees and shrubs are less numerous amongst Monocotyledons, whilst herbaceous types in cultivation are very numerous. Structurally they are all much alike, whether herbs, shrubs, or trees, of one or many years' duration. Palms, some species of Pandanus, and a few Bamboos are the only plants of tree-like habit or dimensions in the class. The stem of a Palm may be taken to consider details of structure (fig. 22). There is no pith in the centre, nor bark on the outside. There is a skin on the epidermis, and that is permanent. The body of the tree is made up of short-celled ground tissue, and distributed through this are very numerous strands or bundles of fibro-vascular tissue. They are isolated in the ground tissue, and when the cells, fibres, and vessels of which they are composed have reached their full size, and thickened their walls, they can make no further growth, as there is no cambium. Towards the circumference of the stem the fibro-vascular bundles are the most numerous, and as the cells of the ground tissue in that region thicken their walls greatly, it follows that the outside of the stem of a Palm is very hard. A seedling-remains without a stem till the leaves have attained a large size and a certain number, when the stem rises up almost of the same thickness throughout. As Palms generally do not branch or increase the number of their leaves, the thickening of the stem is unnecessary. Some species of Dracaena thicken their stems slightly by some of the cells of the outside of the ground tissue retaining the power of dividing and forming new tissue like the rest. The stem of a Fern consists of ground tissue, with an interrupted ring of woody tissue in the form of curved plates and isolated pieces (fig. 22). In all these cases strands of fibro-vascular bundles pass from the wood of the stem or branches into the leaves.
The functions of the stems and branches of all the above plants are to support the leaves, so that they may be properly spread out to the light, and to convey water and food to the leaves, flowers, and fruits. As the stems of trees must be strong to bear the weight of branches and leaves, so they develop a much larger proportion of woody tissue than herbaceous plants require to do.