Vascular system evolution
Vascularization in higher plants is dependent upon the need for supply of water, minerals and other nutrient via the xylem conduits, as well as the need to provide an efficient pathway through which assimilated carbohydrates may be transported. In primary stems as well as in leaves, these tissues always occur on the same radius in vascular bundles whereas they do not follow this pattern in the root. The micrograph to the left, shows part of the vascular system of the cucurbit, Acanthosicyos horridus. The image shows several files of short and wide reticulately pitted xylem vessels. External phloem (left) and internal phloem (right) occur on either side of the xylem. The large-diameter xylem vessels suggest a trade-off in xylem safety, leading to greater probability of xylem cavitation yet Acanthosicyos is an extreme xerophyte, living on dunes in the Namib desert. Perhaps its survival depends on a long taproot system. This practical focuses attention on some of the rage in form and distribution of vascular supply in the plant.