During the initial growth of leaf primordia, cells rely on diffusion to obtain the resources needed to divide and expand. As the young leaves increase in size, their demand for water and carbohydrates quickly exceeds what diffusion can supply. If they are to grow to even a fraction of their full size, leaves must establish vascular connections with the xylem and phloem in the stem. As we have seen in Chapter 29, for leaves to carry out photosynthesis requires both a supply of water through the xylem and the ability to export carbohydrates to the rest of the plant through the phloem.

When a developing leaf is still very small, files of cells within the leaf begin to elongate, forming discrete strands of procambial cells that extend from near the tip of the leaf to the mature vascular tissues within the stem. Procambial cells ultimately give rise to both xylem and phloem within the leaf veins and the vascular bundles in the stalk attaching the leaf blade to the stem. The common origin of xylem and phloem from procambial cells explains why these two tissues are always found in close association throughout the length of the plant.

The vascular system of an elongating stem is built from the successive addition of vascular bundles that connect each developing leaf to the vascular tissues already present within the stem. Spore-dispersing vascular plants typically have one or several vascular bundles in the center of their stems. In seed plants, the most common configuration is a ring of vascular bundles near the outside of the stem. The bundles have xylem conduits toward the center of the stem and phloem nearer the outside of the stem (Fig. 31.8). Typically, each leaf develops vascular connections with several of the stem’s vascular bundles. The region between the epidermis and the vascular bundles is the cortex, and the region inside the ring of vascular bundles is the pith. The placement of the vascular bundles, which contain the stiff xylem conduits, near the outside of the stem increases the stem’s ability to support its leaves without bending over.