Plants absorb nutrients and water through their roots, but photosynthesis — the process by which plants create their fuel — occurs in the leaves. Therefore, plants need to get fluids and nutrients from the ground up through their stems to their parts that are above ground level.
Just as animals, plants also contain vascular tissues (xylem), which transports water and minerals up from the roots to the leaves, and phloem, which transports sugar molecules, amino acids, and hormones both up and down through the plant.
The leaves of plants also contain veins, through which nutrients and hormones travel to reach the cells throughout the leaf. Veins are easy to see some leaves (a maple tree, for instance). In some plants the veins are hard to see, but they're in there.
Sap is the mix of water and minerals that move through the xylem. Carbohydrates move through the phloem. There are several different "modes of transportation" through the xylem and phloem; their main function is to keep all cells of the plant hydrated and nourished.
Inside the cells of the root, there is a higher concentration of minerals than there is in the soil surrounding the plant. This creates root pressure, which forces water up out of the root through the xylem as more water and minerals are "pulled" into the root from the soil. This force results in guttation, which is the formation of tiny droplets on the ends of leaves or grass early in the morning.
The reason the droplets are seen in the morning is because transpiration — the loss of water from leaves — doesn't occur at night, so the pressure builds until morning. Those droplets are not just water, they're sap. And, those sap droplets are proof that water and minerals get pulled up from the soil and transported through the entire plant.
Guttation may work well for small plants, but gravity works against the upward movement through larger plants, so more active processes are involved.
