seeds pass through the animal GI tract or that get buried), and mechanical disbursement (by expelling the seed).
SOIL UTILIZATION AND PLANT NUTRITION Most plants get their mineral nutrition via the roots in soil. The main minerals, like calcium and potassium, are dissolved in water. Interestingly, less than 1 percent of the water that reaches the leaves participates in photosynthesis and plant growth. Most of it is lost in the transpiration process (to be discussed later). This process forces water up through the stems and cools the leaves. The water from the soil enters the epidermis of the root. It travels through the cytoplasm of root cells, called the symplast, passing from cell to cell through the plasmodesmata that connect the cells. The apoplast is the nonliving spaces between the cells; water transports in the root through this part as well. Apoplastic water needs to enter the cytoplasm of the cells inside the epidermis, called the endodermis. It then passes through the stele and finally into the xylem, which is the “vessel” of the cell. The xylem is part of the apoplast because it isn’t inside the cell itself. Water can pass out of the xylem at any point to nourish the stems and other tissues of the plant. At the leaves, the xylem passes through the petiole of the leaves and into the veins of the leaves. The finest veins of the leaves are where the water exits into the spongy and palisade layers of the leaf. Most of the water is lost through transpiration, with one percent used in metabolism. Minerals enter the root via the active transport into the symplast of epidermal cells. They enter the xylem and tracheids to travel up the roots and stems of the plant. Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. So how does water go up the plant against gravity? Because the tracheids and xylem vessels are lifeless, the transport of water is completely a phenomenon of physics. As It turns out, roots are not absolutely necessary; however, leaves are a necessary part of the process. Water is pulled upward by evaporation (transpiration) that causes negative pressure above the plant, drawing water up the plant. Water is cohesive, which means it clings to itself when confined to tubes of small bore. This is the extra pull necessary to
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