The cells in a fertilized plant egg, called a zygote, divide rapidly. The shape and form of animals are programmed genetically, but plant growth is indeterminate. Indeterminate growth means that a species of tree may have leaves of a certain shape or branches that appear in a general pattern, but there is no predicting where new branches will appear or how many leaves the twigs will have. The structures that support and determine plant growth are different for the two basic types of plants, called monocots and dicots.
Herbaceous monocots include most flowers, vegetables and grasses. The vascular bundles that carry plant nutrients are scattered in the stems of monocots. Monocots have thin, moderately branching roots called fibrous roots that grow near the top of the soil. Their leaves have parallel veins.
Herbaceous and woody dicots include most shrubs and trees. Vascular bundles that carry plant nutrients form rings in the stems and trunks of dicots. Dicots have taproots that grow deep in pursuit of water. The veins on their leaves form a net-like pattern
Meristems are cells that divide and lengthen, making the plant grow. They determine the growth and shape of a specific plant. The tips of roots and shoots contain apical meristems. These dividing cells make roots go deeper into the soil and shoots grow up from the plant. This is called primary growth and occurs in both monocots and dicots.
Lateral meristems make the branches or trunk grow wider; this is called secondary growth. Lateral meristems are found only in dicots.
Water and Nutrients
Plants need water and nutrients to grow.
Dicots have tubular vascular tissue called xylem. The evaporation of water from pores in the leaves and surfaces of plants draws water and nutrients up through the xylem by capillary action. Roots obtain water by osmosis. The pressure of water in the roots aids in upward movement of water and nutrients through the xylem.
Long, tube-shaped cells called phoem carry water and nutrients in monocots and herbaceous dicots. Phoem cells have no nuclei and are joined end-to-end. The connecting cells are sieve-shaped; they have numerous small holes that allow water and nutrients to pass from one cell to the next through capillary action.
Parenchyma cells have thin, flexible cells and usually a central vacuole, an enclosed area that contains water and plant enzymes. The enzymes are responsible for the metabolic functions of a plant, those chemical reactions that make the plant grow. They also heal plant injury. They continue living when both monocots and dicots are mature.
Collenchyma cells have thick cells and often provide support for herbacious monocots. They are the strings in celery.
Schlerenchyma cells have thick walls and stop growing when the plant is mature. Long, slender fiber cells include those found in hemp that are used to make rope. Shorter irregular cells, called Schlerids, are found in the shells of nuts and seeds and in the pits of fruit.