It is primarily composed of parenchyma cells, and it may contain sclerenchyma and collenchyma cells in dicot stems. In the ground tissue, the cells are loosely arranged and there is space between them, which facilitates gas exchange between the stem and the surrounding air. Located in the middle of dicot stems, the pith or medulla is composed of soft, spongy parenchyma cells with spaces between them.
The pith is surrounded by a ring of vascular bundles, containing xylem and phloem. Function: Its primary function is storing water and nutrients and transporting them throughout the plant. In dicot stems, sclerenchyma cells can be found in tissues where growth has stopped. There are two types of sclerenchyma cells, fibers and sclereids, which are dead at maturity and have thick, lignified cell walls. Function: Fibers are long, thin cells that provide strength to vascular bundles in stems, and sclereids are variably shaped cells that provide support for secondary phloem in dicots.
The vascular bundles of dicot stems contain a layer of meristem cells, called the cambium or vascular cambium , located between the xylem and phloem. Function: These cells facilitate secondary growth, dividing to create new xylem and phloem cells, and thus widening the girth of the stem.
Dicot leaves have an upper and lower epidermis, and the lower epidermis contains several small pores, called stomata, which facilitate gas exchange and allow water vapor to exit the leaf. Having stomata only on its lower surface helps the dicot leaf conserve water while having most stomata open. The upper and lower epidermis are separated by ground tissue, called mesophyll, which contains parenchyma cells that carry out photosynthesis as well as sclerenchyma and collenchyma cells that provide structural support.
In dicot leaves, the ground tissue is often arranged into two distinct types, the palisade and spongy mesophyll. The dicot leaf's vascular structures form net-like veins that are connected to the stem's vascular structures.
The leaf's dermal cells produce and secrete a waxy substance, composed of lipid polymers, that forms a protective covering called the cuticle. Function: It acts as a barrier that prevents water loss via evaporation and protects the leaf from light damage and invading pathogens, such as viruses, bacteria, and fungi.
Leaves have two outer dermal tissue layers, the upper and lower epidermis, each composed of a single layer of cells that prevents damage caused by sunlight, pathogens, and herbivores. Most dicot leaves have trichomes, hairs that extend from the epidermal cells of the upper and lower epidermis.
Trichomes can also be present on dicot stems, as well as monocot leaves and stems. Depending on the type of plant, trichomes can be unicellular or multicellular, branched or unbranched, and glandular or nonglandular. Function: They protect the plant from damage, which can be caused by herbivores, UV light, and extreme temperatures. Stomata singular: stoma are small epidermal pores that allow gases and water vapor to move between the interior leaf structures and the surrounding air. In dicot leaves, the lower epidermis contains several stomata.
Note: Stomata are also present in photosynthetic stems. Each stoma is surrounded by two guard cells, specialized epidermal parenchyma cells that open and close the pore. Function: These cells regulate photosynthesis and respiration, facilitating gas exchange while open and preventing excess water loss while closed. In dicot stems and leaves, collenchyma cells can be found in tissues where growth is occurring, such as near vascular cambium.
Alive at maturity, these elongated cells have thick cellulose and pectin walls. In monocot and dicot leaves, sclerenchyma cells can be found in tissues where growth has stopped. Unlike most monocot leaves, which have only one type of mesophyll, the ground tissue of dicot leaves is often arranged into two distinct types, the palisade and spongy mesophyll.
In the palisade mesophyll, parenchyma cells are tightly packed, and their shape is usually polyhedral, elongated, or lobed. Function: This structure and the presence of chloroplasts facilitate photosynthesis. In the spongy mesophyll, parenchyma cells are more loosely arranged, with spaces between them, and their shape tends to be spherical or stellate.
Function: This structure allows the leaf to receive carbon dioxide from the air and to release oxygen and water vapor into the air. A bundle sheath surrounds each vascular bundle, or vein, in monocot and dicot leaves. It consists of tightly arranged parenchyma cells in dicots.
Function: These cells protect the vascular structures and play a role in photosynthesis. See more from our free eBook library. Monocot and Dicot Overview. Monocot and Dicot Roots. Monocot and Dicot Stems. Monocot and Dicot Leaves. In addition to distributing nutrients, vascular tissues also provide structural support. In fact, the material commonly known as "wood" is actually xylem. After a time, the xylem at the center of older trees woody dicots ceases to function in transport and takes on a purely supportive role; this nonfunctioning xylem is called heartwood.
The newer xylem closer to the surface remains active in the vascular system, and is called sapwood. SparkTeach Teacher's Handbook. Summary Stem and Trunk.
Transport The vascular system in the stem is continuous with that of the roots, providing an easy route for water and nutrients to flow throughout the plant body.
Monocot Stems In monocots, xylem and phloem are organized in vascular bundles scattered throughout the stem. Dicot Stems The vascular system found in dicots is somewhat more complex than that found in monocots. Previous section Problems Next section Problems. Within each bundle, the phloem is located closer to the outside of the stem, and the xylem is located closer to the inside of the stem.
Like dicot roots, dicot stems are protected by an outer layer of dermal tissue called the epidermis. Then, also similar to dicot roots, dicot stems have a layer of ground tissue called the cortex beneath the epidermis.
The vascular bundles in the stem are arranged around a ring of cambium , which contains cells that divide to expand the girth of the stem. This is similar to the function of the cambium in dicot roots. Within the vascular bundles, the xylem is located interior to the cambium ring, and the phloem is located exterior to the cambium ring, accompanied by sclerenchyma ground tissue. The ground tissue located interior to the cambium ring is referred to as the pith.
Primary growth occurs in all groups when a plant grows taller and when it develops roots, branches, leaves, and flowers. Plants that undergo only primary growth and have soft, green, non-woody stems are known as herbaceous plants. They typically have an annual, biennial, or perennial life cycle, meaning that they partially or completely die after a season and have to re-grow. Secondary growth occurs when dicot stems and roots grow wider.
In general, monocots do not undergo secondary growth. An increase in girth without secondary growth is referred to as anomalous thickening. See more from our free eBook library. An article from Georgia Tech summarizing the differences between primary and secondary growth.
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