References:

Plants and People, pages 36-46.

On Food and Cooking, pages 144-147.

Learning Objectives:

What is a stem?

What is a root?

What is a leaf?

What is a monocot?

What is a dicot?

What are the differences between monocot and dicot stems, leaves and roots?

The stem of a plant is an above ground support structure. In many plants (green plants) photosynthesis occurs in stems. The roots of a plant are the below ground support and water/nutrient absorption organs of the plants. Roots often function as nutrient storage organs. While stems vary greatly among plants, roots are vary similar among plants. The collective term for the vascular tissues of stems and roots is the STELE. In all flowering plants the stele is divided into a series of VASCULAR BUNDLES in cross section. In reality the stele of flowering plant is a series of tubes that branch off and fuse together such that if one makes a single cross section, one sees the group of vascular bundles.

The two major groups of vascular plants are MONOCOTS and DICOTS. We will look at the reasons for these names in a later lecture. Today, however, we will look at differences in the stems and leaves of monocots and dicots. Dicot steles branch in one cylindrical plane such that in a cross section the vascular bundles are seen in a ring around the outer edge of the stem. Vascular bundles in dicot steles often are capped with a cluster of fibers called the BUNDLE SHEATH. The steles of monocots do not branch in any one plane, but branch throughout the inside of the stem, and in cross section the vascular bundles appear throughout the stem. A bundle sheath can also be found in monocot stems. All stems have an epidermal layer and internal parenchyma, often forming a central PITH.

The differences in the organization of the steles is of great significance, because it allows dicots to develop WOOD, which is large masses of secondary xylem tissue. These large masses can develop because they originate from the inside of the stem, and then grow outward. If, as in monocots, the stele were scatted throughout the stem, increases in xylem tissue from secondary growth would produce loose clusters of secondary xylem in parenchyma tissue, not solid masses of wood.

The apical meristems of stems are found at the tips of stems, and form BUDS. Stems bear appendages called leaves. The point where a leaf and a stem meet is called the AXIL of the leaf, and buds are present in the axils of leaves. Stems branch from buds in the axils of leaves.

Roots

There are two types of roots, TAPROOTS and FIBROUS ROOTS. Taproots have one main root with small side roots, like a carrot. Taproots often serve as storage organs, such as turnips, beets and carrots. Fibrous roots are much smaller, and much more numerous roots that actually originate from stems. Monocots have fibrous roots while dicots have one or the other, depending on the species. The tips of all roots have a ROOT CAP, which is a layer of hard cells that protect the root as it pushes through the earth. The apical meristem in roots produces two types of cells, the root cap and cells that form the functional body of the root.

The stele of a root is at the center of the root. The stele is surrounded by a layer of cells called the PERICYCLE. Branch roots originate at the pericycle. Around the pericycle of most roots is a layer called the ENDODERMIS. The endodermis is a layer of cells whose outer edges are filled with SUBERIN. The layer of suberin is called the CASPARIAN STRIP. The Casparian strip seals spaces between cells to prevent anything from getting into the vascular cylinder without passing through the cells. This allows the cells to regulate what does and does not get into the cell.

Leaves

The basic function of a leaf is food production. As we have learned, plants produce food (glucose) by photosynthesis. Leaves have, however, been modified into many other structures in plants. A food producing leaf is called a TROPHOPHYLL. The other main function of leaves is achieved through their association with sporangia. Leaves that produce sporangia are called SPOROPHYLLS. We will focus on trophophylls in flowering plants in this lecture.

The leaves of dicots are surrounded by an epidermis. The parenchyma in dicots have 2 layers,. The upper layer is called the PALISADE MESOPHYLL  and the lower layer is called the SPONGYMESOPHYLL. The role of the cells in the palisade mesophyll is light harvesting, and the cells are packed in tightly. The spongy mesophyll is loosely packed to allow room for gases like water vapor and carbon dioxide to pass in and out of the cells.

The leaves of monocots also have an epidermis, but do not have  2 types of parenchyma. Some of the epidermal cells in monocot leaves are modified to allow the leaves to curl up. These cells are called BULLIFORM CELLS.

The leaves and stems of both monocots and dicots have cells called GUARD CELLS in their epidermal layer. Guard cells alter their shape, causing holes called STOMATA to open and close in response to the plants needs to control the moving of water out of the leaves and the movement of carbon dioxide into the leaves.

The flat part of a leaf is called the BLADE. The stem of a leaf is called the PETIOLE. At the base  of the leaf, on the stem there are often small structures called STIPULES, which may vary in shape from leaflike to thornlike. The place on a stem where al leaf originates is called a NODE. The part of the stem between nodes is called the INTERNODE. IF there is only one leaf present at a node, the arrangement of leaves on the stem is called ALTERNATE. If there are two leaves at a node, the arrangement of leaves is called OPPOSITE. If there are more than two leaves at a node the arrangement of leaves on the stem is called WHORLED.

Leaves that are not divided into leaflets are called SIMPLE LEAVES. Leaves divided into leaflets are called COMPOUND LEAVES. Sometimes it can be difficult to tell a compound leaf from a stem with simple leaves. The way to tell is that a leaf always ends with a leaflet, whereas a stem always ends with a bud. There are two types of compound leaves, PALMATELY COMPOUND, where all the leaflets arise at one point, and PINNATELY COMPOUND, where leaves arise from several different points along the main vein of the leaf.

Vascular tissue in leaves forms VEINS. There are two main patterns of veins. In monocots, veins are PARALLEL. In dicots, veins are RETICULATE (net). Parallel venation refers to veins that run parallel to each other, and are equal in size. Reticulate venation refers to venation where there is/are main vein(s) with smaller and smaller side veins.