Stems Who am I I am the part

Stems

Who am I? I am the part of the plant that grows upward from the roots. I hold the plant up and carry water from the roots to the other parts of a plant. I am made up of three simple cell types: parenchyma, collenchyma, and sclerenchyma cells. If I’m a secondary structure I’m called a petiole.

I am a …. Stem

Instructions: Observe the differences in structure of the cross section of a monocot and dicot stem.

Observe the differences in structure of the cross section of a monocot and dicot stem. Cross-section of a dicot stem Cross-section of a monocot stem

Stem Anatomy The stem and other plant organs are primarily made from three simple cell types: n Parenchyma cells are the most common plant cells. They are found in the stem, the root, the inside of the leaf, and the pulp of the fruit. They are responsible for metabolic functions, such as photosynthesis. They also help repair and heal wounds and store starch. n Collenchyma cells are elongated cells with unevenlythickened walls. They provide structural support, mainly to the stem and leaves. Many of its cells are alive at maturity. n Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at maturity.

Stems in Dicots

Observe the differences in structure of the cross section of a monocot and dicot stem. Structure Vascular Appearance in dicot monocot bundle Cambium Cortex and pith Sclerenchyma

Observe the differences in structure of the cross section of a monocot and dicot stem. Cross-section of a dicot stem Structure Vascular bundle Cambium Cortex and pith Sclerenchyma Appearance in dicot smaller number arranged in a ring it is found in between the xylem and phloem and allows secondary thickening they are clearly developed it forms a fiber cap atop the phloem

Internal Structure of the Stem of a Typical Dictoyledonous Plant Internal Structures: n n n Pith: Large central area for storage & support. Cambium: Found as a circle around inner stem & outer surface. Forms woody secondary tissue for support. Cortex: Storage area between cambium and epidermis. Epidermis: Thin outermost layer of the stem that lacks chloroplast and aid in protection. Xylem: Water conduction up. Phloem: Sap (organic molecules) conduction, usually down to roots.

Internal Structure of the Stem of a Typical Dictoyledonous Plant Internal Structures: Epidermis: n The outermost layer of the stem. n It is single layered and lack of chloroplast. n Outer side of epidermis a layer is present which is made up of cutin is called cuticle. n Epidermis plays a significant role in protection.

Internal Structure of the Stem of a Typical Dictoyledonous Plant Internal Structures: Pith: n This is a well developed region that is clearly distinguished. n Spreads from the ring of vascular bundles to the centre. n The cells of this region are mainly made up of parenchyma n Function of pith: storage of water and food

Internal Structure of the Stem of a Typical Dictoyledonous Plant Internal Structures: Cortex: n This is a well developed region that is clearly distinguished.

Internal Structure of the Stem of a Typical Dictoyledonous Plant Internal Structures: Vascular Bundles: n The vascular bundles (wedge shaped) are arranged in a ring. n Each vascular bundle is made of phloem, cambium and xylem.

Stem- Dicots n n The vascular bundles are found near the outer edge of the stem The xylem is found towards the inside of each vascular bundle, the phloem is found towards the outside In between the xylem and phloem is a layer of cambium Cambium is a layer of meristem cells that divide to make new xylem and phloem

Stems in Monocots

Observe the differences in structure of the cross section of a monocot and dicot stem. Structure Vascular Appearance in dicot monocot bundle Cambium Cortex and pith Sclerenchyma

Observe the differences in structure of the cross section of a monocot and dicot stem. Cross-section of a monocot stem Structure Appearance in monocot Vascular bundle Large number scattered in the ground tissue Cambium Cortex and pith There is no cambium Sclerenchyma it forms a sheath that encase the vascular bundle There is no distinction, all ground tissue

Internal Structure of the Stem of a Typical Monocotyledonous Plant Internal Structure: • Epidermis: the outer most single celled thick layer. It is covered with thick cuticle. Multicellular hairs are absent & stomata are also less. • Ground tissue: The entire mass of parenchyma cells. There is no differentiation of ground tissue in monocotyledon stem. It means ground tissue is not differentiated into endodermis, cortex, pericycle etc.

Internal Structure of the Stem of a Typical Monocotyledonous Plant Internal Structures • Vascular Bundle: are scattered in the ground tissue and are generally oval shape. Vascular bundles lies towards the center are large in-size and-less in number. Vascular bundles surrounded by the layer of sclerenchymatous fibre are known as bundle sheath. • Pith: Pith is undifferentiated in monocotyledon stems.

Stem Functions n Stems have the following 3 functions or jobs: n n n Support Transport of substances Storage

Support n n Stems support the plant’s leaves, flowers, and fruits. The sclerenchyma cells of the stems are hardened by lignin to make them woody and therefore rigid and strong to hold the plant upright.

Transport of Substances n n n Like roots, stems contain vascular tissue (xylem and phloem). Xylem cells transports water and minerals to every part of the plant. Phloem conducts the sugars created during photosynthesis.

Storage n n Stems are sometimes modified to store food and (or) water. Plants that store large amounts of water in their stems or leaves are called succulents.

Benefits of Bark n n n Protects the cells of the stem It allows the plant to grow taller because of the strong support. Plants can support or withstand winds because of the strong yet flexible support Habitat for small animals There is economic importance e. g. bark slack for medicine, rubber, fibres, dyes Allows longevity

Modified Stems

Succulents n n n In extremely dry environments, often the leaf becomes reduced and non-photosynthetic, and the stem becomes the primary photosynthetic organ of the plant. A barrel shape provides a low surface to volume ratio which reduces water loss therefore allowing the plant to store large amounts of water. Examples: Cacti, common desert plants

Tuber n n n A potato is a special type of underground stem called a tuber. Tubers are swollen regions of stems that store excess carbohydrates manufactured during photosynthesis. Don’t confuse tubers with tuberous roots. Tuberous roots are root modifications, tubers are modified stems. Irish potatoes are tubers. Although the common potato is a stem, the sweet potato is a modified root!

The Potato is a Tuber mature tuber

Rhizomes n n n An underground stem that grow near the soil surface. Usually grows horizontally. Capable of producing new shoots and roots. Store food for renewing growth of the shoot after periods of stress. Examples: iris, ginger, canna Iris Canna lily

Stolons n n Aboveground stems that “run” across the soil surface. Horizontally growing. Produce new shoots and roots. Example: number of grasses have stolons; Strawberries Strawberry stolons

Corms n n n Compressed, swollen vertical stem. Papery covering. Found underground. Gladiolus corms

Bulbs n n n Underground stems surrounded by fleshy leaves that store nutrients. A bulb consists of a stem from which modified fleshy leaves develop. The fleshy leaves are circular and hollow; new leaves develop within the older leaves. Leaves are attached to the 'stem' at nodes and spaces between nodes are called internodes. The presence of nodes and internodes means that the bulb is actually a modified stem. Tunicate bulb Scaly bulb

Bulbs n Two different kinds: n Tunicate bulbs have a papery outer covering. An onion is an example. n Scaly bulbs lack a papery outer covering. Lily bulbs are an example. Tunicate bulb Scaly bulb

Parts of a Bulb Scales: modified leaves that store carbohydrates Apical bud: sits on top of the basal plate. Will eventually form a shoot bearing leaves and flowers Basal plate: modified stem

Offset Bulbs Original bulb Offset bulbs

Activity n n n Compare and contrast the internal features of monocot and dicot stem Compare the external appearance of the dicotyledonous stem with modified root. Explain the function of a stem.

Answer these questions: 1. Describe 4 internal features of a cross section of a dicotyledonous stem. 2. Describe 4 internal features of a longitudinal-section of a dicotyledonous stem. 3. Explain 3 advantages of bark on stems. 4. Compare 4 internal features of a monocotyledonous and dicotyledonous stem. 5. Compare the external appearance of the dicotyledonous stem with 6 modified stems.