Modern Biology: Section 31- 3 Plant Stems. In contrast to roots, which are mainly adapted for absorption of and anchoring, stems are usually adapted to.

Plant StemsPlant Stems

Modern Biology: Section 31-3

Modern Biology: Section 31-3

Plant Stems

In contrast to roots, which are mainly adapted for absorption of and anchoring, stems are usually adapted to support leaves. Whatever their sizes and shapes, stems also function in transporting materials and providing storage.

Stems: An Overview

Types of Stems

There are many types of stems, each with it’s own special adaptations. Strawberry stems are not very strong, meant to only transport, as the plant lies along the ground. Cactuses have green fleshy stems that store water and carry on photosynthesis. Some stems have thorns or other extras that keep predators away.

Stems transport and store nutrients and water in a plant. They also support the weight of the leaves and other structures.Carbohydrates, plant hormones, and other organic compounds are regularly transported through the stem.

Movement of carbohydrates goes from the source, or location they were made or stored, to the sink, where they are either stored or used. This process is called translocation.

Stem Functions

Xylem:

Xylem carry water from the roots to the upper parts of the plant. The movement is powered solely by osmosis pressure. Water evaporates from the top, and water below moves to replace it.

Phloem:

Phloem carry sugar and water from upper parts of the plant to lower parts to supply nutrients to the rest of the plant.

Transportation within Stems

Stems conatain a variety of structures that aid it in it’s functions.

Stem Structures

• Stems care divided into internodes.• At each end of an internode is a node.• A node is a structure on a stem at which two or more

leaves are attached

Nodes and Internodes

• At the point of attachment of each leaf the stem bears a lateral bud.

• A bud is capable of developing into a new shoot.• Buds contain an apical meristem enclosed by specialized

leaves called bud scales.

Buds

Contain parallel veins• The shoot apex in monocot stems is more

elongated. Leaf sheathes grow up around it, protecting it. Monocot leaves have continuous vascular tissue going down from a shoot or stem.

Types of Stems: Monocots

• Dicot stems have a pith in the center with vascular tissues in a distinct ring visible in a cross section.

Types of Stems: Dicot

• Primary growth in stems consists of the shoot apical meristem and the primary tissues produced from this meristem. The primary shoot will eventually grow to support leaves, tendrils, and early fruit.

• The epidermis is composed of epidermal cells, trichomes, and lenticels.

• The cortex and pith are composed largely of parenchyma cells.

• The vascular bundles are in a ring. They are composed of primary xylem and primary phloem.

•The primary vascular bundles of Vitis are collateral, meaning that phloem only occurs on one side of the xylem. Primary xylem transports water and nutrients. Its component cells include vessel elements, and fibers. Primary phloem transports photosynthates and is composed of sieve tube members, companion cells, parenchyma cells, and primary phloic fibers.

Primary Growth

• In vascular plants, secondary growth or, perhaps more accurately, secondary thickening is the result of the activity of the vascular cambium. The latter is a meristem that divides off cells: the cells on the inside of the meristem (the adaxial side) will become secondary xylem, while the cells on the inside (the abaxial side) will become the secondary phloem. This growth increases the girth of the plant, rather than its length, hence the phrase "secondary thickening". As long as the vascular cambium continues to produce new cells, the plant will continue to grow more stout. If this is kept up over many years, this process produces wood.

• Because this growth ruptures the epidermis of the stem, woody plants also may have a cork cambium that develops among the phloem. The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss. If this is kept up over many years, this process may produce a layer of cork. In the case of the cork oak it will yield harvestable cork.

Secondary Growth

• In the early part of the growth season the cells produced (called early growth) are larger than the late growth.  Gradually, as the growth season winds on, the cells produce become smaller and more closely packed as the tree moves from producing early growth to late growth.  Eventually the growing season ends and the tree enters a dormant stage.  The change between early and late growth in a particular season is gradual and the change isn’t easily noticeable.  However, when the next growth season comes around the tree again produces the large cells of the early growth of the next beings there is a very sudden change between the two types of cells.  This sharp change is what we see when we look at the rings in tree trunks.

Annual Rings

Important Terms

Primary tissues: Tissues generated from the growth of an apical meristem.

Cambium: A lateral meristem constituting a sheet of cells. Growth of these cells increases the girdth of the plant organ involved.

Secondary tissues: Tissues generated from the growth of a cambium.

Vascular Cambium: A cambium that gives rise to secondary xylem to the inside, and to secondary phloem to the outside.

Periderm: A structure that consists of a cork cambium (phellogen), with cork tissue (phellem) to the outside, and in some cases a layer of cells derived from and to the inside of the cork cambium called phelloderm. Functions to limit dehydration and block pathogens after the epidermis is disrupted by the onset of secondary growth.

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