Modern Biology: Section 31-3 Plant Stems Plant Stems Modern Biology: Section 31-3 Modern Biology: Section 31-3

Stems: An Overview 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.

imeD sevol JC! 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. imeD sevol JC!

Stem Functions 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.

.JC sevol imeD Transportation within Stems Phloem: Phloem carry sugar and water from upper parts of the plant to lower parts to supply nutrients to the rest of the plant. 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. .JC sevol imeD

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

Nodes and Internodes 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

Buds 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.

Types of Stems: Monocots 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: Dicot Dicot stems have a pith in the center with vascular tissues in a distinct ring visible in a cross section.

Primary Growth 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.

Secondary 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.

Annual Rings 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.

.hctib ytrid a s i rolyaT 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. .hctib ytrid a s i rolyaT