The Stem: The Shoot System I

Functions & Organization of the Shoot System -The shoot system is composed of the stem and its lateral appendages: leaves, buds and flowers. -Stems are made up of the 3 tissue systems (Ground Tissue System mainly in the form of cortex, pith and parenchyma; Dermal Tissue System mainly in the form of epidermis and periderm: Vascular Tissue System in the form of vascular bundles containing mainly xylem and phloem conducting tissues. -Vascular tissue runs the length of the stem in vascular bundles. Each vascular bundle contains xylem tissue on the inside, phloem tissue on the outside and meristem tissue in between the two. -Two groups of flowering plants, dicots and monocots, have major differences in the distribution of their stem tissues.

Anatomy of A Dicot Stem Vascular bundles are arranged in a ring around the edge of the stem.

Anatomy of A Monocot Stem Vascular bundles are scattered throughout the stem. The terms pith and cortex usually are not used when the bundles are scattered; instead the term ground tissue is used for all parenchyma tissue surrounding the vascular bundles.

Primary Growth and Stem Anatomy Primary tissues of dicot stems develop from the primary meristems. The SAM is composed of dividing cells. It is responsible for the initiation of new leaves and buds and for making the 3 primary meristems.

The Pattern of Primary Vascular Development in a Dicot Stem The primary meristem, Procambium, gives rise to primary xylem and primary phloem. The primary meristem, Protoderm, gives rise to epidermis. The primary meristem, Ground Meristem, gives rise to pith and cortex. All resulting are primary tissues in the stem of the primary plant body. Parenchyma cells in the outside region of the cortex sometimes contain chloroplasts for photosynthesis. The pith region of the stem in some plants may become hollow by the breakdown of the centrally located parenchyma cells. In some plants exhibiting secondary growth, some procambium between the primary xylem and phloem remains undifferentiated; such cells are called residual procambium.

The Distribution of the Primary Vascular Bundles Depends on the Position of Leaves. Leaf Arrangement Patterns The Vascular Bundles that network into the attached leaves are called leaf traces. The organization of primary vascular bundles in stems depends on the number and distribution of leaves and on the number of traces that branch into the leaves. Primary Vascular System

Primary Growth Differs in Monocot and Dicot Stems Monocot stem diameter is about the same near its apex as its base. Monocots have primary thickening meristems (PTM), which are absent in dicots. The PTM does elongation and lateral growth, a characteristic resulting in an umbrella-like shape. Iris (Monocot) Shoot Tip Monocot Shoot Apex in More Detail The SAM and the primary meristems also are present in these shoot tips. Some monocots, like wheat are hollow in the stem internodes rather than having ground tissue there.

BIO 141 Botany with Laboratory This product is sponsored by a grant awarded under the President’s Community-Based Job Training Grants as implemented by the U.S. Department of Labor’s Employment and Training Administration. The information contained in this product was created by a grantee organization and does not necessarily reflect the official position of the U.S. Department of Labor. All references to non-governmental companies or organizations, their services, products, or resources are offered for informational purposes and should not be construed as an endorsement by the Department of Labor. This product is copyrighted by the institution that created it and is intended for individual organizational, non-commercial use only.