Botany Basics - Stems Part 1

Botany Basics - Stems Stems  Stems are a crucial part of plant physiology and perform the following functions for plants  Offer physical support for the leaves  Serve as conduits for moving water, minerals, and food throughout the plant Stem Terminology  Shoot – a young stem (1 year old or less) with leaves  Twig – A young stem (1 year old or less) that is in the dormant winter stage (no leaves)  Branch – A stem that is more than 1 year old typically with lateral stems radiating from it  Trunk – A woody plant’s main stem Stem Vasculature  The vasculature system of a stem includes 3 main components –  Xylem – carries water and dissolved minerals up the plant  Phloem – carries food down the plant  Vascular Cambium – the cambium is a layer of meristematic tissue that separates xylem and phloem  The Cambium also produces the xylem and phloem tubes and creates the new tissue that is responsible for the change in girth of a stem  E.g. cambium would create the rings of a tree trunk Importance to Gardeners  The vascular cambium is important to gardeners  For example, in grafting, the tissues must line up so that phloem is to phloem and cambium is to cambium  Addtionally, careless weed trimming can strip the bark off of a tree, injuring the cambium and causing tree or shrub death Monocots vs. Dicots  In a monocot, the xylem and phloem are arranged in bundles and dispersed throughout the stem  In dicots, the vascular system forms continuous rings inside the system  The ring of phloem is near the bark and eventually becomes part of the bark in mature woody stems  The xylem forms the inner ring; in woody plants, it becomes the sapwood and heartwood  This information is key to gardeners; for example, herbicidal action is specific to monocots and dicots  E.g. 2 4-D kills only dicots by targeting the continuous vascular system Nodes  A node is an area on a stem where the buds are located  Nodes are a site of great cellular activity and growth, where small buds develop into leaves, stems, or flowers  When pruning, it is important to locate a plant’s nodes  Generally you want to cut just above, but not too close to a node  This encourages the buds at that node to begin development  The area between two nodes is called an internode Internodal length  Internodal length can be an easy visible indicator of a plant’s health and productivity  Several factors can affect internode length  Reduced soil fertility decreases internode length  Applications of high-nitrogen fertilizer can greatly increase internode length  Lack of sunlight, or too low intensity of lighting, can increase internode length, causing a spindly stem  A situation where plants have a spindly stretched stems is called Etiolation  This often occurs in seedlings started indoors and in houseplants that get too little light  Internode length varies with the season  Early-season growth has long internodes; late season growth has longer internodes  If a stem’s energy is divided among three or four side stems, or is diverted into fruit growth and development, internode length is shortened  Plant growth regulators and herbicides will also change internode length Types of Stems  All stems must have buds to be classified as stem tissue  This distinction is important because stems can run underground, making them sometimes easy to confuse with roots  Some plants have specialized aboveground stems known as crowns, spurs, or stolons  Crowns are compressed stems with leaves and flowers on short internodes  Spurs are short, stubby side stems that arise from the main stem  These are the fruit bearing stems on a pear, apple, and cherry tree  If major pruning is done close to the fruit bearing spurs, they can revert to nonfruiting stems, eliminating that year’s crop  Stolons are elongated horizontal stems that often lie along the soil surface.  E.g. the runners of a strawberry are stolons  Roots often develop from these stolons, creating daughter plants  If left unchecked, this is an easy way to increase the size of a strawberry patch