More Botany for Gardeners

Connie Holland
Adams County Master Gardener

This is the second article in a series about botany for gardeners. I have always been in awe of the mystery of what goes on inside plants. Every winter I stare at my gardens with nothing showing and wonder just how the plants and seeds underground will return. Just as a human embryo develops specialized functions like a heart, brain, spine, etc. from cells, so does an embryonic seed sprout and develop roots, stems, leaves and flowers from specialized cells. My first article about botany described the basic development of seeds and roots, including an understanding of tap roots. Todayís article introduces stems and the functions of some of its unique cells.

The above ground part of a sprouted seed is the shoot system consisting of a plantís principal stems, branches and leaves. All arise from the stemís apical meristem whose growing tip makes the stem grow longer via cell division and initiates the orderly arrangement and development of stems and leaves. This apical meristem also forms axillary buds located at the intersection of a leaf and a stem. These very tiny buds can be seen with close-up viewing.

One of the stemís functions is to support the leaves to capture the most sunlight for photosynthesis. As a stem grows, internodes stretch and spread the leaves and axillary buds apart to ensure each leaf gets maximum exposure to the sun and good air circulation. Leaves are arranged around a stem in one of three basic forms, alternate, opposite or whorled in a circle about the stem. Shoots formed are capable of producing many branches; however, only a limited number are created at any one time.

Axillary buds get activated to form new stems by pruning or "pinching back" to stimulate stem branching. Axillary buds can be dormant for seasons or years waiting to spring into action creating a new stem if the apical buds are pruned or destroyed by disease, animals, etc. Stems can be herbaceous, having soft, green tissue with little woody tissue such as Petunias or Foxgloves, or woody having dense tissue such as a tree or shrub.

When looking at a new soft green young tree stem, one wonders how it becomes a woody tree trunk. Inside the stem are bundles of vascular tissues that transport water, minerals and food. The inner half of this vascular bundle contains the water conducting cells called xylem from the Greek word for wood. Water is conducted only in an upward direction. Toward the outside of the bundle are the food transporting cells called the phloem from the Greek word for tree bark.

Between these bundles is the vascular cambium tissue responsible for the sideways increased growth in diameter of a stem. Complicated changes in these vascular bundles form the interior "heartwood" support of a tree while providing transport of food and water and the formation of protective bark. Everyone has seen cross sections of tree trunks where various growth rings are visible. Look closely at the rings and you can see the various "sections".

No one precisely knows how this process of cell differentiation takes place. There are many unresolved mysteries in the developmental process in plants. The progress and success of a plantís growth are in response to the environment in which it is living. Adequate water, optimum temperatures, sunlight and food are just some of the variables that affect growth and survival.

Plants have developed survival attributes to ensure their continued existence. Future botany articles will explore why and how plants respond to light, how they can climb, defensive mechanisms, protection by other creatures such as ants, wound healing, insectivorous plants and other fascinating botanical facts about the plants in our yard and gardens.

Latin for Gardeners: a Brief Pronunciation Guide

A Primer on Plant Nomenclature

Read other articles by Connie Holland