BOTANY 101—THE STEM APEX
By Wayne L. Handlos, Ph.D.
l.s.Coleus stem1 Coleus apical meristem 2 Stem apical meristem3 Diagram: Pelargonium stem apex4
Diagram of a stem5 Root tip (apical meristem) 6
All living things have the ability to grow. This ability is due to the capacity of the cells to divide and
produce two new cells where only one existed before. In flowering plants (and higher animals) not
all cells retain the ability to divide and to be involved in the growth of an organism. In most higher
plants there are areas in the plant where the cells do retain the ability to divide. Such areas are called
MERISTEMS. The most notable of these meristems are the ones found at the tips of stems and
roots. These are calledAPICAL MERISTEMS because they are at the apex or end of the stems and
roots. Because the tip of the stem is usually markedly different from the tip of the root in flowering
plants, a distinction is made to distinguish which meristematic area you are referring to: the stem apical
meristem (SAM) and the root apical meristem (RAM). These two meristems are primarily responsible
for the growth in length of the stems and roots. (In the flowering plants known as dicots,
the continuing growth in diameter is due to two lateral meristems called theVASCULAR CAMBIUM
and theCORK CAMBIUM. These two groups of cells divide repeatedly and add cells to the
thickness of stems and roots.)
After many years of study, it has been found that the stem apex of most flowering plants consists of
several layers of cells each of which maintain their position to each other over time. This arrangement
is referred to as theTUNICA-CORPUS organization of the stem apex. The tunica frequently
consists of two layers of cells, lying parallel to each other and to the surface of the apex of the stem.
(But, the tunica may consist of one to three or even more layers of cells.) The outermost layer
(referred to as L I) contributes cells only to theEPIDERMIS (the surface layer of cells of all plant
parts). This happens by divisions that are perpendicular (called anticlinal) to the surface of the apex.
In other words, the new cell wall between the two new or daughter cells is at right angles to the surface
of the stem. The epidermal cells are usually colorless, that is, they are without chlorophyll.
However, they are genetically capable of producing green chloroplasts but this normally occurs only
in the guard cells of the epidermis. In flowers, the epidermal cells contain the pigments which are
primarily responsible for the colors in the flowers.
The next layer of cells in the tunica is called L II. Divisions in this layer are also primarily anticlinal
or perpendicular to the surface. These cells contribute to theCORTEX of the stem. In the leaf these
cells compose the cells of the mesophyll (the green cells) at the edge or perimeter of the leaf. In the
flower the cells derived from L II give rise to the bulk of the tissues inside the epidermis. In particular,
the reproductive cells of the flower (the pollen grains and the egg cells) develop from this layer.
The corpus of the stem apical meristem is the mass of cells at the heart of the apex, that is
inside the parallel, superficial cells of the tunica (L I and L II) and is referred to as L III. Cell divisions
within the corpus may be in any plane and add to the bulk or volume of the stem in all directions. Cells
derived from the corpus give rise to the vascular system and pith of the stem. In the leaf, derivatives of L III
produce the mesophyll at the center of the leaf. In the flowers and fruit, L III contributes only to the vascular
tissue at the base of the flower.
The root apical meristem does not show quite the same organization as the stem apical meristem. The number
of layers is not the same. In addition, the root normally has an outer protective layer at its tip—theROOT CAP—
composed of a renewable and constantly replaced group of cells outside of the central area of meristematic cells.
As the root has little significance in our next topic (PLANT CHIMERAS), it will not be elaborated here.
In our next newsletter, we will describe the structure of chimeras, the peculiar arrangement of different genetic
tissues within the same plant.
Chimeras are responsible for many of the interesting variations that we see in Pelargonium cultivars. The structure
of the stem apical meristem and the tissues derived from it are key to understanding the nature and effects of chimeras.
Be sure to be here next month.
1. Coleus stem tip ( l.s.= longitudal section=stem cut along its axis, not across) - www.doctortee.com
2. Coleus apical meristem showing a 2-layered tunica - www.sbs.utexas.edu/mauseth/
3. Lonicera SAM ( Stem Apical Meristem) showing the continuous nature of L1 with the epidermis of the leaf - www.zmirr.info/eng/
4. Source unknown
5. Primary meristems - www.doctortee.com/
6. Zea root tip showing a distinct root cap - www.doctortee.com/