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BOTANY 101—THE STEM APEX

By Wayne L. Handlos, Ph.D.

  l.s. Coleus stem apex with labels             stem apical meristem showing 2-layered tunica               close-up stem apical meristem             diagram Pelargonium stem apex           

l.s.Coleus stem1     Coleus apical meristem 2  Stem apical meristem3  Diagram: Pelargonium stem apex4                                    

                            diagram of stem showing meristems                    root tip (apical meristem)  showing root cap

                      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 called APICAL 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 the VASCULAR CAMBIUM

and the CORK 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 the TUNICA-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 the EPIDERMIS (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 the CORTEX 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—the ROOT 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.

Credits:

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/