Stems-Primary Growth

Seedless Plants like ferns, typically have Apical Cells as their Apical Meristems. This is similar to what we saw with Roots. These apical cells are shaped like an inverted pyramid and divide Anticlinally to produce Derivative Cells. These, continue to divide in various planes and produce the shoot system.

The stems of Seed Plants do not have apical cells. Some appear to have have Initials similar to those we saw in some root apical meristems.

However, Shoot Apical Meristems (SAMs) of gymnosperms and angiosperms are clearly Multicellular.

The apical dome is usually convex or flat and its surface is smooth.

However, small protrusions can be seen when the macroscopic leaves are carefully removed. These  small protrusions are Leaf Primordia. The smallest of these is the one most recently produced.

The term Node is used to indicate that place on the Stem where a Leaf is inserted.

An Internode is that portion of a stem between successive nodes. This terminology will be reviewed below.

The "Meristem Proper" is that part of the shoot apex which is ABOVE the youngest leaf primordium.

The term "Shoot Apical Meristem" (SAM) refers to the Meristem Proper.

The "Shoot Tip" would include the youngest leaf primordia and the SAM (Meristem Proper).

This is an important consideration for plant propagation via "tissue culture". 

Bacteria and Viruses can penetrate the shoot tip but can't penetrate the SAM, because there is no Vascular Tissue that links it to the young stem. 

Pathogens are able to travel through the Vascular Tissues into young leaves & older leaf primordia but NOT into the SAM.

Meristem Culture refers to the excision and culture of the pathogen-free Meristem Proper (SAM). 

Shoot Tips can also be cultured and they are easier to grow than true apical meristems. However, the chances for contamination are much greater.

The differences in productivity between pathogen-free plants and contaminated plants is staggering, and makes it well worth the extra effort for some horticultural crops.

Thus, this terminology has basic scientific significance because it allows us to readily and unambiguously discuss these closely-related parts of the shoot system. It also has practical value in terms of pathogen-free propagation.

A careful examination of the shoot apical meristem (SAM) shows that leaf primordia are produced in precise geometric patters. 

The term phyllotaxy is used to refer to these geometric patterns. Phyllotaxy is  best seen in buds which have preformed leaf primordia like those in Spruce (Picea).

It is possible to trace the progression of primordia that will produce helical files of mature leaves once the bud has expanded. These tracings are called Contact Parastices and can be used to describe the Phyllotaxy of the plant.

Each Apex has two opposite sets of Contact Parasticies. 

The precise phyllotactic patterns of different plants have been used to design clever experiments on the control of leaf origination and differentiation.

The Leaf Primordia are closely inserted at the apex and there is little internodal elongation in the Shoot Tip.

However, as older leaves are displaced by new leaves, the former begin to grow rapidly along with the stem where internodal elongation occurs in a coordinated manner. We will say more about leaves later.

Tunica Corpus

Flowering plants (Angiosperms) have a type of SAM architecture called Tunica-Corpus organization.

TUNICA

The outermost layer of cells divide anticlinally to produce the Protoderm.

The Protoderm is the Primary Meristem that produces the Epidermis. 

Cell layers that only divide Anticlinally are called Tunica because they cover what lies beneath it, just as a toga or tunic covers the body.

In some cases there is only one Tunica layer.

However, there may be more than one Tunica layer!

The Tunica, thus, covers the Corpus (body).

CORPUS

Cell divisions in the Corpus occur in various planes, and produce the Ground Meristem and Procambium. These two Primary Meristems produce the rest of the Primary Tissues in the shoot.

Scientists love simplicity and consistency and it would be great if this was the only type of apical organization seen in seed plants. However, there are a few complications.

In some cases there may be two or more Tunica layers. However, only the outer layer contributes to the Epidermis. The other two contribute to interior tissues.

Tunica Tunica on the Wall .....................

Gymnosperms like Ginkgo or Norfolk Island Pine (Araucaria) are also seed plants. Their apical organization can appear to be complex when the cytology of their cells, and their organizational patterns are considered.

Meristematic cells are supposed to be dividing. Dividing cells are relatively small, densely cytoplasmic & microvacuolate. 

The latter are Cytological/Histological Traits.

However, the Central Cells in gymnosperm apices are often very large with noticeable vacuoles, and they appear to divide rarely.

Various regions of these apices have received names based on their apparent division planes, their frequencies of cell division & their Cytological/Histological Traits. (Cyto = Cell & Histo = Tissue). Meristematic Cells -> Meristematic Tissue.

The Tunica-Corpus model is not sufficient to describe the architecture of these apices.

The large center-most cells in the "Corpus" have been called "Central Mother Cells".

The most apical "Tunica" Cells may have similar traits as the Central Mother Cells.

The example (Ginkgo biloba) given below  is relatively simple compared to most gymnosperms. However, it is sufficient to illustrate the general features of gymnosperm SAMs.

It is hard to delimit regions of the SAM based on cell size and staining. The outer Tunica layer is obvious in virtually all SAMs.

One can usually point to the general location of the Corpus because of the rapid enlargement and vacuolation of the ground tissues, and the densely cytoplasmic and elongated procambial cells, which distinguish them from the Corpus. However, this is rather vague and imprecise.

Non-dividing cells constitute the Quiescent Center which is coincident with the Central Mother Cells.

Mitotic cells synthesize DNA rapidly. 

When they are incubated with radioactive Thymidine (remember A:T G:C) mitotic cells accumulate more thymidine than quiescent cells.

When shoot tips are incubated with radioactive Thymidine, the cells at the very Apex, including Tunica & Corpus do not label heavily.

However, subtending cells in both regions are heavily labeled.

Experiments with several species have yielded similar results. 

When the rates of cell division were carefully and painstakingly analyzed in SAMs, it became clear that the most apical cells did not divide frequently. They were Quiescent! However, they did divide given sufficient time. These cells were called the Quiescent Center (QC).

The cells on the periphery of the QC labeled heavily with Thymidine and divided rapidly. This region has been called the Peripheral Zone.

This confounded botanists and appeared to be a contradiction. However, this actually provides a unifying concept, and that is what science truly seeks, at least that is what I believe.

It appears that virtually all multicellular SAMs have a Tunica layer on the surface of the Meristem Proper. This divides Anticlinally and gives rise to the Protoderm.

The Tunica layer(s) covers a multicellular, dome shaped region in which Anticlinal, Periclinal and Oblique divisions occur. The latter is the Corpus or Central Mother Cells (CMC).

Cells at the summit of the SAM, including both the Tunica & Corpus divide infrequently and constitute the Quiescent Center (QC). Subtending cells divide rapidly and form the Peripheral Zone (PZ).

One can distinguish various regions of the PZ based on the planes of cell division and cellular features. However, the latter is not as important as the concept of the Quiescent Center (QC) and the Peripheral Zone (PZ).

One way to think of this would be to imagine that the Quiescent Center is a multicellular Apical Cell from which all other cells are derived. This is probably botanical blasphemy but it is a valid analogy.

Recent work has shown that there is a Quiescent Center in Apical Meristems with Apical Cells! 

Apical Cells also divide infrequently BUT their Derivatives divide rapidly. 

Cells of the QC can be stimulated to divide rapidly by wounding. This can lead to the formation of two shoots. These will eventually form QCs in their SAMs. Consequently, the QC might provide a reservoir of cells to protect against damage to the apex.

Some plants like sunflower have large QCs. These plants form a large terminal inflorescence. Cells in the QC divide rapidly at the onset of flowering and produce the large floral head. This represents a special case, however.

It is known that mistakes are made in DNA synthesis after mitosis. It has been argued that if the "initials" present in SAMs divided frequently, mutations would accumulate in these cells. This could lead to abnormal growth or the production of genetically inferior gametes. The negative consequences of these are obvious.

Thus, cells in the QZ may constitute a genetic vault which produces a few derivatives during the growth of a stem but hoards most of its genetic assets. The derivative cells divide furiously until they are exhausted, at which time the vault releases some fresh currency. When flower production is induced, the vault may release all of its genetic assets which divide and form the floral organs. This would include the gamete-forming tissues in the anther and ovary.

To confound matters, it appears that some plants do not have Quiescent Centers in their SAMs. There is active cell division throughout these shoot apical meristems.

Recent Studies indicate that cell divisions in Apical Meristems are controlled by the Pressures that develop in Shoot and Root Apices.

Root Apical Meristems can have Quiescent Centers .

It is thought that this QC gives the root tip the ability to survive severe injuries to the Root Cap and the Peripheral Zone.

Indeed, isolated QCs of roots can regenerate complete roots.