Stems vary in structure and may be classified into types such as woody vs. herbaceous. This classification is artificial, because there is no sharp line of demarcation between these two, and sometimes the differences are of a purely quantitative kind. The chief differences between stems are based on the spatial relationship between the vascular and nonvascular tissues, and by the relative amounts of secondary growth.

Stems are more complex in structure than roots mainly because of the complexity of the primary vascular system. This is due to the fact that several vascular bundles may diverge into the leaves. These bundles which are outside of the central ring of vascular bundles are called leaf traces. The size and complexity of leaves is correlated with the complexity of the vascular system in the stem. This lab will consider stems which trace their origin directly to the apical meristem. This is known as primary growth. Some plants stop growing at this point but others many continue to grow due to the activity of lateral meristems. This type of growth is called secondary.

During secondary growth the resemblance between roots and stems increases as the secondary xylem and phloem of roots and stems show many similarities.

Gross characteristics of shoot tips. We can study fresh preparations of shoot tips from Elodea, Artocarpus (breadfruit), Ficus, Apium (celery) or Coleus species.

• Dissect the shoot tip of Elodea under a stereo scope.

• Take a vigorous stem and remove a terminal stem of approximately 4-cm.

• Remove the leaves from the basal 2-3 cm..

• Blot this dry and embed it in a piece of modeling clay.

• Attach the clay to a plastic Petri plate base so that the stem is erect, and place this on the stage of a dissecting scope.

• Start removing the most apical leaves carefully with forceps.

• The leaves should get smaller and smaller as you approach the shoot tip.

• Increase your magnification as you proceed.

• You may want to switch to a dissecting needle to remove the smallest leaves by applying gentle pressure.

• You should eventually notice a smooth, shiny, rounded apical meristem with tiny leaf primordia.

• Examine a commercial slide of Elodea and identify the structures cited above.

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The Apical Meristem and the Origin of Tissues

The following tissues and structures may be distinguished:

1. apical meristem;    

2. leaf primordia;    3. protoderm;

                     4. procambium; 5. ground (rib) meristem;

6. axillary buds

• Observe the size increases of successively older cells.

• Note differences between cells of the procambium and ground meristem in terms of shape, size, and contents.

• Also find the rib-meristem which produces the pith.

The term Rib Meristem is applied to Ground Meristem which produces the Pith. It is characterized by transverse divisions which produce very orderly cell files. The Ground Meristem that produces the Cortex is not always easy to pinpoint. In my mind both are Ground Meristem but you should understand the term Rib Meristem.

Apical meristem with a single Apical Cell: Equisetum (horsetail) Seed plants have multicellular apical meristems. However, non-seed plants like ferns can have a large Apical Cell which acts as the source of all cells.

Apical Meristems with Tunica-Corpus Organization.

The tunica, is one or more superficial layers that show only anticlinal (perpendicular to the surface) divisions.

The corpus is a group of cells covered by the tunica. The corpus is characterized by divisions in many planes. This adds volume to the stem as its derivatives enlarge.

The actual number of tunica layers is not always clear. It will be sufficient to discern meristems with One or Several tunica layers.

The corpus can also be hard to delimit. However, when there is a prominent rib meristem, the corpus is circumscribed.

• Elodea has a single-layered tunica.

• Ricinus has two layers in tunica. (No Illustration shown)

• Forsythia, has at least three layers in tunica.

The terms cytohistochemical zonation mean that different groups of cells (cyto) respond in a distinct way to various stains (histochemical) .

• Observe demo of Ginkgo and locate the central mother cells.

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Dicot Stems

Dicot stems usually have one ring of vascular  tissue in stems. The vascular cylinder is usually composed of individual vascular bundles.

• Study Helianthus (sunflower) stems in two stages of development.

• The epidermis is typical and stomata may be present.

• The cortex is composed of parenchyma with abundant intercellular spaces.

• Discrete vascular bundles occur in the young stem.
  Note the immature fiber bundles towards the outside of the phloem. These fibers arise from the same part of the procambium as the primary phloem. The fibers eventually develop lignified secondary walls.

• Note the degree of fiber development in the older stem.

• The Interfascicular parenchyma is present between the vascular bundles.
  The bundles are collateral and contain primary phloem and primary xylem.
  Mature and immature tracheary elements may be present. The immature tracheary elements occur between the differentiated cells of the phloem and xylem.

• Compare the degree of vascular development in young and old stems. In young stems the vascular meristem is called procambium. Later, undifferentiated procambium may divide periclinally (parallel to the surface). This is called the vascular cambium. The vascular cambium is a lateral meristem. It produces secondary xylem and phloem. The accumulation of secondary xylem increases the girth of the stem.

Secondary vascular development is limited or absent in Helianthus. However, the vascular cambium may spread to the interfascicular areas. The fascicular and interfascicular cambia can unite and form a continuous ring which produces cylinders of secondary xylem and phloem. The primary xylem is displaced by the secondary xylem and is pushed into the pith.

The pith is composed of parenchyma cells.

• Study hand sections from the upper and lower parts of Widelia stems.

• Stain with Phloroglucinol and Toluidine Blue.

Widelia Stem stained with Phloroglucinol

Widelia Stem stained with Phloroglucinol & viewed with crossed Polarizers

• Compare these with the commercial slides of Helianthus.

• Do the same for Coleus or another member of the Lamiaceae (Mint Family)

Cross section of Coleus stem stained with Phloroglucinol

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Monocot Stems

• Study prepared slides of Saccharum (sugar cane) or Bambusa (bamboo) stems.

Sugar Cane Stem stained with Phloroglucinol	Sugar Cane Stem seen with Crossed Polarizers

How does the arrangement of vascular bundles compare with dicot stems?

Are the bundles collateral, bicollateral, amphivasal or amphicribral?

• Locate protoxylem and metaxylem? How does the cavity in the xylem develop?

• Try to identify the sieve tubes and companion cells in the phloem. Locate the fibers associated with the vascular bundles.

Cross Section of Makaloa stem: Note the widely distributed Vascular Bundles	Makaloa stem stained with Toluidine Blue

• Examine cross sections of Makaloa (Cyperus laevigatus) stems.

• View unstained sections and sections stained with Toluidine Blue.

Note the widely distributed vascular bundles. This is similar to bamboo and sugar cane.

Sugar Cane Vascular Bundle stained with Toluidine Blue

Makaloa Vascular Bundles stained with Toluidine Blue

Air cavities are present in Makaloa which is a semi-aquatic plant. What term is used to describe tissue like this?

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Makaloa used to make mats for the Hawaiian ali’i.

Piece of a Makaloa Mat displayed at the Bishop Museum: The symbol means hale (house)
The whole stem was used to weave the mats.

The mats have a smooth & spongy quality which makes them good for sleeping.
What anatomical features of makaloa account for the above qualities?

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Taro (kalo) [Colocasia esculenta] is one of the principal food crops of Hawaiians. They use the starch rich stem of mature plants to make poi.

• Examine the demo slide of an immature kalo (Taro) stem.

• Note the abundance of parenchyma in this stem.

• There is little starch in this slide because the stem was so young when it was harvested.

• Locate the vascular bundles and note their distribution.

Is this plant a dicot or a monocot?

Cross section of a young kalo stem: The area indicated by the box is shown at higher magnification below.

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Intercalary Meristems

Many monocots have Intercalary Meristems. Intercalate means to insert between. In this case a meristematic layer occurs BETWEEN the Node and the Apical Meristem.

The Apical Meristem produces a small number of derivative cells towards the base. Those closest to the apical meristem enlarge and differentiate. Those more distal continue to divide and produce derivatives towards the apical meristem. This is the Intercalary Meristem.

These occur just above the nodes and function for a limited period and are thus said to be determinate. They are similar to the basal meristem which produces the strap like leaves of monocots. These are responsible for the re-growth of grass after it is mowed!

• Examine prepared slides of Equisetum.

• Locate the youngest nodes follow the cell files acropetally (towards the tip).

• You should see that the least differentiated cells near the node and the most differentiated cells towards the apex.

• Study the Illustrations on the next page to help you visualize how the intercalary meristem works.

• Follow the cell files indicated by the stars. Start at the node (bottom of the image) and follow cell files towards the top. You should see that cells enlarge as you scan from the bottom to the top. Note the different shapes which occur in the different cell files. Some undergo extensive elongation parallel to the vertical axis. Others enlarge in the transverse plane and some cells enlarge equally in both directions.

Node-Internode of Equisetum: Follow the Cell Files from Bottom to Top (Acropetally)
Embossed Images of Equisetum Node-Internode

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Vascular Bundle Types

Vascular bundles are classified according to the spatial relationships of their xylem and phloem.

Collateral bundles (Widelia, Helianthus, Coleus, Saccharum, and Cyperus) have Xylem on one side and Phloem on the other side.

Bicollateral bundles (Cucurbita) have phloem on both sides of the xylem.

Concentric: There are two types of concentric bundles

• amphicribral - phloem surrounding the xylem (Fern Rhizome)

• amphivasal - xylem surrounding the phloem [Cordyline (ti)].

Amphicribral Bundle stained with Phloroglucinol	Amphicribral Bundle stained with Toluidine Blue	Amphivasal Bundle with commercial staining

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Nodal Anatomy

The study of nodes reveals the association between the vascular system of the stem and the vascular bundles of the leaves. The part of a vascular bundle in the stem that diverges from the vascular cylinder and enters the leaf is called leaf trace. In the location where the leaf trace is bent towards the leaf, a relatively wide parenchymatous area appears in the vascular cylinder of the stem. This area is the leaf gap. Different plants have different numbers of leaf traces per leaf and different number of leaf gaps per node.

• Make serial sections of Nerium (Oleander), Coleus or Hibiscus.

• Cut thick serial sections through a node.

• These transverse sections must include some of the internode above and below the node as well as the node itself.

• Arrange the sections in series on a microscope slide or Perti dish .

• Observe with a dissecting microscope.

• Stain these with Phloroglucinol. This will emphasize the location of the xylem and make the vascular bundles easier to see.

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Finally!!!!!!!! Now that you understand leaf gaps at the macroscopic level, consider them at the microscopic level in the region of the apex where leaves are initiated!!!!
NO!!!! No!!!! I can’t stand any more!!

Remember that the procambium forms a complete circle above the level of leaf initiation in some shoot apices.

The circle becomes interrupted as leaves are formed and develop.

These interruptions are leaf gaps.

The dissected vascular cylinders of dicots arise in this fashion.

• Observe the demo slide of a Coleus shoot tip cut transversely in the region of leaf initiation.

• Locate the ring of procambium and the leaf gaps!

  Cross Section through Coleus Shoot Tip

  Coleus has opposite leaves. Thanks a lot Drill Sargent!!!!!!!