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 represents primary growth.

Some plants stop growing at this point but others many continue to grow due to the activity of lateral meristems. This latter 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. We will study secondary growth later.

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 segment of approximately 4-cm in length.

• 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, lateral pressure so that they snap off.

• 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 parenchyma cells produced by the Rib Meristem have an isodiametric outline and they develop large vacuoles early in their development.

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 easy to locate just above the Rib Meristem.

Observe the following Shoot Apical Meristems

• Equisetum has an Apical cell

• Elodea has a single-layered Tunica.

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

• Forsythia, has at least three layers in Tunica.

The term cytohistochemical zonation means that different groups of cells (cyto) respond in distinct ways to various stains (histochemical) . Put them together and what have you got?

• 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.  We will explore this later!

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.

• Compare these with the commercial slides of Helianthus

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

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

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.

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

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

The mats have a smooth & spongy quality which makes them good for sleeping but hard to weave.

What anatomical features of makaloa might account for the above qualities?

• 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.

• 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

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

• 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.

• 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!!!!!!!