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xercise 2: the stem - variation in structure

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Core Objective:                                                     

 

Understanding basic variations in stem structure

 

 

Introduction                                                   

 

 

 

 

 

   Core specimens                                                    

 

 

 

 

Stems, as is the case with roots, are composed of three tissue systems. These are the dermal system, the fundamental or ground system, and the vascular system. Like all plant parts, the stem is composed of a large number of cells which are organized into distinct tissues. The stem has a more complex structure than the root. In the first place, it bears leaves , and is divided into nodes and internodes. In addition, it has to withstand the elements in the physical environment, and has had to develop or enhance support structures and mechanical tissues to stabilize the stem itself, and to allow resistance to breakage.

 

There are many ways in which mechanical support is achieved -- perhaps the simplest is the formation of a hollow stem, common in many herbaceous plants. Annuals and perennials tend to develop more complex support systems. Many of these mechanical support systems will involve deposition of a secondary wall - this wall could be primary or it could be secondary. Support tissues with secondary walls are more costly to produce. Amongst the monocotyledons and several perennial dicotyledons, a ring of  supporting or mechanical tissue is often found just beneath the epidermis, or forming part of the outer or inner cortex.

 

Development of the primary tissues within the stem are controlled by the shoot apex, The shoot apex is itself, a complex structure. The shoot apex produces primary tissues, from which leaves, stem tissue as well as lateral buds are formed. At first glance, the system seems to contain very similar cells to other regions of a plant. The actual zones that are involved in cell division are very limited. The most important can be recognized in the apical dome, as is illustrated in the longitudinal section, between the two young developing leaf primordia in the Coleus shoot apex illustrated in this exercise.. The meristematic zone is divided into two layers usually, an outer layer that is responsible for formation of the outer regions of the plant, including the first-formed protective layer, called the protoderm and beneath this, a region of thin-walled cells, which will form the cortex. Look carefully, and you should be able to recognize a region of meristematic tissue called the procambium. The procambium is responsible for the formation of the first vascular tissues -- the protoxylem and protophloem -- which occupies the inner region of this young developing stem. This region is called the stele, which forms the central core of the stem. So, whilst the shoot apex appears at first glance to be a relatively simple structure, it is actually a very complex and important functional part of the plant, responsible for forming the many cell types found in the aerial parts of the plant. As a result, the form, morphology and anatomy  of the stem apex may become highly modified to suite the plant.

 

Young stems of conifers and dicotyledons develop a vascular system within the internode, which essentially  looks like a hollow cylinder. This hollow cylinder, forms the boundary between the cortex and the stele.  Adding to the complexity, is that the developing stem apex is intimately bound up with the formation of leaf primordia. Because of the presence of a branching vascular supply to the leaves, the internal structure at the node -- where the vascular traces from the leaf enter the stem, prior to joining the vascular traces of the stem -- is much more complicated than at the internode, which forms the majority of the stem structure. However, it is naive to think that the vasculature of stems is 'easy' to understand. Indeed it is very complex, and the complexity is compounded by the fact that stem anatomy is profoundly influenced by the vascular traces emergent from leaves.

 

The cortex of the stem usually contains more tissue types than the corresponding regions in the root. This is a feature that may be related to the aerial habit of the stem, as well as its mechanical function. Thus we may find stomata in the epidermis, and chloroplasts in the cortical and sometimes even in more deeply-seated parenchyma. Mechanical tissue is usually present. Sometimes abundant, in the form of collenchyma and sclerenchyma. In addition, great variation in the detailed structure of stems of different species may be observed. As all primary stems develop as a result of the programmed cell division that takes place in the shoot apical meristem, it is wise to look first at the structure of a typical shoot apex.

We have included a shoot apex, as this example serves to illustrate several key features of the shoot apex, principal of which is that the  apex lies close to the surface of the developing shoot tip, and is protected by a protoderm only and by the unfolding leaves that have been produced by the apex. A range of anatomies is illustrated here - dicotyledons as well as monocotyledons are included.

As background to this exercise, as well as to that dealing with root and leaf anatomy, we have included short overviews dealing with basic structure and function.

 

 

It is not advisable to look only at at cross sections of stems, leaves or roots in isolation, as one cannot build up a good image of what the structure really looks like, or how it functions. Each of the exercises dealing with the anatomy of stems, roots and leaves, are hyperlinked to short introductions dealing with the structure of the organ, as well as a brief overview of the transport systems

Click here for the stem overview

Click here for the transport overview

 

 

 

The specimens that we have chosen to illustrate, do not necessarily present any strict evolutionary progression or  development in terms of anatomy. They have been chosen simply because we believe that the majority of the species will be available in most teaching environments.

We have also provided additional images of roots, leaves and stems which can serve as a basis of comparison or discussion. These images can be accessed by simply clicking here, or by following the link to 'Digital Plant Anatomy' on the virtual plant contents page.

 

1.  Coleus shoot apex -LS.

2.  Bidens, TS young & mature stem,

3.  Virgilia TS young stem

4.  Trifolium TS young stem

5.  Bignonia TS stem

6.  Pelargonium stem.

7.  Solanum (potato) stem

8.  Nymphoides TS petiole        

9.  Cucurbita stem, TS

10. Zea mays TS stem

11. Pinus (the pine) stem

 REFER TO TECHNIQUES: 2, 3, 4, 7, 8, 9 for information on drawing and sectioning

 

    Roadmap                                                   

                

 

 

 

Simply click on the thumbnail images to the right, to go to the detail pages. coleusapex1.jpg (6284 bytes)

Bidenst1.jpg (5206 bytes)

 

Coleus apex

Young Bidens

Virgilia stem Trifolium pratense stem Bignonia stem
pelargst1.jpg (6649 bytes) solanstm1.jpg (7692 bytes)
Pelargonium Solanum Nymphoides

Cucurbita

Zea mays

Pine

 

 

Clicking on the  images above, will direct you to new pages which contain more detail

STUDENTS: You would need a sharp, new razor-blade if section cutting is contemplated.

  Text Book                                                    

 

 

Cutler, Botha and Stevenson, Plant Anatomy, an Applied Approach and Raven, Evert and Eichhorn, Biology of Plants (6th or later edition) are highly recommended for additional background information.

 

1. Coleus shoot apex                                               

 

 

Click for LP and detail of Coleus shoot apex

CLICK on the image to have a look at a series of high-resolution micrographs which show more details, and try to identify the following:

coleus

1. The  promeristem - is a region of apical initials, plus their incompletely differentiated derivatives.

2. Tunica- note the anticlinal walls

3. Corpus- note anticlinal and periclinal walls Leaf primordia - in various stages of development

4. Protoderm- derived from the tunica and destined to form the epidermis . The procambium -

 columns of narrow, elongated cells, derived from the corpus and destined to differentiate into the

 vascular tissue

5. The ground tissue - in the position which will be occupied by the future cortex and pith

First mature tissue - note particularly the differentiation of the vascular tissues from the procambium.

 

 2. Bidens pilosa stem                                              

 

 

 

Bidens TS young stem,

Bidenst4.jpg (6323 bytes)

Bidens, vascular cambium development

Bidens pilosa is an example of a weedy species and is a member of the Asteraceae. It may grow to 1-2 meters in height, and the stem undergoes limited secondary growth. This means that most of the new cell production is limited to the vascular bundles - sometimes referred to as fascicles. The stems will produce other mechanical tissues - called  which are usually associated with the vascular bundles, just outside of (exarch to) the phloem. - and associated with this, fiber development occurs as well. There is limited interfascicular activity.

The outer cortex in Bidens does not have perivascular fibers as do Virgilia or Pelargonium species.

 

 
 The micrograph to the right shows a stem in which the development of secondary vascular tissues is very evident. Cell division from the vascular cambium had produced a large number of secondary xylem (2XV) - some of these cells are without a secondary wall. Both primary (1P) and secondary phloem (2P) are visible beneath the the deep red-stained primary phloem , which may be referred to as perivascular .

Note the relatively thin cortex and a single-layered epidermis (E).

Click here for other micrographs which show more details of this stem.

There are several points to note about this stem.  -

 

  1. Epidermis. - fairly thick cuticle, with numerous epidermal hairs (trichomes).
  2. Outer cortex - collenchyma, varying from 3 to, at the corners of the stem, 6 cells deep. Examine the water mount and note that the thickened primary walls show up clearly.
  3. Inner cortex - consists of parenchyma, containing numerous chloroplasts. This zone varies considerably in width, but does not usually extend round the corners of the stem.
  4. Endodermis., or starch sheath - a single layer of rather large cells, forming the inner limit of the cortex. Examine the water and iodine mounts - note that the cells contain abundant starch, but do not contain chloroplasts.
  5. A ring of vascular bundles - of varying size, embedded in interfascicular parenchyma and enclosing a parenchymatous pith, the parenchyma is derived from the ground meristem.
  6. Note that the xylem and the phloem lie on the same radius, within the confines of each vascular bundle - this is a totally different arrangement from that in the root.
  7. Primary phloem  - these are derived from the protophloem. This forms the outer tissue (these cells should be stained red in the Fabil section) of each vascular bundle.
  8. Metaphloem- small cells (stained blue) lying between the phloem  and the xylem. Note very small, deeply-stained companion cells and the rather wider sieve tubes.
  9. Metaxylem - (stained red here) of wide-diameter metaxylem vessels surrounded by lignified parenchyma.
  10. Protoxylem - of narrow-diameter protoxylem vessels (stained red here), surrounded by both lignified and non-lignified xylem parenchyma. Note that the protoxylem lies towards the centre of the stem and is thus described as being endarch.
  11. Note the vascular cambium is entire, and has produced secondary phloem -and secondary xylem.
  12. A Pith - of very large, thin-walled parenchyma, with conspicuous intercellular spaces.

 

 

 

   3. Virgilia stem                                                       

 

 
 
Virgilia is an example of the pea family Fabaceae, and is common in many floras. It is very similar in structure to Trifolium as it to, is herbaceous. Click here for a more detailed view of this stem.

 

 

 

 
  4. Trifolium stem                                                     
 
 
 
 

Trifolium (red clover) is an economically-important legume (Fabaceae).  The example shown here is of a mature stem at the end of primary growth, that is, the vascular bundles contain very limited amounts of secondary xylem and  secondary phloem.  The cortex is very narrow and is composed of chlorenchyma. The cortex is separated from the vascular bundles and the underlying pith, by a starch sheath. The pith is parenchymatous.

 

Click here for a more detailed image of this stem

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

   5. Bignonia stem                                                    

 

 

The dicotyledonous family Bignoniaceae contains plants that predominantly vines, but herbs some shrubs and trees also occur. The stems exhibit variable secondary thickening. Click here for more details and an example of the anatomy of a Bignonia stem.

 
 
 
 
 
 
 
 
   6. Pelargonium stem                                             
 
 
 
 
Pelargonium is an example of a herbaceous plant in which secondary growth occurs within the stem and is thus  representative of species that undergo secondary growth. It makes an ideal study plant, as many features such as the development of a periderm, perivascular fibers and interfascicular as well as fascicular  cambial activity can be demonstrated using Pelargonium species. The micrograph to the left shows a cross section of a stem which has completed its primary growth phase, and is at the onset of secondary development.
This image shows a detail of developing perivascular fibers immediately exarch to the primary phloem tissue.

Question: What is the origin of these perivascular fibers?
 
 

CLICK on the image to see this stem in more detail

 
 

7. Solanum tuberosum (Potato) stem                 

 
 
 
 
The potato (Solanum tuberosum)  is one of few families that have phloem which is located external to the metaxylem (and to the fascicular cambium when this is formed) as well as internal to the protoxylem.  If you do not look carefully, you may miss the internal phloem, which is separated from the protoxylem by a few rows of parenchyma. The internal phloem is formed by procambial strands  although it is not clear if these are remnants of the procambial strand that would have differentiated of the more 'normal' collateral component of this vascular bundle, or if the cambial layer is reformed, prior to differentiation of the internal phloem. Click on the image to get a more detailed view of this unusual anatomy in more detail.

Click here for a more detailed look at the vascular tissues

Self assignment: Make notes which highlight  the differences between the structure that you can see in Solanum's stem structure to that which you can see in the structure of the vascular bundle of Cucurbita stem.
 

   8. Nymphoides petiole                                         

 

 
 

Nymphoides is a plant of contrasts. The rooting system and petioles are submerged and functional leaves float on the surface of the water, The adaxial leaf surface is exposed to sunlight and given that the abaxial leaf surface is in direct contact with water, whole leaf surface temperatures are unlikely to become too stressful.  Because of its continual exposure to direct sunlight, this hydrophyte will possibly have a high rate of photosynthesis.

The vasculature is 'typical' of a hydrophyte, in that the xylem is much reduced, and remains functional to traffic transpiration water as well as nutrients. The phloem on the other hand, occupies a significant proportion of the cross-sectional area of the vascular bundle , and contains many large sieve tubes and associated companion cells.

 

 

 

The illustration to the left shows part of the petiole of Nymphaea. Note the large intercellular spaces which occur in the petiole. These airspaces are surrounded and delineated by long finger-like columns of parenchyma cells. The columnar cells contain chloroplasts and are presumably photosynthetic. Part of a large astroscleried can be seen cutting across one of the parenchymatous fingers. These sclereids add some mechanical strength to an otherwise fragile structure.

 

CLICK on the image below to have a look at a series of high-resolution micrographs which show more details

 

 

The section to the left, shows most of the central vascular tissue of the petiole. The upper region essentially mirrors the lower region, in which only part of the primary xylem is visible.  The protoxylem faces the centre and metaphloem occurs exarch to the metaxylem strands.

Why do you think that there is more phloem tissue visible, than xylem?

 Click on this image for a higher resolution view of the same area.

 

Note the following points about this specimen

 

a. The lack of a cuticular layer,

b. The very reduced cortex, immediately beneath which is

c. A ring of vascular bundles.

d. Much of the central region of this section is filled with airspaces, with

e. Thin unicellular strands of specialized parenchyma, termed aerenchyma separating the individual

f.  Airspaces within the stem.

 

 

The vascular tissue occupies the central region of this specimen. Depending on where the section was cut from, the petiole may contain two central mirrored vascular strands.  The vascular bundle in the above micrograph, is surrounded by a starch sheath. If you can access more basal sections, then the vascular tissue may be surrounded by an endodermis which contains  strips. More basal sections will possibly contain a pericycle as well.

 Try to draw a low power diagram, which shows the distribution of the tissues in this section.

 

 

  9. Cucurbita pepo (pumpkin) stem                     

 

 

 

Cucurbita is an herbaceous dicotyledon, but the stem structure differs in many respects from that of Bidens. Some of the features are typical of climbing plants - e.g. - the rather wide-diameter vessels and the broad interfascicular regions. Cucurbita is a fast growing herb, which has hollow stems and petioles, so not much carbon resource is allocated to the production of lignified mechanical tissue.

 

 

cucsievep2.tif (1478552 bytes) cucsievep.jpg (3164 bytes) The external phloem in the cucurbit stem is composed of sieve tubes, companion cells and associated phloem parenchyma cells. Sieve plates  are near transverse, and contain numerous sieve plate pores which can easily be seen in the micrographs to the left.

 

internal phloem Sieve plate, external phloem

 

CLICK on the images above, for more details

 

NOTE the following points about this stem :-

 

  1. The epidermis - multicellular hairs occur at intervals.

  2. The outer cortex, with alternating blocks of collenchyma and chlorenchyma.

  3. Perivascular .

  4. A broad zone of large parenchyma cells, separating perivascular fibers from the phloem.

  5. The stele - contains a number of very large vascular bundles. Each bundle has phloem on both the outer and inner face of the bundle. The bundle is therefore described as being bicollateral.

  6. The phloem - at the outer side of the outer phloem group and at the inner side of the inner phloem group, is the protophloem, consisting of rather small cells, which are somewhat flattened radially. The main bulk of each phloem group consists of metaphloem. Notice the very wide sieve tubes, some of which may show a sieve plate. Note also, the narrower, deeply- stained companion cell cells.

 

The following points should also be noted:

 

  1. The Metaxylem - contains very wide vessels, surrounded by parenchyma.

  2. The Protoxylem - consists of a group of narrower-diameter vessels, in the endarch position, but exarch to the inner phloem.

  3. The Interfascicular parenchyma - thin walled parenchymatic cells.

  4. The Pith - is mostly occupied by an air-filled cavity, as the stem is hollow.

 

Question: Many of the features that you can see in the accompanying micrographs point to occupation of a specific environmental niche. As an example, comment upon features that suggest that Cucurbita does not normally tolerate stressful conditions.

 

 

   10. Zea mays stem                                        

 

 

 

 

Zea mays is an important  crop plant, and its structure has been  well studied.  Maize is a  monocotyledonous plant, and resembles other grasses in the arrangement of tissues in the stem leaf and root.  The stems of monocotyledons generally have a single ring of vascular bundles immediately beneath the epidermis, and internal to this a system of vascular bundles that are scattered throughout the pith. The peripheral vascular bundles are those that immediately join the leaf traces, and, as such any differences in the structure of the superficial vascular bundles and the deeper-seated bundles, might reflect some of the known structural components associate with the leaf vascular bundles. Please look at the leaf assignment, and compare the structure of the maize stem and leaf vascular bundles. How similar are stem and leaf vascular bundles? Are there any obvious differences that you can see?

 

 

 

CLICK on the image to have a look at a series of high-resolution micrographs which show more details

of this stem's structure.

 

The anatomy of monocotyledonous stems is very different from that of the dicots which you have examined up to now.  Zea mays is a fairly 'typical' monocot stem.

Note:
  1. Epidermis - with fairly thick, cuticularized walls.
  2. The vascular bundles - note that they do not form a single ring, but develop in a series of spirals, from near the centre of the stem, out towards the periphery. This arrangement, sometimes referred to as "scattered vascular bundles" is common amongst monocotyledon stems - however there are many (e.g. in the grasses) where just two rings of bundles exist.
  3. Ground parenchyma - surrounding the vascular bundles. As the stem matures, the cells in the outer part of this zone become thick-walled (look at your slides) and are lignified. in an old stem, nearly all the vascular bundles may be embedded in lignified parenchyma.
  4. Bundle sheath - in mature bundles, this can consist of of lignified parenchyma.
  5. Phloem - Sieve tubes (wide in cross section) and companion cells (narrow, and often darkly stained) form a regular crisscrossed pattern, which is again, rather typical of monocots. Most of the phloem is the latter-differentiated metaphloem, but you may also see the remnants of protophloem occurring as an irregular green line, towards the outer face of the phloem, beneath the bundle sheath cells
  6. metaxylem - consisting of wide vessels, a few narrow tracheids between them and also some surrounding non-lignified parenchyma.
  7. protoxylem - consisting of one or two medium diameter vessels and surrounding parenchyma. In some bundles, a cavity will be observed in the protoxylem, sometimes referred to as a protoxylem canal or protoxylem lacuna, which represents the position previously occupied by the first-formed protoxylem elements.

 

Whilst the number of xylem vessels, tracheids, and parenchyma cells varies from bundle to bundle, the  tissue relationship and the  arrangement of cell and tissue types within the vascular bundles pattern remains the same, with phloem exarch to the xylem in this species.

 

   9.  Pine Stem                                                         

 

 

 

 

The Gymnosperms are an important study group, as they are of great economic importance.  Structurally, Gymnosperm stems differ from the dicotyledonous and monocotyledonous examples shown earlier, in that the vascular tissues are more primitive. The phloem for example, lacks sieve tubes and companion cells, where the conduction of carbohydrate is managed by sieve cells. The sieve cells are accompanied by albuminous cells and associated parenchyma cells. In contrast to the dicotyledons, gymnosperm wood contains only tracheids, and no vessels. All gymnosperms contain resins of some kind, which is transported in  resin ducts or resin canals, which are usually associated with the cortical tissues in the stem.

 

The Young pine stem

The transection to the left is of a young Pine stem, cut transversely near the shoot apex. Note the multilayered epidermis (EP). At this stage of development, you can see individual vascular bundles, as well as resin canals (R) above the vascular bundles. 

Click on the image to see a more detailed micrograph.

 

 

The image to the right, shows a resin canal (RC) which is embedded within the secondary xylem. Numerous parenchyma cells (P) occur between the secondary xylem tracheids (T) and connect to the thin walled cells, called epithelial cells (E). These cells are involved in the secretion of the materials that are transported within the resin canals. Current research suggests that resin canals may offer resistance to bark beetles amongst other insects.

 

See Rosner, S; Hannrup, B (2004)  Resin canal traits relevant for constitutive resistance of Norway spruce against bark beetles: environmental and genetic variability. Forest Ecology and Management. 200, no. 1-3, pp. 77-87. 25 Oct 2004. [An interesting paper dealing with resistance in Norway spruce to bark beetles].

 

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