xercise 6: Anomalous Growth in Plants

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  Core Objectives:                                                     

 

 

To learn more about anomalous growth

 

 

   Introduction                                                            

 

 

 

 

Whereas the development, arrangement, activity of the vascular cambium in most woody dicotyledonous and Gymnospermous plants tends to be very similar, there are some alternatives which produce new secondary tissues that do not follow a normal pattern. As a result, the secondary plant structures that are formed are termed  anomalous. Most anomalous growth is associated with the formation of multiple cambia.`

 

 

To improve understanding the concept of anomalous secondary growth in plants,

To improve identification of cells and tissues within anomalous structures.

 

Core Species                                                             

 

  1. Beta vulgaris  root & stem.

  2. Daucus (carrot) root

  3. Dracaena TS stem

  4. Young and old Bougainvillea stem

  5. Beta  TS stem

  6. Campsis TS stem

  7. Boerhaavia TS stem

  8. Dicranopteris TS stem

  9. Serjania TS stem

 

   Text Books:                                                            

 

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

 

   Anomalous Secondary Growth             

 

 

 

The word anomalous means deviating from the general or common order or type. Thus, the term,  anomalous growth reflects a growth condition which is not commonly seen and which is present in a limited number of families or genera. This exercise explores a few examples of anomalous growth, bear in mind, there are many to choose from! The examples here illustrate aspects that are common - and include multiple cambia, included vascular bundles, and multiple vascular cylinders.

 

 

 

 

 

 

 

 

 

  Road Map                                                                 

 

beta_rt1.jpg (6764 bytes)  bougainyst.jpg (7115 bytes) beta4stm.jpg (6658 bytes)

Beta TS root

Carrot root

Dracaena

Bougainvillea

Beta TS Stem

 

Campsis stem

Boerhaavia stem

Dicranopteris TS rhizome

Serjania stem

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   1. Beta TS mature root.                                        

 

 

 

beta_rt1.jpg (6764 bytes)

Examine the accompanying micrograph of Beta vulgaris root. As in the stem, the root of Beta shows anomalous secondary growth patterns. This is evidenced by the formation of successive supernumerary cambia each of which gives rise to a ring of vascular bundles and wide zones of parenchymatous tissue between these bundles. Note that the successive bundles are arranged more‑or‑less along the same radius as preceding ones.

 

 

Click on the image to have a look at high-resolution micrograph of the Beta root

 

   2. Daucus - the carrot root                                   

 

 

Carrot roots undergo limited secondary thickening, but as can be seen in the accompanying photomicrographs, this secondary growth is unlike that seen in normal secondary growth in roots. The carrot, like beetroot, forms successive cambia, and multiple rings of vascular bundles.

 

 

Click on the image to look at a high-resolution micrograph of this root

 

     Anomalous secondary growth in stems        

 

 

 

         Monocotyledons                                              

 

 

 

Examine the accompanying micrograph of Dracaena stem. Can you locate the position of the cambium, secondary cortex, and "secondary" vascular bundles? Dracaena (the Dragon's blood tree) is the only monocot which has been shown to also have secondary growth in roots. Another alternative is to look at  Cordyline stems to get an indication of the distribution of mature secondary tissue. 

 

    Dracaena stem                                                     

 

 

 

Palm trees are monocots that grow quite tall and thick, yet they lack "normal" secondary growth. Dracaena is a monocot but not a true palm, as palms lack the peripheral secondary thickening meristem such as is found in Dracaena and Cordyline. This meristem produces both new vascular bundles and ground tissue (parenchyma). Dracaena is an unusual plant, in that the vascular bundles are surrounded by very prominent fibre bundles. In this sense, Dracaena is not anomalous. The stems undergo a specialized secondary growth, which manifests itself in the production of additional parenchymatous elements. Their later growth pattern is termed diffuse secondary growth, and consists mostly of a proliferation of ground parenchyma cells and additional vascular bundles near the periphery.

 

 

 

       Dicotyledons                                       

 

 

 

    1. Bougainvillea stem                                          

 

 

 

bougainyst.jpg (7115 bytes)

bougain2.jpg (5974 bytes)

young stem

old stem

 

 

 

 

 

 

 

 

 

 

 

 

Click on the images to see high-resolution micrographs for more details

 

Bougainvillea is a member of the Nyctaginaceae and is an example of a dicotyledonous stem which displays anomalous secondary growth.  In this TS, near the centre of the stem, you will see some primary vascular bundles embedded in lignified pith parenchyma. Move the slide towards the outer regions, and you will notice that there has been fairly extensive production of secondary vascular tissue. Look for the vascular cambium. Secondary phloem and secondary xylem lie on either side of it. The secondary xylem is composed of tracheids, fibers and narrow-diameter vessels. Interspersed with the secondary xylem you will be able to see small pockets of phloem and what look like large-diameter metaxylem vessels. These are reminiscent of the primary bundles towards the centre of the stem. These are in fact primary vascular bundles embedded within the secondary xylem, hence the use of the term, anomalous growth in this instance. The phloem is described as being included phloem, which by definition is phloem tissue which lies between  regions of secondary xylem. Whilst the physiological advantage of the formation of included phloem has not yet been studied, one could speculate that in this instance, the included phloem would be well-protected from predators and pests and, of course, be well-supplied with water and nutrient. The anomalous growth results as as a result of differential cambial activity. Newly-produced vascular cambia result in the outer lateral meristem becoming  quiescent, and this cambium returns to activity only when the internal vascular cambium (which produce the individual embedded bundles) become less active. Vascular cambia are said to not produce rays in Nyctaginaceae (lateral meristems do), but do produce vessels and associated, axial parenchyma and sometimes fibres to the inside and variable secondary phloem to the outside.

 

Notice the epidermis, which is composed of small, rather thick-walled cells. In places you may see that periderm initiation has commenced immediately beneath the epidermis and exarch to the chlorenchymatous photosynthetic cortex. (see Carlquist, S. 2004:  Bot. J. Linn. Soc.146:, (2) 129-143) for a discussion on anomalous growth in this family.

Click here for a more detailed look at this stem.

 

     2. Beta  stem                                                         

 

 

 

beta4stm.jpg (6658 bytes)

The beetroot stem is anomalous, in that there are several layers of  primary vascular bundles visible towards the centre of the stem.  An active  vascular cambium  is  capped by secondary phloem strands interspersed with a few secondary phloem elements, which are derived from the cambial ring.

 

In older stems, some primary vascular bundles lie in close proximity to, or are partly embedded in a band of secondary xylem. Click here to see a detail of an older Beta stem

 

Like Bougainvillea, Beta stems show anomalous secondary growth. Notice that this stem contains a number of primary vascular bundles, that are  scattered throughout the central cortex, giving this stem an almost monocotyledonous-like appearance! Exarch to these scattered vascular bundles, normal secondary vascular tissue develops in the fascicular and interfascicular regions. There is no evidence of secondary growth in the internal primary vascular bundles.

 

Double click on the image to see a high-resolution micrograph for more details

 

 

     3. Campsis stem                                                

 

 

 

Campsis radicans (Bignoniaceae) is a climbing vine. At first glance, this stem looks like a typical dicot stem, which is undergoing secondary growth. A broad band of secondary xylem ( 2X) occurs exarch to the primary xylem, and this xylem terminates with protoxylem (PX) internally. External to the xylem, one can see that the vascular cambium has formed some secondary phloem (2P). Strands of primary phloem fibres (PPF) are present. Look carefully at the tissue internal to the protoxylem. A cambial zone is evident, as are areas where internal phloem has developed. This internal cambium forms inverted medullary bundles  in Campsis.

 

Click on the dotted circle, for a higher magnification view of this medullary bundle.

 

   4. Boerhaavia stem                                               

 

 

Cross section of Boerhaavia stem, showing  successive rings of xylem and phloem

Detail, showing part of the stem, note that the xylem contains metaxylem  (MX) as well as protoxylem vessels (PX).

Boerhaavia is a member of the Nyctaginaceae and has been described as having C4 and C3 physiology and mixed anatomy, with some species  showing C4; others C3 type anatomy and related physiology. The stem in Boerhaavia contains well-defined anomalous secondary growth, which is  characterized by the presence of successive rings of xylem and phloem. According to Rajput and Rao, (1998) The cambium is composed of fusiform initials only, which give rise to rayless secondary vascular tissues. The cambium is described as being storied when cell division ceases. Each successive ring of cambium is originated from the outermost phloem parenchyma cells.

The cambial ring is functionally segmented into fascicular and interfascicular regions which produce mostly conducting elements of the xylem and phloem with some parenchyma, the latter to parenchyma cells. The xylem parenchyma cells develop into conjunctive tissue following thickening and lignification of cell walls. In two of the species (B. verticillata and B. rependa) phloem parenchyma cells also undergo lignification, but in B. diffusa parenchyma cells remain primary-walled. Alternate bands of lignified and parenchymatous bands are distinct in the stem.

The xylem vessels elements are short with a simple perforation plate on slightly oblique to transverse end walls. The phloem contains sieve tube members and associated companion cells.

See: Rajput, K. S. & Rao, K. S. 1998: Cambial anatomy and absence of rays in the stem of Boerhaavia species (Nyctaginaceae). — Ann. Bot. Fennici 35: 131–135.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

    5. Dicranopteris stem                                          

 

 

 

 

Click here to see a more detailed micrograph of this rhizome

The structure of the vascular tissue or stele has often been used to separate certain groups of pteridophytes. The simplest form of vascular structure is the protostele, in which a solid vascular core or strand of tissue, which contains xylem towards the centre of the stele, and external to this, a strand of phloem. In other instances the central protostele may contain non-vascular parenchyma cells and this condition is termed a medullated protostele or an ectophloic siphonostele. In this definition, a siphonostele is "any uninterrupted stele with an undifferentiated centre". Where external as well as internal phloem coexist, the structure is known as an amphiphloic siphonostele or sometimes equivalently, a solenostele.

Dicranopteris is known to contain xylem vessels, in which the end walls are clearly perforate, compared with the lateral wall pits which are associated with a pit membrane .

 

For additional information see:

Carlquist S and EL Schneider (2001) Vessels in ferns: structural, ecological, and evolutionary significance American Journal of Botany. 88:1-13

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

    6. Serjania stem                                                    

 

 

 

The Liana, Serjania a member of the Sapindaceae, is curious, because the stem consists of several vascular cylinders enclosed in a common periderm. Each bundle has a separate pith. Metcalfe and Chalk describe this as a compound xylem mass and the structure is also referred to as extra-stelar bundles. The secondary thickening developing from a conventional cambial ring, or may be  anomalous as in this case, forming from concentric cambia. These features are common in the Sapindaceae.

 

See: Johnson, MA, Truscott MH  (1946) On the anatomy of Serjania. Path of the bundles. Amer. J. Bot. 43:7 509-518 .

Metcalfe CR, Chalk, L (1950) The anatomy of the dicotyledons Oxford, Clarendon Press. (-one of the most useful anatomy reference books available).

 

Click here for a more detailed micrograph of the secondary vascular tissue.

 

Click here for further details of apotracheal and paratracheal parenchyma

 

  1. In Doxantha stems, what tissue forms the wedges between areas of secondary xylem? Compare this to Aristolochia stems-is it the same or not?

  2. Campsis stems are also anomalous - can you find the accessory cambium? In what plane (direction) do the phloem and xylem grow in?

 

 

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