Objectives

     Review the internal and external forces that act to produce stresses in plant bodies.  Examine the various anatomical features which act to stabilize plant bodies against these forces.

I.  Adaption to gravity

     Plants like most other organisms on earth must be able to support their own weight under the more or less constant force of gravity which tends to pull things toward the center of earth.  Two aspects of plant construction are fundamental to their ability to support their own bodies:  1) The middle lamella cements the cellulosic cell walls, that surround each cell, to its neighbors thereby forming a polygonal three dimensional network of cellulose in the plant body.  2) Under "normal" conditions, the turgor pressure of the cytoplasm creates a positive hydrostatic force which makes individual cells and hence the entire cellulose network rigid.  The role that these two aspects contribute to plant structure can be assessed by performing the following experiment.

  A.  Compare the rigidity and form of Apium,  Helianthus, Coleus, and Pelargonium stems and leaves under full turgor and in a plasmolyzed (wilted) condition.  The differences you observe between similar organs of the same species are due to loss of turgor pressure against the cell wall network.  Record your observations in the lab exercise sheet.

     There appear to be two basic strategies which plants utilize to further reinforce the structural stability of the turgid primary cell wall network:  1) Addition of more cellulose in the form of either primary cell wall or secondary cell wall.  These additions can either be equally or unequally distributed about the cell walls of individual cells.  2) Addition of other material, like lignin, to the cellulosic substructure of the cell walls.  The following experiments will help illustrate the role that each type of modification plays in providing structural support.

   B.  Select similar organs from two of  the different species of plants used in 1 above that appeared to respond differently to plasmolysis.  Prepare free hand sections of these organs and identify the relative abundance of parenchyma, collenchyma, sclereid and fiber cells in each.  Note that, in general,  cells with similar characteristics occur in distinct patterns within the organs. The differences you observe between different organs and between different species reflect differences in organ geometry and anatomical construction.   What can be deduced concerning the relative strength of cells which are reinforced via unevenly thickened primary cell walls (collenchyma) versus evenly thickened and lignified secondary cell walls (sclereids and fibers) from your observations?  What is the relationship between the geometry of these mechanical cells and the geometry of the plant organ as a whole?  Record your observations in the lab exercise sheet.
 

II.  Adaptations to stress.  Various components of the environment (wind, water, animals) act to place stress on plant organs.  The arrangement of mechanical tissue within plant organs are similar to arrangements of structural parts that have been discovered by humans (mechanical engineers) to work well in human constructions.  There are similar underlying physical reasons why such arrangements work best in both cases.
Study the the basic tissue pattern arrangements of Helianthus stems, leaves,  hypocotyls, and roots which will illustrate what appear to be optimum solutions for conferring resistance to stresses inherent to these organs.  Record your observations in the lab exercise sheet.

     A.  Inflexibility is maximized by H beam arrangement of tissues in aerial stems and leaves.  The mechanically stronger tissues (collenchyma and fibers) are clustered at "flange" regions, while the "webbing" is provided by weaker parenchyma tissues.  This arrangement  confers maximum rigidity to these organs making them resistant to longitudinal compression and tension.

     B.  Resistance to shear stress is maximized by a network of  resistant tissue at right angles to the deforming force.   With leaf lamina the greatest deforming force (wind) is  typically orthogonal to the lamina surface.  The  reticulate pattern of vasculature and associated        sclerenchymatous bundle sheaths, in addition to the geometry of various sclereids, in leaves serve to resist  shearing stresses in these organs.

    C.  Inextensibility is maximized by aggregation of  mechanically stronger tissue in a compact central  mass in roots and other anchoring organs that are  subject to longitudinal tension.

     D.  Resistance to radial compression is maximized by a  cylindrical shell of resistant tissue surrounding an  interior of less resistant tissue.
Materials

Prepared Slides                        Fresh Material

Cucurbita (3.19,3.191)                   Apium stalks
Piper (5.02)                             Zea plants
Hedera (5.03)                            Coleus plants
Peperomia (5.04)                         Pelargonia plants
Sambucus (5.05,5.051,5.052)              Half of above should have
Rheum (3.10)                             received water stress
Helianthus (3.11,3.115)                  for ca. 5 days, other
Hoya carnosa (5.08)                      half should be fully
Camellia (3.12)                          turgid.
Nymphaea (3.13)
Osmanthus (3.14)
Olea (3.15)
Fraxinus (3.17,3.171)
Linum (5.15,5.151)
Cannabis (3.24)
Helianthus stem (3.11,3.115)
Iris leaf (2.08)
Coffea leaf (5.17)
Populus leaf (5.18)
Young and old Ranunculus root (5.19)
Young and old Zea root (5.20,5.21,5.22)