Objectives:

     Study the anatomical features that serve to protect plants against potentially harmful elements in their environments.

I.  Protection against desiccation.

     The outer cell walls of epidermal cells of aerial parts of plants are cutinized and covered with thick cuticle layers and a layer of wax of variable thickness.  All these features serve to inhibit loss of water from the plants' interior.

     1.  Examine free hand sections of the various aerial parts
         of the plants provided to assess the relative thickness
         of the wax and cuticle layers between different organs
         and between species.

     2.  Perform the following experiments on Kalanchoe and
         Picea pungens leaves to assess the degree to which
         wax and the epidermal layers inhibit water loss.

         a.  Obtain at least three leaves of comparable
             size for each species.

         b.  Remove the wax layers of one leaf using
             xylenes, followed by a water rinse.

         c.  Surgically remove the epidermal layer from one
             leaf.

         d.  Use the other leaf as an untreated control.

         e.  Seal the petiole end of each leaf with petroleum
             jelly.

         f.  Record the fresh weight of each leaf then place
             them on the side bench where they'll remain
             undisturbed.

         g.  During the next two weeks, weigh each leaf on a
             periodic basis.

         h.  Calculate the rate of water loss for each leaf
             and compare the effects of each treatment.  You
             may wish to pool data with your colleagues to
             obtain an adequate statistical sample.
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page 26

     Many perennial plant stems and roots expand in circumference over time.  In some cases the continuity of the protective epidermal layer is interrupted.  To compensate for this the phellogen produces phellem which replaces the protective role of the lost epidermis.  These tissues plus any included phloem and cortex tissue comprise the periderm.

     3.  Study Pelargonium (2.03), Tilia (3.05,3.055) and Pinus (3.06,                   3.061) stems with respect to the place of origin and
         the structure of the periderm.  Compare these systems
         with the periderm of Glycine (6.04) and Actea (6.05) roots.

     4.  Perform the following experiment on Solanum tubers
         to assess the degree to which the periderm inhibits
         water loss.

         a.  Obtain at least two tubers of comparable size.

         b.  Carefully peel the periderm from one tuber.
             Examine what you have peeled using free hand
             sectioning and histological techniques.

         c.  Record the fresh weight of each tuber then place
             them on the side bench where they'll remain
             undisturbed.

         d.  Calculate the rate of water loss as in 2 above.

Deciduous trees loose their leaves at the end of the growing season in
an organized manner.  The stem interior is sealed off from the outside
environment prior to leaf fall via a periderm which forms in the
abscission zone.

   5.  Study the periderm formed in the abscission zone of
         Fraxinus (6.06) and Acer (6.07).  Can you deduce where
         the phellogen first forms?

     6.  Many aerial plant organs are involved in gaseous
         exchange which is necessary for photosynthesis and
         transpiration.  Slowing the rate of water loss can be
         critical to plants' well being under certain environ-
         mental conditions.

         a.  Dissect Aesculus hippocastanum shoot buds.  Tight
             seals between winter bud scales serve to limit
             water loss from shoot apical meristems.  Note the
             special trichomes called colleters which secrete
             the resinous material that seals the bud scales.
             The dense trichomes on the leaf primordia within
             the bud probably protect the young leaves from
             excessive insolation by reflecting sunlight.
Plant Anatomy VI
Page 27

         b.  Examine the dense mats of trichomes on the surface
             of Cephalocepeus.  Dense mats of trichomes are
             interpreted to protect organs against desiccation
             by providing a layer of still air above surfaces of
             the organs.

III.  Protection against predation.

     Various anatomical features of plants are interpreted as providing protection against herbivory and seed predation.

     1.  Examine the stiff trichomes on the leaf surfaces of
         Pelargonium, Begonia and African Violet.  These
         appear to function by mechanically preventing many
         insects from reaching the epidermis of these leaves.
         In some cases insects may become entangled in these
         trichomes and starve to death millimeters from a
         potential food source.

     2.  The specialized stinging hairs of Urtica have
         silicified distal cell walls which readily break
         away from the calcified proximal part of the hair
         to provide a jagged hypodermic needle which injects
         histamine and acetyl choline plus unidentified
         substances into the offending animal's epidermis.
         Study the prepared slides of these species to learn
         how this defense mechanism works.

     3.  The sharp stiff prickles of many Rosaceous species
         and stiff spines of many Cactaceae species provide
         strong deterrents to potential herbivores.
         Carefully prepare hand sections of the examples
         provided to study the cellular details of these
         structures.  What makes these structures so stiff?

     4.  Many seeds are resistant to the digestive enzymes
         of animals that eat the fruit in which they are
         found.  Make free hand sections of Phaseolus (3.128) and
         Pisum testa to study the epidermal macrosclereids
         and subepidermal brachysclereids and
         osteosclereids which convey this resistance to
         these seeds.
Plant Anatomy VI
Page 28

Materials
 

Prepared Slides                      Fresh Material

Pelargonium (2.03)                    Kalanchoe leaves
Tilia (3.05,3.055)                   Picea pungens leaves
Pinus (3.06,3.061)                   Pelargonium stems w/cork
Glycine (6.04)                       Tilia stems w/cork
Actea (6.05)                         Pinus stems w/cork
Phaseolus (3.128)                    Solanum tubers
Faxinus (6.06)                       Fagus shoot buds
Acer (6.07)                          Cephalocepeus plant
                                     Pelargonium leaves
                                     Begonia leaves
                                     African Violet leaves
                                     Urtica stems and leaves
                                     Rosaceous stems
                                     Cactaceae stems
                                     Phaseolus seeds
                                     Pisum seeds
 

                                     Other

                                     Petri dishes
                                     Digital scale