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.

A.  Perform the following experiment on Malus sylvestris fruits to assess the degree to which wax and the epidermal layers inhibit water loss.  Record your hypotheses, data, and conclusions in the lab exercise.

         1.  Obtain at least three Malus sylvestris fruits, all of comparable size.

         2.  Remove the wax layers of one  fruit using  xylenes, followed by three water rinses.

         3.  Surgically remove the epidermal layer from one  fruit.   Examine what you have removed using free hand sectioning and histological techniques.

         4.  Use the other  fruit as an untreated control.

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

         6.  During the next two weeks, weigh each  fruit on a  daily basis.

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

     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.

 B.  Perform the following experiment on Solanum tuberosum tubers  to assess the degree to which the periderm inhibits  water loss.    Record your hypotheses, data, and conclusions in the lab exercise.

         1.  Obtain at least two tubers of comparable size.

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

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

         4.  Calculate the rate of water loss as in A above.

C.  Use the twig slides you have prepared to study the activity of the cork cambium.  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.  Make longitudinal sections through leaf scar regions on your stems to study the relationship between the periderm associated with the abscission layer and the periderm of the internode regions of your stem.   Record your best image of the abscission zone in the lab exercise.

II.  Protection against predation.

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

     A.  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.   Determine whether the trichomes are unicellular or multicellular.  Record your observations in the lab exercise.

     B.  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?   Record your observations in the lab exercise.

     C.  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  and Pisum testa to study the epidermal macrosclereids and subepidermal brachysclereids and osteosclereids which convey this resistance to these seeds.   Determine what type of sclereids are found in these seeds.  Record your observations in the lab exercise.

III.  Protection of dormant and newly expanding shoot apical meristems.

    A.  Dissect the terminal shoot buds of the twigs you have been collecting.  Tight  seals between winter bud scales serve to limit  water loss from shoot apical meristems.  Note any resinous material that may be present to better seal the bud scales.   The dense trichomes on the leaf primordia within  the bud probably protect the young leaves from  excessive insolation by reflecting sunlight during early stages of shoot expansion.   Estimate the trichome density on the leaves within your shoot bud.  What happens to trichome density as the leaves expand?  Record your observations in the lab exercise.

Materials
 

Prepared Slides                      Fresh Material

Pelargonium (2.03)                   Malus sylvestris fruit
Tilia (3.05,3.055)
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