Objectives

     Study the anatomical features that are involved in the absorption of water, minerals and carbon dioxide.

     Plants are autotrophic organisms that live by converting the electromagnetic energy of light into chemical energy.  Although they are independent from external sources of chemical energy they do require external sources of water and carbon dioxide to perform the light and dark reactions of photosynthesis.  In addition plants require external water and essential mineral elements in order to grow and maintain vital processes.
 

I.  Absorptive processes providing water and mineral elements

    A.  Carefully remove barley from the soil matrix in  which it is growing.  Wash the roots with running  water.  Notice that the extreme tip and more proximal  parts of the roots wash clean of soil particles but  that in the portion of the root in between these  regions the soil particles adhere to the root.  Examine  these regions under the dissecting microscope.  Why do  the soil particles adhere to the root?  This region  corresponds to the region of active absorption of water and mineral elements.  Record your observations in the lab exercise sheet.

    B.  Study the Zea and Pisum roots that have been immersed in a solution of neutral  red or a weak solution of eosin for 4, 12, and 24 hours by preparing free hand  transverse sections at various places along the  longitudinal axis of the root.  Where do you find the  dye?  Is dye penetration into the interior of the  root uniform throughout the longitudinal axis?  What  do your observations tell you about the absorption  function of roots?  Locate the endodermis by staining the casparian strip in one of the root samples.   Record your observations in the lab exercise sheet.

    C.  Some epiphytic plants lack roots (Tilandsia) or have aerial roots (many Bromiliads and Orchids) which have a multiple epidermis called a velamen.  These plants absorb water and elements from rainfall and dews which fall or condense on their aerial surfaces.   Record your observations on one of the following examples in the lab exercise sheet.

    1.  Make observations using the dissecting microscope and  free hand sections of Tilandsia leaves and Orchid roots that are dry and that have been exposed to neutral  red or a weak solution of eosin.  Comparison of the anatomy of these structures  subject to the two conditions will reveal how water  and elements are absorbed in these organs.

    2.  Perform similar experiments with Pinus fascicles.  Pay special attention to what happens in the region of  scale leaves surrounding the proximal part of the  fascicles.
 

II.  Absorptive processes involving gases

    The CO2 utilized in the dark reaction of photosynthesis must diffuse into the chloroplasts from the external atmosphere.  The greater the exposure of surface area of chlorenchyma cells, the more rapid this will occur.  On the other hand, however, the greater the exposed surface area of these cells, the more rapid the loss of water will be to the atmosphere.  Plants have evolved specialized guard cells within the aerial epidermis to regulate this gaseous exchange, necessary to their existence.  Record your observations in the lab exercise sheet.

    A.  Make epidermal impressions of two species for which  fresh leaf material is provided using the cellulose acetate peel technique.  Determine what type of stomata these species have.   The features that these different types share in common  reveal the necessary functional aspects of stomata,  whereas the differences between them will give you an  idea of the biological variation that occurs within these  functional constraints.

In plant organs, like woody stems, that lack chlorenchyma  tissue, oxygen, necessary for respiration, may become limiting.  Specialized openings in the phellem, termed  lenticels, facilitate gaseous exchange through this otherwise  impervious tissue.  This gaseous pathway can be illustrated by blowing air into the base of a twig that has been immersed in a beaker of water.

    B.  Study the geometry and distribution of lenticels on the twigs you have collected.  Prepare free hand sections through these lenticels to determine their relationship with living internal tissues, the surrounding impervisous cork tissue, and the phellogen, or cork cambium.   Can you recognize filling or complementary cells and closing cells within your lenticel preparation?   Record your observations in the lab exercise sheet.
 

Material

Prepared Slides                       Fresh

Coffea (5.17)                           2 wk old plants of
Dianthus (7.02)
Nerium (7.03)                           Zea
Zea (3.07)                              Pisum
Pinus (3.09,3.095)
Sambucus (5.05,5.051,5.052)             Tilandsia
Tilia (3.05,3.055)                      Orchid roots
Betula (7.07,7.071)                     Pinus fascicles

                                      Leaves of:
                                         Ranunculus
                                         Kalanchoe
                                         Dianthus
                                         Bignonia
                                         Nerium
                                         Zea

                                      Twigs of:
                                         Zelkova
                                         Prunus
                                         Quercus
                                         Pinus

                                      Other:
                                         Cellulose acetate
                                         Acetone