Objectives

     To study the continuity of the long distance transpiration and translocation pathways in plants.

I.  Xylem

     The general flow of water and elemental solutes during the growing season is through the xylem tissue.  The xylem pathway can be visualized as beginning at the absorptive region of the roots; extending upward through more proximal portions of the roots; passing through the transition region of the hypocotyl on into the leaf trace network of the stem; until finally, it passes into the leaf vein network to successively smaller and smaller veins.  In order for transpiration to work the water within this pathway must be in physical continuity throughout, even when secondary growth processes produce secondary xylem in the more proximal regions of roots and stems.

     A.  Use Pisum and Zea roots that have been placed in 1% solutions of basic fuchsin  for 12, 24, and 48 hours, allowing this dye to transpire upward toward  the leaf tissue.  Prepare hand sections at various  levels along the root, stem and leaf to examine the  pathway that the dye has taken.  NB:  Since this dye  is water soluble,  don't leave your sections  in the water baths too long, and, of course, work as rapidly as possible.   Identify protoxylem and  metaxylem regions and consider their relative  arrangement in terms of endarch vs. exarch.  How many xylem archs do you observe in the roots?  What is the arrangement of the vascular bundles in the stem?  How are the vascular bundles in the stem related to those of the petiole and/or leaves? Record your observations in your lab exercise sheet.

    B.  Use Pelargonium stems to study the activity of the vascular cambium in the generation of secondary xylem.   Once you have located vascular cambium activity, examine the longitudinal extent of this activity using longitudinal sections of your twigs.  Record your observations in your lab exercise sheet.

    C.  The normal pathway for xylem transpiration is analogous to going up a series of rivers in a watershed.  That is xylem solution can follow any number of different pathways in the vascular network to various leaves, just like a canoeist travelling upstream determines their destiny by selecting a direction to follow as each tributary joins the river they are travelling.   If one wants to learn the exact path in the xylem that interconnects a given leaf, then one must mark that path in the opposite direction of the transpiration flow.  This is analogous to floating downstream with the current flow in a water shed.  That is, a canoeist just floating with the current will only follow one given path as they travel within a watershed.  By way of example, you know that you'll end up in the Gulf of Mexico at the mouth of the Mississippi River if you put a canoe into the Ohio River and just float with the current.   However, if once you got there, you turned around and paddled back upstream, there would be no guarentee that you would return to your starting point unless you made the correct choices at each tributary junction in the Mississippi River Watershed.  Given one set of choices you could end up in Montana!

Use the transverse sections of Populus stems in which dye has been pulled through the xylem of the central  leaf trace associated with leaf number 17 prior to sampling to follow the path that xylem solution would have to take in order to enter this particular leaf.   Be sure to  observe the relative location of the dye in the primary and secondary xylem regions of the stem to help you visualize the functional continuity of the  transpiration pathway between secondary xylem and the  primary vasculature leading to a single leaf.Record your observations in your lab exercise sheet.

  D.  Examine the major and minor vein relationships in cleared leaves and cross sections of petioles.   What happens to the rate of water flow as it enters successively smaller leaf veins? Record your observations in your lab exercise sheet.

II.  Phloem

     The general flow of photosynthate within the phloem is from sources of sugar production (i.e. leaves) to sinks of sugar utilization or storage.  The location of sinks relative to sources within the plant is dependent on what stage of ontogeny the plant is in and to some extent its environment, especially in temperate latitudes.  This being said, the pathway of the flow of photosynthesis is determined by the anatomical organization of the phloem tissue in the leaves, stems and roots of a plant.  In general, the pattern of phloem tissue follows that of xylem tissue in both primary and secondary tissues, but the flow within these two tissue need not have any relationship with one another.  Translocation of radioactive carbon is commonly used to track movement of photosynthate within plants.

     1.  Beginning with ID above and working backwards,  reexamine the slides in exercises ID through IA.  This  time, however, make observations on the primary and  secondary phloem tissues.    Make sure you distinguish between primary and secondary phloem.  Compare the relative arrangement of primary phloem and xylem tissue in the various organs, by using  the terms collateral vs. alternate where applicable.  Record your observations in your lab exercise sheet.
 

Material

Prepared Slides                         Fresh
                                                         Pelargonium
   Ranunculus (8.01,8.15)            2 week old plants of
                                           Pisum
   Zea (5.20,5.21,5.22)                         Zea
   Pinus (8.03)
   Glycine (6.04)
   Picea (8.05)
   Populus (two unlabelled boxes)     Other
   Helianthus (3.11,3.115)
   Pinus (3.06,3.061,3.065)                         Basic Fuchsin
   Aristolochea (8.08,8.081,8.082)
   Tilia (3.05,3.055)
   Olea (3.15,3.155)
   Cucurbita (3.19,3.191)