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Laboratory 27 - Magnoliophyta: Vegetative Morphology IV
The primary organization of monocots and dicots is dramatically different. Dicots have a well
developed primary root. Additional orders of branching in dicots usually originate directly from the
primary root. Monocots, however, have a rather short-lived primary root, with most roots originating
from the base of the stem and additional roots erupting from the stem at nodes. Additional orders of
branching in monocots emerge from the adventitious root system. What is the normal pattern of root
production in the various plant groups that we have looked at over the course of the semester--is it
more similar to dicots or monocots or is it mixed for different characteristics?
1.
General organization
Observe the organization of the primary (tap) root in any dicot. What relationship does the tap root
have to the radical during seed germination? How many orders of branching do you observe? How
do the laterals originate in dicots and monocots--from the root apex or distant from the apex?
Superficially (exogenously) or subsuperficially (endogenously)?
Now, observe the fibrous organization of the roots of a monocot. Can you identify the radical in the
structure of the mature monocot plant? How many orders of branching can you identify? Does this
differ from the dicot? Where are root hairs expected to occur? What is their function and
morphological nature?
2.
Specialized root organizations
A.
Contractile roots
Contractile roots are those which become shorter with age, pulling the plant downward into the
anchoring matrix in which its secondaries are located. Perhaps the most familiar example is
Taraxacum officinale (the dandelion). Observe the young and older roots. How is the contractile
behavior of these roots indicated on the surface. Make a longitudinal section of one of the
contracting roots and observe it as a wet mount in the compound microscope. What tissues are
involved in this behavior? Although this example shows strong contractile behavior in the tap root,
there also are monocots with contractile roots.
B.
Aerial roots
Aerial roots in orchids are highly differentiated organs that play a role not only in binding the plant to
its support matrix but also in obtaining water and nutrition. Observe the aerial roots of some
epiphytic orchids. Note that the surface of some of these roots is covered with a highly modified
water-retaining material known as velamen--part of the highly modified multiseriate epidermis.
Slides of this material are available for observation. Once water is retained on the surface, it is
rapidly absorbed by subjacent layers of the root.
C.
Prop and stilt roots
Prop roots are those that are ramified to support the plant. A classical example is that of Rhizophora
mangle, the mangrove, which grows in the intertidal zone on seashores and in estuaries. This plant
has developed the remarkable adaptation of premature germination of the seeds on the plant, resulting
in the primary root elongating to 10-15 cm before release from the plant. When the seed is shed, the
primary root is dropped into the mud, establishing the apex above water. This root continues to
branch to stabilize the growing plant, producing an extensive system of prop roots. Observe the
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living specimen, noting the mode of branching and origin of the initial branch of the root and its suborders.
Stilt roots originate regularly from the node and are typical of monocots. Observe a corn plant to
determine the origin and general organization of the stilt roots. Are these absorptive organs or
strictly architectural in function in this plant?
D.
Root nodules
The formation of stable root nodules in legumes is the result of a stable symbiosis of modified short
laterals that have become infected with nitrogen-fixing bacteria, frequently of the genus Rhizobium.
This provides organic nutrients and a protective matrix for the bacteria and enhanced ability to
accumulate mineral nutrients (particularly nitrogen) for the legume--obviously the basis for a very
successful symbiotic relationship. Observe the organization of the roots in these representatives:
Coronilla varia and Trifolium pratens, both common weeds, and other examples as available. Where
is the bacterium located within the nodules? What is the morphological basis of the nodule (what
organ forms the root nodule)?