| Patterns of Root Growth
To Root or Not to Root: Spirodela, Landoltia
and Lemna develop roots. The maximum number initiated per
frond depends on the species. Wolffiella and Wolffia
form
no roots, and roots may occasionally be absent in the other groups. Lemna
fronds make just one root, while Spirodela and Landoltia
fronds initiate more. According to Landolt (1986) Spirodela
can have as many as 21 roots per frond. We do not know what
environmental
factors govern the number of roots initiated.
The regulation of root length depends on environmental
conditions
(Landolt, 1986). Except in extreme conditions, low levels of
nitrogen,
phosphate or trace minerals encourage longer roots, while high
fertility
results in very short roots, or even an absence of root. Far-red
light shortens roots, suggesting that phytochrome
is involved. Gibberellins
may also have a role.
|
Photo: A view from below, roots of Spirodela as collected
from a wetland. The reflection at the water surface makes some of
the roots appear to grow upward. Debris from the wetland still
clings
to some roots but will fall off as the roots grow in culture. |
The structure of duckweed roots is very simple. Unlike
most
plants, duckweed roots do not show secondary growth or branching, and
do
not sprout root hairs. As a result, duckweed roots are very
slender,
less than 0.5 mm. Like other primary roots, they can be divided
into
four zones: the root cap, the meristematic region, the elongation
zone, and the zone of mature cells.
Anatomy of a Developing Lemna
Root.
[ Click on the image for a
better
view. ]
Left: Longitudinal Section. The arrow
(just below level B) indicates the level where mature sieve
elements first can be seen. RA = root apex, RC
= root cap
Right: Cross-sections at three levels.
The levels of the corrections are marked A,
B,
and C on the longitudinal section.
 |
Level A:
A cross-section at this level is shown in 2. The central cell ( X
) is a mature tracheary element. It is surrounded by a ring of
phloem
tissue. Two mature sieve elements ( SE ) occur on opposite sides
of the stele. The vascular tissue is surrounded by first by
endodermis
cells ( E ), and then by the root cortex parenchyma cells ( PC) .
An overall view of the region of this cross-section is shown in the
insert. |
| Level B:
A cross-section at this level is shown in 3. At this
stage,
the two sieve elements ( SE ) are mature, whereas the tracheary element
( X ) is not yet mature. The sieve element on the right contains
three characteristic plastids. A partial face view of the sieve
plate
is evident in the sieve element on the left. Note the relatively
dense cytoplasm of the two companion cells (arrows). An overall
view
of the region of this cross-section is shown in the insert. |
| Level C:
A cross-section at this level is shown in 2. Closest to the root tip,
all
cells are immature at this stage. One of the phloem mother cells
(arrow)
has divided tangentially to give rise to cells that can be identified
at
this very early stage as a sieve element and its companion cell, based
on their position in the stele. An overall view of the region of
this cross-section is shown in the insert. |
| Reference: Melaragno, J.E. and Walsh, M.A.
(1976) Ultrastructural
features of developing sieve elements in Lemna minor L. --The
protoplast. Amer.
J. Bot. 63(8):1145-1157. |
What
is the function of the
duckweed
root?
That question may seem obvious if one knows plant physiology, but
duckweeds
are exceptional. Research has shown that duckweeds mostly absorb
water and nutrients through the lower surface of the frond, not the
roots.
An experiment you can try:
Paul Gorham tried painting the lower
surface
of Lemna fronds (but not the roots) with lanolin, a soft
fat.
The thin lanolin coating prevented the fronds from making direct
contact
with the water. He found that coated fronds grew very slowly
and their roots elongated more than the roots of control plants.
For another
control, he painted some fronds with lanolin on the upper
surface.
Those fronds were unaffected. Another experiment showed that Lemna
will dry up if the fronds are held above the water but with
their root tips still submerged.
In the water, long duckweed roots become entangled with one
another.
If one tries to gently pick up one plant or frond colony, often the
result
is to pick up several others whose roots are entangled with the
first.
In nature, this helps duckweeds to form loosely- connected mats on the
water
surface. Landolt (1996) points out that the a duckweed root
provides
stability to help keep the individual plants orientated rightside-up in
the water as they encounter wind and water turbulance. The root
is
like a sea-anchor.
A sea anchor does not touch bottom, but prevents a sailor's boat from
drifting
in the wind. In the same way, the duckweed root slows the plant's
movement when the wind blows.
Duckweed roots are very sticky. This is key to another
root function: dispersal. Ducks and other aquatic birds
land
among duckweeds and eat the fronds. When they do, some of the
fronds
may adhere to their feathers. This allows duckweeds to hitch a
ride
on the bird to the next pond. In this way, duckweeds spread from
one place to another.
Test this experiment yourself with a duckweed frond
that has one or more long roots:
Pick up a frond with a stainless steel spoon or spatula. Tip
the
spoon and let the water drain off for just a minute. Try gently
shaking
the frond off. Did it still cling? Take a paper towel or
tissue
and gently wick the water from the root. Does it still
cling?
Cut the root(s) off the frond and repeat the experiment. Can you
think of a way to measure the force it takes to pick up a frond
clinging
to a smooth surface?
Link for more experiments and projects.
On-Line References in Root
Development
and Function
Basic
information on roots by by Mark Brundrett, CSIRO Forestry
and Forest Products, from the book, Working with Mycorrhizas in
Forestry
and Agriculture by Mark Brundrett, Neale Bougher, Bernie Dell, Tim
Grove and Nick Malajczuk. Australian Centre for International
Agricultural
Research, 1996.
A
series of lectures on root anatomy and development by David T.
Webb, University of Hawaii, with wonderful illustrations.
Topics include the following and more:
Diagrams
of zones in corn root development: Corn Root Tip, Rapid
cell
division, and Cell elongation from a course at the University of
Calgary,
Alberta.
Root
development by Philip N. Benfey and Ben Scheres (2000) Current
Biology 10(22):R813-815.
Glossary
Proto: prefix that refers to a part or stage that comes
before or at first
Rhizodermis: the dermal (outer) tissue of the root
covers
all underground plant parts. Root hairs are often found on the
surface
of this tissue in most plants, but duckweed roots do no form root
hairs.
[ read
more ]
Root cap: a structure at the tip of the root. It
is cone- or thimble-shaped and consists of concentric layers of cells
covering
the root apical meristem. In terrestrial (land) plants, the root
cap produces a mucilage and sloughs off its oldest tissues to provide
lubrication
as the root pushes through the soil. It is not known if duckweed
roots produce a mucilage, but they are sticky.
Root apical meristem: meristem located at the
apex
of a root where new root cells are produced.. Cell division
at the apical meristem produces new root cap cells in the outward
direction
and new root cells in the inward direction.
Zone of elongation: Tissue behind the apical meristem.. In this
area,
the new cells are enlarging and differentiating into specialized root
tissue.
Sieve elements: Cells of the
phloem,
consisting of sieve cells and sieve-tube members. They typically
have sieve plates instead of final walls. Sieve elements of angiosperms
are associated living companion cells. [ read
more ]
References:
Gorham, P.R. (1941) "Measurement of the response of Lemna to
growth-promoting substances." Amer. Jour. Bot. 28: 98-101.
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Revised: August 14, 2006
