| 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.
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.
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.
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
|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.
Photo right courtesy of Dane Deal, Deal Associates, Roxboro, NC 27573
An experiment you can try: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.
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.
Test this experiment yourself with a duckweed frond that has one or more long roots:Link for more experiments and projects.
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?
On-Line References in Root Development and Function
Basic information on roots by Mark Brundrett, CSIRO Forestry and Forest Products, from his website, Mycorrhizal Associations: The Web Resource.
Genetics of Primary Root Development by R M Twyman, from a book on plant developmental biology.
Root development by Philip N. Benfey and Ben Scheres (2000) Current Biology 10(22):R813-815.
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
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: June 8, 2013