|Themes > Science > Botanical Sciences > Plants and their Structure > Mature Root Cross Section|
|This tropical Avicennia
or black mangrove is demonstrating a specialized root structure known as a
pneumatophore. The black mangrove lives in a swamp of anaerobic mud
sediments. In order for its root system to survive here, it must provide a
pathway for oxygen to supply respiration in the roots. These
pneumatophores extend above the sediments; lenticels in their bark provide
entry of oxygen and interior aerenchyma provides an internal pathway for
oxygen to get all the way to the root tips. In a sense, then,
pneumatophores are similar to a snorkel.
Storage of nutrients: The cortex of
a root is a parenchyma tissue that can store large amounts of starch,
sugars, minerals, and other biomolecules for long periods of time. During
the late summer and fall, trees sense the changing daylength signalling
the approach of winter. They degrade the polymers in their leaves
(senescence) and return the amino acids, sugars, and essential minerals,
etc. to the root via the phloem. These remain in the trunk or root system
all winter and are returned to the shoot in the spring sap flow. We tap
this sap from maple trees, boil it down (40:1) and produce maple syrup.
Root Apical Meristem
The root grows from it tip and here we see
two longitudinal sections of a young root tip:
The Root Cap: The very tip of the root ends in a thimble-like covering, the root cap. This cap has a column of cells in its interior that are meristematic...they divide rapidly to make more cells. The derivative cells of these divisions are pushed outward by additional divisions and ultimately will slough off from the root cap surface. These slough cells assist in reducing friction and coating the rough surfaces of soil particles through which the root must grow. In addition, the root cap cells secrete mucilage. This chemical is strongly hygroscopic (attracts and locks up moisture) forming a gel. This mucilage acts as a lubricant for penetration in the soil. Humans use mucilage for the adhesive on stamps and envelope flaps...yes you have licked the mucilage the plants use to penetrate the soil.
The Zone of Mitosis: Immediately proximal to the root cap is a cluster of cells that do not actively divide. This pad of cells is often called the quiescent center. These cells probably represent a reserve of cells to be recruited later in time for the meristem. As such they serve as corrections for proliferating somatic mutations. Just proximal to the quiescent center are cells that divide rapidly by mitosis, adding new cells to the length of the root. This is of course just one contribution to elongation of the root. Just for your review, here is a table of mitosis:
The Zone of Elongation: Just proximal to the zone of mitosis is a zone of cell elongation. In this part of the root the newly created cells expand in their long dimension to push the meristem and root cap through the soil. The addition of the cells and their elongation are the tandem contributors to root elongation. This elongation involves resculpting the wall, growth of the cell within, a coalescence of the vacuoles to form a single large vacuole, and maturation of the organelles in these cells.
The Zone of Maturation: As we keep moving proximally (away from the root apex), we find that the cells that are elongating are also differentiating. They are becoming distinguishable from each other. Some are destined to be typical parenchyma cells, while others will mature to be sclerenchyma cells. Here is a view of adjacent cells in a root that have become differentiated...the bluish parenchyma cells lay right next to some reddish sclerenchyma cells:
The difference between these cells is a matter of how much division occurs to make the cells, how much elongation occurs after the cells were made, and then how the interior of the cells matured. In the case of sclerenchyma cells the primary wall (picks up bluish dye) is joined by additional secondary cell wall layers that become lignified (pick up the red dye). This incorporation of lignins into the wall material makes the cell wall exceedingly hard and even brittle. Eventually the cytoplasm is programmed for senescence and death, leaving behind an empty, hollow cell, with just the wall. As these cells are stacked end to end along the plant and the end wall degenerates, they form a kind of plumbing for the plant. These tracheary elements in the xylem will conduct water and minerals from the soil to the leaves. Yes, even dead cells can serve important functions. In addition to water and mineral conduction, the layers of xylem represent substantial support in the form of wood for tree trunks. From a human point of view, what dead cells are critical in human physiology? Think skin!
On the exterior of the root in the zone of maturation, the epidermal hairs elongate out into the soil particles as root hairs. Here is a photo of part of the root-hair area in the zone of maturation:
|These root hairs increase the
surface area of the root tremendously. They assist in soil water intake.
They are also critical for secreting acids onto soil particles to initiate
cation exchange of soil minerals. Thus the uptake of materials occurs in
these very fine young-root areas. People who do not understand this
attempt to move large trees without taking the soil ball with them. They
will knock off the soil or hose it out, etc. to lighten their
transplanting burden without realizing that they have doomed all the root
hairs to desiccation. The tree will suffer a strong set-back shock and may
even perish because of this. It helps if the moves are made in the cool of
late fall or early spring.
Mature Root Cross SectionBelow is a cross-section of a "typical" dicot root.
|This is a list of the tissues
within that root:
Root Hairs - cell extensions of epidermis increase absorptive surface
A monocot root has a similar structure, but of course the pericycle does not produce periderm in the typical case. Also monocot roots are typicaly polyarch as shown here.
Koning, Ross E. 1994. "Roots". Plant Physiology Website.
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