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Understanding the basic variation in root structure. Identifying cells and tissues of the cortex and stele
Primary roots have not been the subject of as many, or such full studies as has been the case of stems or leaves. They do, however, show a wide range of variation which is influenced both by environment, in terms of ecological adaptation, as well as by the genotype. Compared with stems and leaves, fragments can be difficult to identify in the primary state. This is not entirely because they are relatively undescribed or poorly represented in reference microscope slide collections, but partly since there is, overall, less variation.
Roots have to take strains or pulling forces. It rarely has to bend or flex, since it is usually found in a more or less solid medium. In consequence of this, the main strengthening tissues are positioned in the central region of the root and function like a rope.
Ranunculus root, TS.
Salix young root TS
Pisum young root TS
Zea mays root, TS
Helianthus old root
Iris root, TS
Zea mays mature root TS
Young Pine root TS
Zea mays- origin of lateral roots, TS and LS.
PLEASE REFER TO TECHNIQUES: 3, 7, 8 & 9.
|Click on the thumbnail images, to go directly to the detail of that specimen|
double click on the images to have a look at high-resolution micrographs and more details
Raven, Evert and Eichhorn, Biology of Plants is highly recommended
roots have not been the subject of as many, or such full studies as
has been the case of stems or leaves. They do, however, show a wide
range of variation which is influenced both by environment, in terms
of ecological adaptation, as well as by the genotype.
Whilst they attract less attention, they are an important component
of the plant, and may account for 25 - 60% of the plant's structure!
Roots are highly specialized, with two principal functions:-
The root is an organ which has to take strain or pulling forces. It rarely has to bend or flex, since it is usually found in a more or less solid medium. In consequence of this, the main strengthening tissues in young roots are positioned in the central region of the root and the structure will function like a rope.
2. Uptake of water and nutrients.
In all except aerial roots and the non-anchored roots of aquatic plants, root hairs are usually present a short distance from the growing apex. These develop from the rhizodermis or root epidermis. Root absorbtion is a relatively simple process. Examination of the younger parts of roots (those especially, that carry root hairs) present few barriers to absorption by plants. Whilst it is theoretically possible for absorption to occur anywhere it is most likely that absorbtion is greatest where root hairs exist, and the outer protective layer (the rhizodermis), and is most efficient just a few millimeters behind the root tip.
Click here for more background information about root anatomy.
A. Dicotyledonous roots
root illustrates the relatively simple structure of a young dicotyledonous
root. The xylem is tetrarch, and four strands of phloem alternate with the
protoxylem. This root is just beginning to undergo limited secondary
growth, with a cambial zone .
Click here for a low power image of root.
Please note the following points:
Salix (willow) root
Salix is another example of a dicotyledonous root. The micrograph to the right shows that the xylem is composed of four strands, with protoxylem exarch immediately beneath the pericycle. Phloem, occurs between each of the xylem strands,
This example of a transection of a pea root has three protoxylem poles (triarch) It has started to undergo secondary growth, with limited secondary xylem (2X) and secondary phloem (2P) opposite the metaxylem (click here for a more detailed view). The cambium is spreading towards the protoxylem poles. The protophloem (double arrowheads) is crushed, and the remnants of this tissue are located between the protoxylem poles, at the periphery of the developing secondary vascular cylinder. The central core of this young root is composed of metaxylem vessels.
|The transection to the left shows the central core of a mature Helianthus root and illustrates the extensive secondary growth that can take place in a sunflower during one growing season. In the detail micrograph below, you can see that the xylem contains large-diameter secondary xylem vessels (V), arranged radially in files, and these are interspersed with narrower vessels and tracheids (T). Variable width parenchyma rays separate the xylem. .|
|Click here to see the central primary xylem core in more detail||You should try to draw a representative portion of the root of Helianthus using the micrographs provided.|
The Iris root
|Monocotyledons usually have a large number of alternating groups of xylem and phloem. If you look at the photomicrograph which shows part of the root of Zea mays below, this statement should become obvious to you. Note the regular alternation of xylem and phloem, and the distinct junction between the cortex and the stele. Which tissue forms the boundary layer, endodermis or pericycle ?|
Again the Iris root is another example of a monocotyledonous root. In this example, we can see a very prominent endodermis -- seen here, as the layer of cells with striking thickening of the radial and inner tangential walls. The endodermis is the innermost layer of the cortex. The wall thickening forces water and other molecules to take a symplasmic route from the cortex to the stele, and vice versa, through the unthickened passage cells.
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CLICK on the image of the Iris root above to see high-resolution photomicrograph to get more detail
(Note: not all the details are visible in the accompanying micrographs)
The endodermis is the innermost layer of the cortex, which separates the vascular cylinder from the cortex. It occurs regularly in roots. Sometimes there is a band of suberin called the Casparian strip running round the radial and tangential walls of the cell. Sometimes, the whole wall is suberized. It is an important barrier, and the presence of the Casparian strip, as well as suberization, will regulate apoplasmic transport across the cortex to the pith, and from the pith to cortex. Passage cells such as those illustrated in the Iris root (click here to review this image) which generally lack thickened walls, are involved in symplasmic transport between the cortex and stele of root systems.
|The portion of a Zea mays root shown in the micrograph to the left, illustrates the arrangement of the separation of the cortical from the stelar tissues. Zea, like all primary roots, has an endodermis that forms the boundary between the cortex and the stele, and a layer immediately beneath this, the pericycle that is the outermost layer of the stele. This is an example of an atactostele.|
The micrograph (above left) shows
protoxylem (PX) and metaxylem (MX
vessels), and between these, a phloem strand. Detail from a mature root.
The phloem in monocot roots is often reduced to a few sieve tube members, (S)
surrounded by companion cells and associated phloem parenchyma. Note the
conspicuously thickened endodermis, (EN)
and beneath this, the pericycle, (Per)
which forms the outermost later of the vascular cylinder.
The Zea mays root is a typical monocotyledonous root. Try to identify the following (not all are visible in the accompanying micrographs):-
single-layered pericycle (the walls of which may or may not be sclerified),
the numerous strands of xylem, which alternate regularly with;
small, inconspicuous phloem strands and
The first root to emerge at seed germination, is the radical or primary root. This may persist and grow into a tap root, or as frequently happens in grasses and other plants, it may remain small, or disappear as the plant develops. The primary root may give rise to other roots, the lateral roots (sometimes called branch roots, or secondary roots that develop from meristems within the root, specifically from the pericycle. Lateral roots may themselves, also give rise to other lateral roots. All of these root types, constitute the root system of the plant.
|Anatomically, Gymnosperm roots are similar to those of the dicotyledons. They have a central core, which, in the young toot, is occupied by metaxylem elements. The protoxylem is exarch to the metaxylem, and the xylem poles in young roots are also, like the dicotyledons, few in number. Secondary growth, like that in the maturing dicotyledonous root, is initiated between the metaphloem and the metaxylem, with the cambium spreading laterally, until it forms a complete circle.|
Gymnosperm roots are generally very similar in appearance to dicotyledonous roots, with one striking exception. The xylem contains tracheids only, and therefore, by association, the phloem tissue is composed of sieve cells, albuminous cells and phloem parenchyma cells. If sections are cut near the tip of a pine root, then it is likely that you will see the characteristic diarch stele. In this example, the two xylem poles composed of tracheids (T) are visible. Each xylem pole is associated with a resin duct (RD). The xylem is this described as being diarch. The phloem (P) alternates with the xylem poles and is partly crushed in this section. Note that a narrow cambial zone exists between the xylem and the phloem in this root. The endodermis and pericycle (not clear in this micrograph) separate cortex from stele. Click here for a more detailed view
The Origin of Lateral Roots
|You should make appropriate drawings and notes and consult
texts such as Raven, Evert and Eichhorn; Biology of Plants for more details.
Note the following:-