Another problem with only having primary growth is that the source of water the youngest parts of roots keeps getting further from the place that needs water, the shoot tips where the leaves are and where new growth occurs.
While branch roots, or adventitious roots may be created to shorten the route, the fact still remains that primary growth separates water sources from parts that need water. Additionally, both the conducting cells of the xylem and those of the phloem can fail for a variety of reasons. Because repair of existing cells is often not possible and because primary growth does not allow for the production of replacement conducting cells, the ability to make stems wider, and in particular make them wider with the addition of transport cells and structural support cells, provides some clear advantages, including but not limited to longevity.
Radial growth is possible in plants that produce what are known as lateral meristems. These meristems are capable of increasing the girth of roots and shoots beyond what is produced by primary growth. Cell division in these embryonic regions, followed by expansion of the new cells, allows stems and roots to increase in girth in a type of growth defined as secondary growth.
Consequently, radial growth in roots and stems requires two lateral meristems, one, the vascular cambium, responsible for most of the increase in girth, and one, the cork cambium, responsible for making a new skin. The vascular cambium produces new vascular tissue and is responsible for most radial expansion of it. In a cross section of a stem or root the vascular cambium exists as a circle of cells, only a few cells in width. In three dimensions the vascular cambium is a cylinder.
Developmentally the vascular cambium originates from undifferentiated cells located between the xylem and phloem that were produced by the apical meristem. Recall that the primary growth of stems produces xylem and phloem in bundles that, for all groups other than monocots which do not exhibit secondary growth , occur in a ring within the stem.
To make the vascular cambium a continuous ring requires that cells between the vascular bundles be stimulated to start dividing. The vascular cambium may also develop in roots, again originating from cells located between the xylem and phloem and additional cells to form a continuous ring. Whether any particular cell produced by the action of the vascular cambium differentiates into secondary phloem or secondary xylem depends on its position, a common factor controlling cellular differentiation.
In the simplest case, when a vascular cambium cell divides it produces one cell that remains embryonic does not expand or differentiate and one cell that is destined to expand radially and differentiate. If the maturing cell is to the outside of the cell that remains meristematic it is destined to become a phloem cell: a sieve tube member, a parenchyma cell, or a fiber. H Nymphoides crenata — pentarch with pith, endodermis with CBs only and exodermis with CBs and SL, astrosclereids in mid-cortex with aerenchyma.
I Artemisia lavandulaefolia — diarch primary root, no pith, endodermis with CBs only, faint CB staining in hypodermis; photograph courtesy of Chaodong Yang. J Gentiana asclepiadea — root with dilatated endodermis and exodermis in early secondary growth; photograph courtesy of Alexander Lux. The cortex is delimited internally by the endodermis which varies as much in eudicots as it does in monocots, and often passage cells with CBs are opposite protoxylem and SL cells are opposite the protophloem Fig.
Air spaces in the form of aerenchyma are found most commonly in aquatic eudicots e. There is an exodermis in many eudicots e. Many eudicots, especially the many trees and shrubs, have secondary root growth, even if very limited as in small herbaceous plants e. Secondary root growth is probably accompanied by a dilatated endodermis and exodermis in many species, as in Gentiana Fig. Resin canals in roots are known but relatively little studied French, a.
Laticifers are more a feature of eudicots e. Ipomoea purpurea ; Seago, ; and Lactuca sativa , J. Root laticifer development has been studied e. Seago, , and crystalliferous and tanniniferous cells, especially in rootcap or cortex, are also well known e. Seago and Marsh, Aerenchyma types have been presented by several researchers e. Justin and Armstrong, ; Evans, , but the explanations for the development of intercellular spaces into aerenchymatous lacunae by Seago et al.
Based on this feature, Seago et al. Clearly, the earliest root aerenchyma in angiosperms was most probably by expansigeny Fig. Particularly in monocots, various kinds of lysigeny arose in more derived families of several orders.
The occurrence of diaphragms across aerenchymatous lacunae has been noted and even studied in detail e. The presence or absence of diaphragms has not been widely studied across monocots and eudicots Seago et al.
Aerenchyma, air cavities. B Schizogeny — cell wall separations, in Typha glauca ; photograph from Seago et al. In legumes, vascular cavities can be found in the pith of some triarch Pisum roots Fig. Legumes do not have the cortex development or structures that allow easy formation of aerenchyma Seago et al. Such cavities are not considered aerenchyma. Secondary aerenchyma, aerenchymatous phellem derived from phellogen, can also occur in wetland plants Lythrum salicaria ; Stevens et al.
From the concepts of Barlow , on increasing complexity and quiescence, to Clowes on epidermis origins, the possible evolutionary path of RAM organization has been presented in three major studies by Clowes , Groot et al. The latter authors presented an analysis of RAMs with several manifestations of closed and open types and reported that some specimens of Amborella trichopoda and the magnoliids contain common initials for most meristematic tissues of the root.
As stated above, Heimsch and Seago further related the open and closed RAMs with cortex and epidermis association in the nymphaealean families Cabombaceae, Nymphaeaceae and now the Hydatellaceae to the monocots. In Friedman et al. In overcoming some of the questions which Les and Schneider posed about the lack of solid evidence for nymphaealean and monocot phylogenetic connections, we argue that there is no stronger anatomical evidence for a Nymphaealean—monocot connection than the RAM and cortex, because such a type of anatomy is not found in Amborellaceae, magnoliids and eudicots.
These patterns clearly arose in the ancestors of monocots, i. Further, there appears to be a clear association between RAM organization and the patterns of lateral rootcap cells and their sloughing Hamamoto et al. Open RAM produces more cells and releases individual living border cells, whereas closed RAM releases sheets or groups of dead cells. The fate of lateral rootcap cells in the tiered or closed RAMs of Cabombaceae and Hydatellaceae, as well as the open transversal RAMs of Nymphaeaceae, need to be examined to determine if the same relationships holds for RAMs of these basal angiosperms.
The differentiation of epidermal cells, especially in simple tiered RAMs, has received enormous attention in just a select few species Bruex et al. The endodermis is a well-defined structural feature of angiosperm roots Kroemer, ; Van Fleet, ; Wilson and Peterson, ; Seago and Marsh, ; Seago et al. A hypodermis is the outermost cell layers of the cortex derived by periclinal divisions in the outer ground meristem Seago and Marsh, Multiseriate exodermis is much more common in monocots than in eudicots Seago et al.
Two different cell types can occur in exodermis — long cells and short cells; Shishkoff reported no dimorphic hypodermis in Nymphaeales see also Seago et al. Dimorphic hypodermis as seen in Allium cepa Fig. The exodermis and its passage cells can have major effects on root—fungus associations e. Baylis, ; Brundrett, There have been analyses of root structures with regard to their application to systematics e. French, a , b ; Keating, , but differences in interpretations present some problems.
Kauff et al. Hydrocharis Fig. The similarities between members of the Nymphaeales and the Acorales have been noted with regard to xylem cell structure Schneider and Carlquist, , ; Carlquist and Schneider, as well as cortex structure Seago et al.
In the basal angiosperms, two of the families, Cabombaceae and Hydatellaceae, have predominantly monarch roots, while the Nymphaeaceae are dominated by species with mainly polyarch roots, as are Acorus Acoraceae , sister to the rest of the monocots, and the Araceae Keating, Most of the remainder of the monocots are polyarch, except for aquatic families such as Hydrocharitaceae Seago et al. According to Metcalfe and Chalk a , b , Popham , Esau and Metcalf , eudicots are generally depicted as having two to six poles or strands of primary xylem and phloem often, apparently, diarch in young lateral roots; Byrne and Heimsch, ; Byrne, It seems that diarchy is more common in primary roots derived from radicles , at least in the basal angiosperms.
That some wetland eudicots at the base of the core eudicots Gunnera and near the base of the basal eudicots Nelumbo are strikingly polyarch, such as Nymphaeaceae and the vast majority of monocots, raises interesting questions. The greater the number of poles or strands of xylem and phloem heptarchy and above , the less likely it is that secondary growth may occur, whereas diarch to hexarch patterns can lead more easily to secondary growth.
Since so many species, especially among basal angiosperms including Nymphaeales, e. Friedman et al. Such is clearly not the case; and, too many basal angiosperms and magnoliids have patterns other than diarchy. Another aspect of development and structure which should be examined more closely is the state of embryo development and structure at the time of maturation and germination.
Amborellales, Nymphaeales and Austrobaileyales have very small embryos with little differentiation Martin, ; Tobe et al. This might be important to the balance between a primary root system and adventitious root systems, to the relative state of development in primary roots vs.
Most eudicots, when producing adventitious roots, form them from more or less typical eudicot vascular patterns in stems, bundles in one ring with a remnant procambial strand or incipient vascular cambium. Most monocots, on the other hand, form adventitious roots from stems with scattered vascular bundles or two or more rings of vascular bundles, so that one could argue that it is the number of available vascular bundles that produces the greater number of xylem and phloem poles in monocot roots.
Thus, vascular patterns in embryonically produced roots might reflect vascular bundle distributions of their stems. The contributions of molecular genetics will have a major impact on our understanding of evolution of vascular patterns in roots see Scarpella and Meijer, On the matter of mycorrhizae, after Baylis , Simon et al.
Brundrett offered possible root structural features needed to accompany the evolutionary pathways. Mycorrhizal roots can sometimes be extremely modified e. Imhof, , For nodules, the study of Soltis et al. A recent study by Markmann et al. Heimsch and Seago and Seago et al. It should be noted that these families with nodulating roots are not closely associated with basal angiosperms or basal eudicots, and none is found in the monocots where epidermal origin is associated with cortex, not lateral rootcap; roots with bacterial symbioses seem likely to represent a derived condition in angiosperms.
In summary, root anatomy offers many interesting perspectives on developmental patterns, systematics and evolutionary relationships but, since their structure can vary depending on the type of experimental conditions, their importance is often less appreciated. However, when roots are examined based on their typical habitats, they can be useful when comparing groups of plants. Therefore, based on the information presented in this overview, there appears to be a general trend in angiosperm root structure see summarized information in Table 1 and, in general, we note that the Amborellales and magnoliids have many root structural features like those of eudicots, whereas the Nymphaeales roots are strikingly similar to those of the monocots, especially basal monocots such as Acorales.
The Austrobaileyales are enigmatic and have root structural features which do not align easily to either monocots or eudicots. Clearly, the basal angiosperms require far more anatomical examination to corroborate the findings of molecular phylogenetic analyses. Summary of selected, but typical anatomical features of angiosperm roots including RAMs from Heimsch and Seago, The late Charles Heimsch is specially noted because he instilled in J.
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JoVE Core Biology. Previous Video Next Video. Next Video Embed Share. As a plant grows, the shoots and roots lengthen through primary growth and, in woody plants, thicken through secondary growth. Please enter your institutional email to check if you have access to this content. Please create an account to get access. The periderm substitutes for the epidermis in mature plants.
The combined actions of the vascular and cork cambia together result in secondary growth, or widening of the plant stem. These structures are illustrated below:. In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis.
A new layer of xylem and phloem are added each year during the growing season. The interior xylem layers eventually die and fill with resin, functioning only in structural support. The interior, nonfunctional xylem is called heartwood. The newer, functional xylem is called sapwood. The exterior layers of phloem eventually become crushed against the cork cambium and are broken down. Thus a mature tree contains many interior layers of older, nonfunctional xylem deep within the stem, but only a small amount of older phloem.
The layers of tissues within a mature tree trunk. The activity of the vascular cambium results in annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened.
This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood.
This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem shown below. An examination of the number of annual rings and their nature such as their size and cell wall thickness can reveal the age of the tree and the prevailing climatic conditions during each season.
The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth. Seasonal changes in weather patterns can also affect the growth rate, causing the rings vary in thickness. CC BY 2. It is the faith that it is the privilege of man to learn to understand, and that this is his mission.
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