Doubtnut is not responsible for any discrepancies concerning the duplicity of content over those questions. Study Materials. Why use Doubtnut? Instant Video Solutions. Request OTP. Question : What is the venation of leaf in fern? Related Answer. What is venation? Describe various types of leaf venation. What is common between a fern leaf and a Cycas leaf.
Some leaves are attached to the plant stem by a petiole. Leaves that do not have a petiole and are directly attached to the plant stem are called sessile leaves. Small green appendages usually found at the base of the petiole are known as stipules.
Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce veins of vascular tissue. The edge of the leaf is called the margin.
Figure shows the structure of a typical eudicot leaf. Within each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the venation pattern. Monocots and dicots differ in their patterns of venation Figure. Monocots have parallel venation; the veins run in straight lines across the length of the leaf without converging at a point.
In dicots, however, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. One extant plant, the Ginkgo biloba , has dichotomous venation where the veins fork. The arrangement of leaves on a stem is known as phyllotaxy. Leaves are classified as either alternate, spiral, or opposite.
Plants that have only one leaf per node have leaves that are said to be either alternate—meaning the leaves alternate on each side of the stem in a flat plane—or spiral, meaning the leaves are arrayed in a spiral along the stem. In an opposite leaf arrangement, two leaves arise at the same point, with the leaves connecting opposite each other along the branch. If there are three or more leaves connected at a node, the leaf arrangement is classified as whorled. Leaves may be simple or compound Figure.
In a simple leaf , the blade is either completely undivided—as in the banana leaf—or it has lobes, but the separation does not reach the midrib, as in the maple leaf. In a compound leaf , the leaf blade is completely divided, forming leaflets, as in the locust tree. Each leaflet may have its own stalk, but is attached to the rachis. A palmately compound leaf resembles the palm of a hand, with leaflets radiating outwards from one point. Examples include the leaves of poison ivy, the buckeye tree, or the familiar houseplant Schefflera sp.
Pinnately compound leaves take their name from their feather-like appearance; the leaflets are arranged along the midrib, as in rose leaves Rosa sp. The outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper and lower epidermis, respectively.
Botanists call the upper side the adaxial surface or adaxis and the lower side the abaxial surface or abaxis. The epidermis helps in the regulation of gas exchange. It contains stomata Figure : openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing. The epidermis is usually one cell layer thick; however, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration.
A waxy layer known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves may have small hairs trichomes on the leaf surface. Trichomes help to deter herbivory by restricting insect movements, or by storing toxic or bad-tasting compounds; they can also reduce the rate of transpiration by blocking air flow across the leaf surface Figure.
The palisade parenchyma also called the palisade mesophyll has column-shaped, tightly packed cells, and may be present in one, two, or three layers. Below the palisade parenchyma are loosely arranged cells of an irregular shape. These are the cells of the spongy parenchyma or spongy mesophyll. The air space found between the spongy parenchyma cells allows gaseous exchange between the leaf and the outside atmosphere through the stomata.
In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts.
Guard cells are the only epidermal cells to contain chloroplasts. Like the stem, the leaf contains vascular bundles composed of xylem and phloem Figure. The xylem consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the photosynthetic products from the leaf to the other parts of the plant.
A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues. To this end, the mechanical stability in the early land plant Aglaophyton major from the Devonian, was achieved by turgor pressure in cortical parenchyma; turgor also supported Rhynia spp.
As effective as turgor-based mechanics may have been for these plants, reliance on water for support restricted them to mesic habitats, capped their height at less than 40 cm and limited the lateral reach of their stems Speck and Vogellehner, , ; Bateman et al. The appearance of the hypodermal sterome in early seed-free vascular plants is a prime example of how modest modification of existing tissues, in this case the progressive lignification of parenchyma, can change the developmental trajectory and functional potential of a plant structure.
The sterome is a layer of sclerenchyma fibers that are mostly dead Dickison, , and present in the majority of ferns just beneath the cuticle of a rhizome or leaf petiole Figure 1. It has largely been lost in angiosperms Rowe and Speck, , but some exceptions exist, such as among lianas in young Aristolochia and Manihot stems that rely on the sterome for self-support prior to their transition to climbing e.
That early tracheophytes transitioned from small-statured plants with either enations, simple branch systems, or microphylls to plants with potentially huge leaves can be attributed, in no small part, to the appearance of the sterome not only because it stiffened axial organs, but because it created a central, mechanically stable neutral zone in which vascular tissues could be cushioned by surrounding parenchyma Bateman et al.
Figure 1. Photos and micrographs of selected Pteridaceae ferns, their petioles and steromes. These species were chosen on account of their diverse habitats, interesting stele arrangements, wide range of leaf sizes and variable steromes, if present.
A—C Polytaenium citrifolium , an epiphyte without a sterome; D—F Bommeria hispida , a short, desert-dwelling species with a thin sterome composed of thick-walled fibers; G—H Pityrogramma ebenea , a tropical species with an average sterome but a pronounced adaxial groove in the petiole; I—K Pteris livida , a 2 m tall tropical upland species with relatively large yet poorly reinforced sterome fibers. Informed permission to include student in I was granted by written consent.
The evolutionary transition from parenchyma to sclerenchyma tissue for support may have been relatively simple, requiring little more than the development of a secondary cell wall, cell elongation and enhanced lignification. Fossil evidence of a thick walled sterome characterizing Cooksonia axes dates back to the Silurian Edwards et al. Similarly, the early Devonian rhyniophytes show some cortical differentiation Lang, ; Kidston and Lang, ; Rowe and Speck, but this is complicated by the arbuscular fungal growth within the cortex.
Certainly by the late Devonian, hypodermal steromes were mechanically significant in a range of tracheophytes such as Psilophyton and early lignophytes such as Tetraxylopteris Speck and Vogellehner, Recent phylogenetic analyses place the origin of ferns in the early Devonian Pryer et al.
The goal of this study was to examine the anatomy, and the geometrical and biomechanical contribution of the hypodermal sterome to leaf support across the highly diverse Pteridaceae family of ferns. The Pteridaceae have origins in the Jurassic, but divergence and expansion are believed to have occurred in the Cretaceous Schneider et al. Species' life history and morphology is equally diverse; desert adapted cheilanthoids are frequently desiccation tolerant, typically less than 20 cm tall, and active only during the humid seasons Hevly, ; Nobel, , while similarly-sized tropical epiphytes are more or less perennial.
Other taxa can be enormous: the cloud forest understory species Pteris livida and P. All of these plants have steromes.
Yet equally compelling are members of the Pteridaceae that lack steromes, including the epiphytic vittarioids, and the aquatic ceratopteridoids, that rely on parenchyma and petiole geometry for support and bending resistance. What governs the presence and the radial thickness of the hypodermal sterome in ferns, and how does this trait vary with petiole geometry and leaf size?
The findings challenge the simplest hypothesis that increasingly larger leaves rely on proportionally greater support from the sterome. Rather, it appears that the sterome is a nuanced structure that is related to both support needs and life history traits: in some ferns, its presence may be critical to survival, while in others, it's absence is not a detriment but rather a cost saving.
Figure 2. A phylogeny of the selected Pteridaceae ferns examined in this study, with leaf areas mapped onto the branches. The colors of species' name indicate their native habitats. A bootstrap value is associated with each node. Complete leaves of 20 Pteridaceae species were collected from habitats ranging from aquatic to xeric, and included both terrestrial and epiphytic taxa Table 1 ; Figure 2.
The tropical, terrestrial Dennstaedtia cicutaria was chosen as the outgroup because it belongs to the Dennstaedtiaceae, an earlier-branching sister clade to the Pteridaceae.
Table 1. List of species studied, the clade to which they belong within the Pteridaceae, habitat, and their name abbreviation. A two-gene plastid atpA and rbcL dataset was assembled in order to resolve relationships among the focal taxa Figure 2. For each taxon, previously published sequences were obtained from GenBank; accession numbers are provided in Supplementary Data Sheet 1. In five instances, due to the unavailability of suitable sequences from the focal species, it was necessary to use sequences from a closely-related species in the same genus Supplementary Data Sheet 1.
The atpA and rbcL sequences were manually aligned separately in AliView version 1. Each of the single-gene alignments was phylogenetically analyzed using a maximum-likelihood approach in RAxML version 8.
These analyses employed the GTRGAMMA model of sequence evolution and involved 1, rapid bootstrap inferences followed by a thorough maximum likelihood search. The resulting trees were examined for significant conflicts.
Only one such conflict was uncovered among species within the genus Jamesonia. The atpA and rbcL alignments were ultimately combined and analyzed in unison as above, but with parameters independently estimated for each gene. The resulting tree was rooted with the single included representative of the Dennstaedtiaceae, D.
Schuettpelz et al. Ferns inhabiting the upland montane cloud forests were gathered from the Parque Nacional Los Quetzales 9. Leaves of the brackish adapted Acrostichum aureum were collected from the shorelines of Lake Sontecomapan Veracruz, MX; Dry-adapted Cheilanthoid taxa were sampled from populations in the Mt. Graham region of the Pinolenos Mountain Range in southern Arizona All samples were placed in at least two plastic bags with wet paper towels, and transported to the lab within 2—4 days after collection.
Cross-sections were made in the mid-petiole region of the leaf. Fern petioles can taper significantly from the leaf base to the rachis mid-point and can show a variety of geometries from the point of insertion through to the tip of the main axis.
Since the goal of the study was to employ a comparative measure of petiole geometry that minimized complexities related to the petiole insertion point, the petiole mid-point was retained for the geometrical and mechanical comparisons. This is appropriate for assessing the diversity and functional roles of the sterome in a clear manner; potential problems arising from confounding variation in the petiole properties of leaves from 21 species are thus largely eliminated, revealing patterns that are distinguishable and importantly, readily comparable within the clade.
Mid-stipe cross sections were excised by hand, stained with phloroglucinol to highlight lignified tissues, and mounted in glycerin. The sections were then photographed with a Moticam 2, digital camera attached to a Motic BA compound microscope www.
ImageJ software Schindelin et al. No discernible differences were evident in the sterome structure within a cross section, so sterome attributes were measured from three regions that included the adaxial, abaxial and lateral sides. Sterome thickness was defined as the radial distance between the segment cuticle and the innermost extent of the sclerenchyma fibers, just as the tissue transitioned to ground parenchyma Figures 1 , 3. The sterome thickness of each of these sectors was averaged for each leaf, and species' mean sterome thickness was computed as the average of three leaves.
Figure 3. A schematic of a petiole cross section, indicating how sterome thickness a , fiber lumen area b , wall thickness c and cell distance to cuticle d were measured.
Lumen area measurements were converted to diameters D by treating them as area-equivalent circles. The fiber cell wall thickness t was calculated as the average of two measures, one at the thinnest and another at thickest portions of the cell wall. This is a simplified version of a proxy used to estimate the mechanical strength of hollow cells such as xylem conduits Hacke et al.
The FWF was subsequently computed by dividing the fiber cell wall area by the total fiber area. The fiber wall fraction serves as a proxy for each species' carbon investment in the sterome.
Lastly, leaf length, leaf area and dry mass were determined on the same leaves that were used for the sterome analysis. Some leaves suffered handling mishaps, so leaf attributes were measured on digital photos of herbarium specimens collected as close as possible to the sampling sites Department of Botany Collections, Smithsonian Museum of Natural History, www. Photos and field notes guided the selection of specimen size to most closely match taxa observed in situ.
A two step approach was used to first measure tissue geometry properties and then explore how different geometries influence theoretical calculated values of EI with the assumption that the petiole tissue elements a xylem and phloem, b cortical parenchyma and c sterome sclerenchyma were the same between taxa.
Different species are unlikely to develop the same Young's modulus for equivalent tissues but it was outside the scope of this study to minutely examine all tissues for all species. The current approach represents a first step prior to more detailed studies in which more tissue-specific mechanical approaches can be used.
Complete petiole cross sections were photographed see Supplementary Image 1 and outlines of each tissue area were drawn manually Figure 3. The macro establishes the center of mass for the entire cross sectional area and then calculates I mm 4 of each tissue area with reference to a theoretical neutral line neutral plane of bending that passes through the center of mass for the two orthogonal directions relative to the x horizontal and y vertical planes of bending.
The macro thus computed I for all four tissue types and the entire I for both the vertical and horizontal directions. Contributions of each tissue to cross-sectional area and to the axial I total , a proxy for petiole bending resistance, were computed for both the vertical and horizontal orientations.
Like units of cross sectional area mm 2 units of second moment of area mm 4 are additive, thus the entire axial second moment of area represents the sum of each second moment of area of each tissue Equation 1. In the second step of this analysis, the theoretical values of rigidity EI theor Equation 2; N m 2 were computed for both vertical and horizontal orientations for each species using tissue-specific values of Young's modulus according to Niklas , who worked with Psilotum nudum.
Tissue properties can differ within a plant axis and certainly among species. For example, the mechanical attributes of branch segments of P. Although much smaller in absolute terms, the E tissue of primary xylem and parenchyma is also context or species-dependent Niklas, , , It is beyond the scope of the project to identify the species-specific tissue moduli for 21 species of ferns, but given that: 1 sclerenchyma tissue consists purely of mechanically functional and densely packed fibers, 2 all leaves were sampled at full maturity, and 3 P.
The fractional contribution of each tissue to theoretical flexural rigidity of the petiole, EI contr was calculated per Equation 3 ,. E composite is based on attributed values of each tissue type and calculated values of axial second moment of area.
Solid beams are susceptible to deformation or collapse under excessive loading, a phenomenon known as Euler buckling, which can be successfully modeled in plants Niklas, This model requires that EI be invariable along the length of the petiole due to constant material and geometric properties, but even if some variation exists, the model remains a valuable tool with which to evaluate the comparative mechanical limits of leaf petioles at first pass.
Hence, L max was computed per Niklas and Spatz :. Statistical analyses were performed in the R environment Team, All data were checked for normality with the Shapiro-Wilks test and log-transformed for analysis, if necessary. SI units were used for all analyses Table 2. Table 2. A comparative approach to studying the function and evolution of continuous traits acknowledges that taxa are descendants of a common ancestor, and that taxon relatedness violates the key statistical assumption that data are independent Felsenstein, ; Garland et al.
Phylogenetic independent contrasts PICs compensate for non-independence by accounting for tree topology and branch length in calculations of phenotypic differences between related taxa Felsenstein, ; Garland et al.
Petioles in this species were too large and brittle to generate a stitched, composite photograph of the entire petiole cross-section with any confidence. The phylogeny in Figure 2 illustrates the four major clades in the Pteridaceae, highlighting the wide range of leaf sizes among the focal taxa as well as their habitat diversity.
Leaf area varied by over three orders of magnitude from 5. With the exception of the tropical Hemionitis palmata , which had a leaf area of Species that occupy the paramo—a high-elevation habitat in which plants are exposed to extreme temperature changes, high winds and high levels of solar radiation—are also small and grow close to the ground such as Gaga spp. There was no clear pattern with respect to leaf type: cheilanthoid taxa were just as likely to have highly divided leaves bipinnate pinnatifid as the large Pteris species.
Significant linear correlations were observed between petiole diameter and leaf length, leaf area and dry mass with R 2 values of 0. The results from phylogenetic independent contrasts analyses PICs corroborate these findings Table 2. Taken together, the data indicate that leaf biomass scales predictably with petiolar dimensions and sterome thickness; this accords with the commonsense expectation that the sterome of the petioles serves to mechanically support the leaf laminae.
Figure 4. Log-log plots of stipe diameter in relation to leaf length A , leaf area B , and leaf mass C. Table 2 provides the scaling and correlation coefficients. Figure 5. Log-log plots of petiole diameter vs. The cellular composition of the sterome may determine in part its mechanical properties, so the lumen diameter and single wall thickness of the fiber cells were measured, as was the cells' distance from the cuticle.
This pattern was evident across the three major Pteridaceae clades. Figure 6. Figure 7. This implies that the relative carbon investment in sterome composition increases in small-statured species and this is indeed the case. Ferns with narrow stipes develop denser steromes on account of a higher fiber wall fraction, relative to larger-leaved taxa Figure 8.
For example, with an average stipe diameter of 0. Figure 8. A log-log plot of sterome thickness vs. Measurements of petiole axial second moment I varied by six orders of magnitude, from 0. Sclerenchyma occupies the highest percentage of the petiole and contributes most to I in small species such as M. The second moment is solely a function of petiole geometry calculations of I ignore material properties so the sterome itself is not the sole anatomical trait necessary to achieve high second moment of area.
However, its peripheral position, combined with its high E tissue enhances its mechanical contributions. This combination of geometry and mechanical strength presents a functional advantage with respect to petiole flexural rigidity, EI.
Figure 9. The percentage of sclerenchyma and parenchyma tissue occupying a cross-section of species' petioles in relation to each tissue's contribution to the second moment of area. The inset shows the relationship between petiole diameter and the second moment of area.
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