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Veins in Plant Leaves (terminology)

Veins in Plant Leaves (terminology)


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Why do we call the vascular bundles in plant leaves "veins"? I think its probably because the xylem and pholem serve circulatory function analogous to the human body's circulatory system with arteries and veins. But why don't we call the vascular bundles arteries instead or veins?

In the human body, we call a blood vessel "vein" when it travels blood to the heart and an "artery" when a blood vessel travels blood away from the heart. Is there a similar reason or significance as to why we call vascular bundles "veins" instead of "arteries"?


Distinguishing Characteristics Between Monocots & Dicots: Leaf Veins

Angiosperms, or flowering plants, can be classified as either a monocot or a dicot. The terms monocot and dicot actually refer to the cotyledons, or embryonic leaves that first appear on the plant embryo. A monocot has only one cotyledon and a dicot has two. However, the number of cotyledon is only one of five distinguishing characteristics. Another identifying trait is the leaf veins. Monocots generally have parallel leaf veins whereas the leaf veins of dicots are generally multibranched.

This picture is of Fakahatchee grass (Tripsacum dactyloides). The leaf clearly displays the parallel leaf veins of a monocot.

This picture shows the leaf of a wild coffee plant (Psychotria nervosa). The veins of this leaf branch out, revealing the plant to be a dicot.


1.14: Plant Morphology - Leaves

  • Contributed by Michelle Nakano
  • Faculty (Horticulture) at Kwantlen Polytechnic University
  • Sourced from KPU Zero Textbook Cost Program

Leaves are specialized structures for photosynthesis that provide plants with energy. Leaves arise at nodes just below an axillary bud on woody stems and are usually petiolate, that is composed of a blade and stalk-like petiole. Petioles may have stipules, two small leaf-like flaps that are attached at the base. In some cases, stipules on leaves and stems may become modified into spines, thorns, or prickles. Some leaves are sessile, that is, they lack petioles and have blades directly attached to the stem. When a bud is located in the axil of a single leaf and the stem, as shown in Figure 14.1 the leaf is classified as simple.

However, when a bud is located in the axil of a structure with more than one leaf (leaflet) on attached to the axis (rachis), the leaf is classified as compound. As shown in Figure 14.2, even or odd numbers of leaflets may be pinnately compound that is, arranged along a central axis (feather-like), or palmately compound from one point on the tip of the petiole, (like fingers on an out-stretched hand). Compound leaves may undergo double (bipinnate) or triple (tripinnate) compounding into finer segments or leaflets.

Phyllotaxy, the arrangement of a leaf or bud in relation to another leaf or bud along a plant stem is a useful basis for classifying plants. Figure 14.3 illustrates common leaf arrangements where leaves and buds on a stem are opposite (directly across from each other on the stem), alternate (spaced alternately along the stem axis), whorled (three or more leaves and buds are positioned at a node), or basal (emerging from the base). Leaf arrangement may also be described as spiral, clustered, decussate (alternating pairs at right angles), and imbricate (overlapping scales).

Leaf venation refers to the patterns of veins within the leaf blade. In eudicot plants, leaf venation is typically either pinnate or palmate and may have multiple branching that gives an overall netted appearance. In contrast, monocots will have parallel leaf venation. Additional morphological features for description include leaf shape, tip and base features, and margins (edges). Leaf surface characteristics vary and some may be smooth (glabrous) or with hairs (hirsute or pubescent), wrinkles (rugose), pustules (verrucose) or other interruptions of the surface. Additional leaf surface terms are defined at this link to Leaf[New Tab] [1] .

Figure 14.4 and Figure 14.5 illustrate components of a leaf morphology chart commonly used for plant identification. More detailed information about the external characteristics of leaves is available at this link to Leaf Morphology[New Tab] [2] .


Function of the Leaf

As one of the most important constituents of plants, leaves have several essential functions:

Photosynthesis

The primary function of the leaf is the conversion of carbon dioxide, water, and UV light into sugar (e.g., glucose) via photosynthesis (shown below). The simple sugars formed via photosynthesis are later processed into various macromolecules (e.g., cellulose) required for the formation of the plant cell wall and other structures. Therefore, the leaf must be highly specialized to combine the carbon dioxide, water, and UV light for this process. Carbon dioxide is diffused from the atmosphere through specialized pores, termed stomata, in the outer layer of the leaf. Water is directed to the leaves via the plant’s vascular conducting system, termed the xylem. Leaves are orientated to ensure maximal exposure to sunlight, and are typically thin and flat in shape to allow sunlight to penetrate the leaf to reach the chloroplasts, which are specialized organelles that perform photosynthesis. Once sugar is formed from photosynthesis, the leaves function to transport it down the plant via specialized structures called the phloem, which run in parallel to the xylem. The sugar is typically transported to the roots and shoots of the plant, to support growth.

Transpiration

Transpiration refers to the movement of water through the plant, and subsequent evaporation via the leaves. When the stomata open to accommodate the diffusion of carbon dioxide into the plant for photosynthesis, water flows out. This process also serves to cool the plant via evaporation of the water from the leaf, as well as regulate the plant’s osmotic pressure.

Guttation

Guttation refers to the excretion of xylem from the edges of leaves and other vascular plants due to increased levels of water in the soil at night, when the stomata are closed. The pressure caused at the roots results in the leakage of water from the xylem out of specialized water glands at the edges of leaves.

Storage

Leaves are a primary site of water and energy storage since they provide the site of photosynthesis. Succulents are particularly adept at water storage, as evidenced by the thick leaves. Due to the high levels of nutrients and water, many animal species ingest the leaves of plants as a source of food.

Defense

In general, the types of leaf can be divided into six major types, although there are also plants with highly specialized leaves:

Conifer Leaf

Conifer leaves are needle-shaped or in the form of scales. Conifer leaves are typically heavily waxed and highly adapted to colder climates, arranged to dispel snow and resist freezing temperatures. Some examples include Douglas firs and spruce trees. The images below illustrate this type of leaf.

Microphyll Leaf

Microphyll leaves are characterized by a single vein that is unbranched. Although this type of leaf is abundant in the fossil record, few plants exhibit this type of leaf today. Some examples include horsetails and clubmosses. The image below illustrates this type of leaf.

Megaphyll Leaf

Megaphyll leaves are characterized by multiple veins that can be highly branched. Megaphyll leaves are broad and flat, and generally comprise the foliage of most plant species. The image below illustrates this type of leaf.

Angiosperm Leaf

Angiosperm leaves are those found on flowering plants. These leaves are characterized by stipules, a lamina, and a petiole. The illustration below shows an example of an angiosperm leaves.

Fronds

Fronds are large, divided leaves characteristic of ferns and palms. The blades can be singular or divided into branches. The image below presents an example of a frond.

Sheath Leaf

Sheath leaves are typical of grass species and monocots. Thus, the leaves are long and narrow, with a sheathing surrounding the stem at the base. Moreover, the vein structure is striated and each node contains only one leaf. The image below presents an example of a sheath leaf.

1. The primary function of a leaf is:
A. Water evaporation for cooling
B. Photosynthesis
C. Provide shade to the shoot and root structures of the plant
D. Transpiration

2. Which of the following statements is TRUE regarding guttation:
A. It typically occurs at night.
B. It occurs when the stomata are closed.
C. It results from increased water pressure in the soil.
D. All of the above


6.2) Leaf structure

Tip: allows the water to drip off and not block light or damage leaf.

Mid-rib: contains the xylem and phloem.

Vein: contains the xylem and phloem.

Lamina: the site of photosynthesis and production of useful substances.

Cuticle: Made of wax, waterproofing the leaf. It is secreted by cells of the upper epidermis.

Upper epidermis: These cells are thin and transparent to allow light to pass through. No chloroplasts are present. They act as a barrier to disease organisms.

Palisade Mesophyll: a layer of palisade cells which carry out most of photosynthesis

Spongy Mesophyll: a layer of spongy cells beneath the palisade layer, they carry out photosynthesis and store nutrients.

Vascular Bundle: it is a group of phloem and xylem vessels that transport water and minerals to and from the leaves. (called translocation)

Lower epidermis: This acts as a protective layer. Stomata are present to regulate the loss of water vapour (called transpiration). It is the site of gaseous exchange into and out of the leaf.

Stomata: Each stomata is surrounded by a pair of guard cells. These can control whether the stoma is open or closed. Water vapour passes out during transpiration. Carbon dioxide diffuses in and oxygen diffuses out during photosynthesis.


Scaling of xylem vessels and veins within the leaves of oak species

General models of plant vascular architecture, based on scaling of pipe diameters to remove the length dependence of hydraulic resistance within the xylem, have attracted strong interest. However, these models have neglected to consider the leaf, an important hydraulic component they assume all leaves to have similar hydraulic properties, including similar pipe diameters in the petiole. We examine the scaling of the leaf xylem in 10 temperate oak species, an important hydraulic component. The mean hydraulic diameter of petiole xylem vessels varied by 30% among the 10 oak species. Conduit diameters narrowed from the petiole to the midrib to the secondary veins, consistent with resistance minimization, but the power function scaling exponent differed from that predicted for stems. Leaf size was an organizing trait within and across species. These findings indicate that leaf vasculature needs to be included in whole-plant scaling models, for these to accurately reflect and predict whole-plant transport and its implications for performance and ecology.

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Types of Calyx

The calyx may differ in size, structure, loss or persistence in different plants, depending upon which the calyx is classified into the following types.

Depending upon the size of the calyx

  1. Regular: The sepals of the calyx exhibits the same size, as in China rose.
  2. Irregular: Here, the sepals exhibit different sizes, as in Cliteria sp.

Depending upon the structure of calyx

  1. Tubular: The sepals appear tube-like, as in Nicotiana species.
  2. Infundibuliform: Calyx resembles a funnel shape, like in Atropa belladonna.
  3. Urceolate: The sepals appear as urn-shaped, like in Hyoscyamus species.
  4. Bilabiate: It consists of two lips, as in Ocimum and Salvia species.
  5. Campanulate: Calyx seems like a bell shape, as in Lathyrus odoratus.
  6. Cupulate: The calyx seems cup-like, as in Gossypium species.

Depending upon the fusion of sepals

  1. Gamosepalous: In this type, the sepals are fused. Examples: Datura, Hibiscus etc.
  2. Polysepalous: Here, the sepals are free. Examples: Anona, Tomato etc.

Depending upon the loss and persistence of a calyx

  1. Caducous: In this type, sepals fall as the flower blooms, like in the poppy plant.
  2. Deciduous: In this type, sepals fall along with the petals after fertilization, like in the mustard plant.
  3. Persistent: In this kind, the sepals remain intact with the fruits, like brinjal, chillies etc. It is further categorized into two groups:
    • Marcescent: When the persistent calyx acquires a shrivelled appearance and shows no growth after fertilization, known as marcescent calyx. Example Guava.
    • Accrescent: In this type, a persistent calyx simultaneously grows in size with the increasing fruit size even after fertilization and termed as the accrescent calyx. Example Physalis.

Depending upon the colour of sepals

  • Sepaloid: In this type, the sepals are green in colour.
  • Petaloid: In this type, the sepals are non-green or colourful like petals. Example: Mirabilis etc.

Depending upon the calyx-modification

The structure of calyx modifies into various appendages, and it is categorized into the following types:

  • Pappus: Here, the sepals transform into feather-like or hairy structures. Example Sunflower, Sonchus etc.
  • Spurred: The sepals transform into beak-like structures. Example Delphinium etc.
  • Petaloid: Sometimes, the sepals become enlarged and appear brightly coloured like petals. Example Mussaendra etc.
  • Hooded: Here, one of the sepals extends and transforms into hood-like structures over the flower. Example Aconitum etc.
  • Spinous: In this kind, the sepals modify into spines. Example Trapa etc.

Conclusion

Therefore, the calyx is a group of sepals that functions as an outermost whorl, which harbours the flower from its budding phase to the blooming or flowering stage against harsh environmental conditions desiccation, and sometimes modify to perform special tasks.


Definition of Shoot System

A shoot refers to the plant’s main stem or the complex network of various structures like branches, leaves, buds, flowers, and fruits attached to the main stem. A shoot or a shoot system always grow upwards to the ground and performs multiple functions like photosynthesis, storage, reproduction, transport, hormone production etc.

Characteristics of Shoot System

A shoot or seedling originates from the plumule of the seed’s embryo and shares the following morphological features:

It functions as a skeleton by constituting a major part of the shoot system and firmly supports the other components like leaves, buds, flowers and fruits. The main stem originates through the direct prolongation of the embryo’s tigellum and gives rise to the lateral stems, leafy appendages, buds etc.

Leaves

These are the flattened structures that hit the node of the main stem and the region in the middle of two nodes or internode. The angle forming between the leaf at the node section and the vertical stem commonly refers to the “Leaf axil”.

Leaves have a basipetal or acropetal pattern. They emerge out via differentiation of the shoot apical meristem. A leaf mainly comprises of three elements, namely:

  • Leaf base: It fixes the leaf to the stem’s node.
  • Petiole: It is a stalk-like appendage that joins a leaf base to the leaf lamina.
  • Leaf lamina: It is the leaf blade that comprises the midrib, veins and veinlets.

Axillary bud

It commonly refers to a lateral bud or lateral meristem attached to the leaf axil. An axillary bud has two types:

  • Type-I or Vegetative kind: It promotes the growth of the vegetative branch.
  • Type-II or Floral kind: It gives rise to the flowers from the rudimentary reproductive tissues.

Apical bud

It commonly refers to a terminal bud or terminal meristem found at the plant’s shoot apex. It appears small, compact, and contains apical meristematic tissues. Leaf primordia surround the apical bud.

Apical bud includes three meristematic layers of cells, namely protoderm, procambium and ground meristem. The ground tissues of the apical meristem further divide and differentiate to form vascular tissues that serve the conduction of food materials.

Flower

It constitutes the reproductive part of the shoot system and belongs to the members of angiosperms that are meant to reproduce sexually. A flower involves four characteristic whorls, namely:

  • Calyx: The arrangement of sepals collectively refers to the calyx that appears green-coloured, leaf-like and present towards the flower’s base.
  • Corolla: The arrangement of petals collectively refers to the corolla that is present above the calyx and appears bright-coloured.
  • Androecium: Stamen consisting of filament and anther will collectively constitute androecium or the male reproductive part.
  • Gynoecium: Carpel consisting of stigma, style and ovary will collectively constitute gynoecium or the female reproductive part.

Based on the function of floral parts, a flower is classified into the following two types:

  1. Accessory organs: It includes calyx and corolla that attract the pollinators like a honey bee, butterflies etc.
  2. Reproductive organs: It includes androecium and gynoecium that encourages the growth and fertilization of the flower.

Fruit

It is the reproductive structure that indicates the maturity or the age of the plant. “Parthenocarpic fruits” lack seed or reproductive parts and produce fruits asexually. An ovary or ovule differentiates into pericarp and seed, respectively, after complete fertilization. When a pericarp is thick, it differentiates into three distinct layers, namely outer epicarp, middle mesocarp and inner endocarp.

Cotyledons

They are the seed leaves or true leaves that germinate after the fertilization of the ovule from the plumule of the mature embryo.

Shoot System Functions

A shoot system of the plant body performs specific functions like:

Protection: In some plants, a stem comprises hairy or spiny structures on its surface that harbour a plant from predators. Some plants like bracken produce toxic materials that also keep away the grazing animals.

Support: The ground tissues like sclerenchyma and collenchyma also provide strength and rigidity to the stem. Thus, the stems withstand straight and embrace various components of the shoot system like leaves, lateral branches, buds, flowers etc.

Photosynthesis: Leaves of the shoot system possess chlorenchyma tissue, which contains a high chlorophyll pigment that absorbs light energy to produce sugar. Through photosynthesis, mechanism leaves maintain the proper metabolism.

Transpiration: Both leaves and stems undergo transpiration via stomata and lenticels, which allow gaseous exchange between the plants and the surrounding.

Conduction: A main stem of the shoot system participates in preparing food by the leaves to the other parts via phloem vessels. Besides, the root system facilitates the absorption of water and minerals to the other components via xylem vessels.

Hormone production: A shoot tip produces auxin (a growth regulatory hormone) that stimulates the vertical growth or height of the plant and restricts the growth of axillary bud.

A cytokinin is also a growth-regulating hormone that conquers the inhibitory effect of auxin by stimulating side branching or the growth of axillary bud. The cytokinins increase the diameter or thickness of the plant and give a bushy appearance.

Therefore, both the growth-regulating hormones can manipulate the plant’s growth pattern and can be used widely in the field of agricultural science.

Shoot Development

A shoot develops after embryogenesis, where a zygote inside an ovule goes through successive mitotic division to form a mature embryo. A mature embryo comprises five distinct regions that you could see in the diagram below:

A top green part will produce seed leaves or cotyledons, in between which a shoot apical meristem is present. The shoot apex or the terminal bud will stimulate elongation of the plant. The region below the apical meristem will encourage the growth of the main stem. The below two layers will give rise to the root system of the plant.


Leaf veins inspire a new model for distribution networks

A straight line may be the shortest path from A to B, but it's not always the most reliable or efficient way to go. In fact, depending on what's traveling where, the best route may run in circles, according to a new model that bucks decades of theorizing on the subject. A team of biophysicists at Rockefeller University developed a mathematical model showing that complex sets of interconnecting loops -- like the netted veins that transport water in a leaf -- provide the best distribution network for supplying fluctuating loads to varying parts of the system. It also shows that such a network can best handle damage.

The findings could change the way engineers think about designing networks to handle a variety of challenges like the distribution of water or electricity in a city.

Operations researchers have long believed that the best distribution networks for many scenarios look like trees, with a succession of branches stemming from a central stalk and then branches from those branches and so on, to the desired destinations. But this kind of network is vulnerable: If it is severed at any place, the network is cut in two and cargo will fail to reach any point "downstream" of the break.

By contrast, in the leaves of most complex plants, evolution has devised a system to distribute water that is more supple in at least two key ways. Plants are under constant attack from bugs, diseases, animals and the weather. If a leaf's distribution network were tree-like and damaged, the part of the leaf downstream of the damage would starve for water and die. In some of the Earth's more ancient plants, such as the gingko, this is the case. But many younger, more sophisticated plants have evolved a vein system of interconnected loops that can reroute water around any damage, providing many paths to any given point, as in the lemon leaf. Operations researchers have appreciated that these redundancies are an effective hedge against damage. What's most surprising in the new research, according to

Marcelo O. Magnasco, head of the Laboratory of Mathematical Physics at Rockefeller University, is that the complex network also does a better job of handling fluctuating loads according to shifts in demand from different parts of the system -- a common real-world need within dynamic distribution networks.

"For decades, people have believed that the tree-like network was optimal for fluctuating demand," Magnasco says. "These findings could seriously shake things up. People will have to take another look at how they design these kinds of systems."

In a paper published as the cover story of the January 29 Physical Review Letters, Magnasco, lead researcher Eleni Katifori, a fellow at Rockefeller's Center for Studies in Physics and Biology, and colleagues lay out a model that assigns a cost to each section of leaf vein proportional to how much water it can carry. They looked for networks that suffered the least strain in the face of two challenges common in both leaves and human-built networks: damage to a randomly chosen segment of the network and changes in the load demanded by different parts of the network. In both scenarios, they found the most robust system was a complex, hierarchical network of nested loops, similar to the fractal-like web of veins that transport water in leaves. This loopy network design is also found in the blood vessels of the retina, the architecture of some corals and the structural veins of insect wings.

Katifori is now extending the research to delve more deeply into how distribution networks handle fluctuating loads, guided by nature's own solution in the leaf.

"It is tempting to ignore the loops, because the central veins stand out and have a tree-like form," Katifori says. "But they are all connected, and the loops are right there to see, if you just look at the leaf."

Story Source:

Materials provided by Rockefeller University. Note: Content may be edited for style and length.


Results

We evaluated four metrics related to the reticulate structure and redundancy of leaves: VLA, loopiness, meshedness and robustness. VLA (mm −1 ) varied from a minimum value of 1.37 to a maximum of 10.76, with a mean of 3.17 (Figure 2). Loopiness (# of areoles/mm 2 ) varied from a minimum value of 0.19 to a maximum of 5.20, with a mean of 1.40 (Figure 3). Meshedness which ranges from 0 (a "tree" structured graph without any loops) to 1 (a network which has a maximum number of loops. Meshedness varied from a minimum value of 0.06 (i.e. more like a tree) to a maximum value of 0.26 (more like a maximally connected planar graph) with a mean of 0.14 standard deviation of 0.04. Robustness, estimated based on the difference between reticulate nets and MSTs weighted for conductance, varied from a minimum of 0.026 to a maximum of 0.150 thus leaves varied in their robustness to disturbance (Figures 2 and 3, Additional file 1: Figure S1–S339). Hence, whereas VLA provides a measure of vein investment per unit area, loopiness is a better indicator of features like distance from non-photosynthetic tissue meshedness provides a strong indicator of the shape of the network and its tendency to be redundant, and robustness provides a measure of a network's ability to remain connected under perturbations that damage the network. Note, our VLA values are lower on average, but overlap those previously reported [29],[30]. This is due to methodological differences, primarily the fact that our images were not magnified and of lower resolution (see discussion in [28],[31],[32]).

Positive correlations among the four network measures we quantified in this study VLA, loopiness, meshedness and robustness. As leaf networks become more like planar networks and less like trees, their loopiness, VLA, and ability to buffer disturbance increases. Note, in this and Figure 3 a single VLA value of 10.76 is not shown for figure clarity.


Watch the video: Μαλακά και ρυτιδιασμένα φύλλα. Τι έχουν να μας πουν τα φύλλα της ορχιδέας μας. (December 2022).