Cross Section Of Lily Leaf

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Leaves, the main site of photosynthesis by most plants, function in light interception and gas exchange. Since the uptake of carbon dioxide through leaf stomata must be balanced against water loss, leaf structure usually shows clear connections to prevailing environmental conditions. For instance, plants in arid environments often have thick leathery leaves, with fewer stomata, and a dense covering of hairs that reflect light and may reduce water loss from the leaf surface. On an individual plant, leaves that develop in the shade (shade leaves) are generally thinner and have fewer leaf hairs than leaves that develop in the sun (sun leaves). Leaves are also often modified chemically or physically for defense against herbivores.

adaptations that allow them to thrive under such conditions. Many have thick, leathery leaves and fewer stomata, or stomata that are sunken below the surface in special depressions, all of which reduce loss of water through transpiration. They also may have succulent, water-retaining leaves or no leaves at all (with the stems taking over the function of photosynthesis), or they may have dense, hairy coverings. Pine trees, whose water supply may be severely restricted in the winter when the soil is frozen, have some leaf modifications similar to those of desert plants. The modifications include sunken stomata, a thick cuticle, and a layer of thick-walled cells (the hypodermis) beneath the epidermis (Fig. 7.12). The leaves of compass plants face east and west, with the blades perpendicular to the ground, so that when the sun is overhead, it strikes only the thin edge of the leaf, minimizing moisture loss (Fig. 7.13). The submerged leaves of plants that grow in water usually have considerably less xylem than phloem, and the mesophyll, which is not differentiated into palisade and spongy layers, has large air spaces. Other modifications are described in the sections that follow.

Tendrils

There are many plants whose leaves are partly or completely modified as tendrils. These modified leaves, when curled tightly around more rigid objects, help the plant in climbing or in supporting weak stems. The leaves of garden peas are compound (Fig. 7.14), and the terminal leaflets are reduced to whiplike strands that, like all tendrils, are very sensitive to contact. If you lightly stroke a healthy tendril, there is a

Open Stomatal Pore Leaf Cross Section

Figure 7.12 A pine needle in cross section. (Photomicrograph by G.S. Ellmore)

resin canal mesophyll transfusion tissue xylem phloem endodermis sunken stomatal pore hypodermis epidermis

Figure 7.12 A pine needle in cross section. (Photomicrograph by G.S. Ellmore)

Transfusion Tissue Pine Needles
Figure 7.13 A compass plant (Lactuca serriola). The leaves of this and other compass plants such as Silphium laciniatum have their blades parallel to the sun and are oriented toward the east and west.
Cross Sectiuon Flowering Plant
Figure 7.14 The terminal leaflets of this garden pea plant leaf are modified as tendrils.

sudden rapid growth of cells on the opposite side, and it starts curling in the direction of the contact within a minute or two. If the contact is very brief, the tendril reverses movement and straightens out again. If, however, the tendril encounters a suitable solid support (e.g., a twig), the stimulation is continuous, and the tendril coils tightly around the support as it grows.

Whole leaves of yellow vetchlings are modified as tendrils, and photosynthesis is carried on by the leaflike stipules at the bases. In the potato vine and the garden nasturtium, the petioles serve as tendrils, while in some greenbriers, stipules are modified as tendrils. In Clematis, the rachises of some of the compound leaves serve very effectively as tendrils. Members of the Pumpkin Family (Cucurbitaceae), which includes squashes, melons, and cucumbers, produce tendrils that may be up to 3 decimeters (1 foot) long.

As the tendrils develop, they become coiled like a spring. When contact with a support is made, the tip not only curls around it, but the direction of the coil reverses (see Fig. 11.8); sclerenchyma and collenchyma cells then develop in the vicinity of contact. The sclerenchyma cells provide rigid support, while the collenchyma cells impart flexibility. This makes a very strong but flexible attachment that protects the plant from damage during high winds. The tendrils of many other plants (e.g., grapes) are not modified leaves but develop instead from stems.

Spines, Thorns, and Prickles

The leaves of many cacti and other desert plants are modified as spines. This reduction in leaf surface correspondingly reduces water loss from the plants, and the spines also tend to protect the plants from browsing animals. In such desert plants, photosynthesis, which would otherwise take place in leaves, occurs in the green stems. Most spines are modifications of the whole leaf, in which much of the normal leaf tissue is replaced with sclerenchyma, but in a number of woody plants (e.g., mesquite, black locust), it is the stipules at the bases of the leaves that are modified as short, paired spines. Like grape and other tendrils, many spinelike objects arising in the axils of leaves of woody plants are modified stems rather than modified leaves. Such modifications should be referred to as thorns to distinguish them from true spines. The prickles of roses and raspberries, however, are neither leaves nor stems but are outgrowths from the epidermis or cortex just beneath them (Fig. 7.15).

Storage Leaves

As previously mentioned, desert plants may have succulent leaves (i.e., leaves that are modified for water retention). The adaptations for water storage involve large, thin-walled parenchyma cells without chloroplasts to the interior of chlorenchyma tissue just beneath the epidermis. These nonpho-tosynthetic cells contain large vacuoles that can store relatively

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Plant Spines Without Leaves

Figure 7.1 SB Thorns (modified stems) produced in the axils of leaves.

Prickles Raspberry
Figure 7.1 SC Prickles of a raspberry stem. The prickles are neither leaves nor stems; they are outgrowths from the epidermis or the cortex just beneath it.

substantial amounts of water. If removed from the plant and set aside, the leaves will often retain much of the water for up to several months. Many plants with succulent leaves carry on a special form of photosynthesis, discussed in Chapter 10.

The fleshy leaves of onion, lily, and other bulbs store large amounts of carbohydrates, which are used by the plant in the subsequent growing season.

Flower-Pot Leaves

Some leaves of Dischidia (Fig. 7.16), an epiphyte (a plant that grows, usually non-parasitically, on other plants) from tropical Australasia, develop into urnlike pouches that become the home of ant colonies. The ants carry in soil and add nitrogenous wastes, while moisture collects in the leaves through condensation of the water vapor coming from the mesophyll through stomata. This creates a good growing medium for roots, which

Flowering Trees For Pots
Figure 7.16 A flower-pot leaf of Dischidia. The leaf has been sliced lengthwise to reveal the roots inside that have started to grow down from the top.

develop adventitiously from the same node as the leaf and grow down into the soil contained in the urnlike pouch. In other words, this extraordinary plant not only reproduces itself by conventional means but also, with the aid of ants, provides its own fertilized growing medium and flower pots and then produces special roots, which "exploit" the situation.

Window Leaves

In the Kalahari desert of Botswana and South Africa, there are at least three plants belonging to the Carpetweed Family (Aizoaceae) that have unique adaptations to living in dry, sandy areas. Their leaves, which are shaped like ice-cream cones, are about 3.75 centimeters (1.5 inches) long (Fig. 7.17) and are buried in the sand; only the dime-sized wide end of a leaf is exposed at the surface. This exposed end is covered with a relatively transparent, thick epidermis with few stomata and a waxy cuticle. There is a mass of tightly packed, transparent water-storage cells below the exposed end; these allow light coming through the "windows" to penetrate to the chloroplasts in the mesophyll, located all around the inside of the shell of the leaf. This arrangement, which keeps most of the plant buried and away from drying

Carpetweed Kalahari
Figure 7.17 A window plant (Fenestraria). Note the transparent tips of the window leaves.
Thick Leave Flowering Plants
Figure 7.18 A leaf of an air plant (Kalanchoe), showing plantlets being produced along the margins.

winds, allows the plant to thrive under circumstances that most other plants could not tolerate. Window leaves also occur in succulent plants of a few other families.

Rep roductive Leaves

Some of the leaves of the walking fern are most unusual in that they produce new plants at their tips. Occasionally, three generations of plants may be found linked together. The succulent leaves of air plants (Fig. 7.18) have little notches along the leaf margins in which tiny plantlets are produced, complete with roots and leaves, even after a leaf has been removed from the parent plant. Each of the plantlets can develop into a mature plant if given the opportunity to do so.

Floral Leaves (Bracts)

Specialized leaves known as bracts are found at the bases of flowers or flower stalks. In the Christmas flower (poinsettia), the flowers themselves have no petals, but the brightly colored floral bracts that surround the small flowers function like petals

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Cross Section Flower
Figure 7.1Q Apoinsettia (Euphorbia pulcherrima) "flower." There are several flowers without petals in the center. The most conspicuous parts of the "flower" are the colored bracts (modified leaves) surrounding the true flowers.

in attracting pollinators (Fig. 7.19). In dogwoods and a few other plants, the tiny flowers in their buttonlike clusters do have inconspicuous petals. However, the large white-to-pink bracts that surround the flower clusters, which appear to the casual observer to be petals, are actually modified leaves. In Clary's annual sage (Salvia viridis), large colorful bracts are produced at the top of flowering stalks, well above the flowers (Fig. 7.20).

Insect-Trapping Leaves

Highly specialized insect-trapping leaves have intrigued humans for hundreds of years. Almost 200 species of flowering plants are known to have these leaves. Insectivorous plants grow mostly in swampy areas and bogs of tropical and temperate regions. In such environments, certain needed elements, particularly nitrogen, may be deficient in the soil, or they may be in a form not readily available to the plants. Some of these elements are furnished when the soft parts of insects and other small organisms trapped by the specialized leaves are broken down and digested. All the plants have chlorophyll and are able to make their own food. It has been demonstrated that they can develop normally without insects if they are given the nutrients they need. The following plants represent four types of insect-trapping mechanisms.

Pitch er Plants

Cross Sectiuon Flowering Plant

Figure 7.20 Clary's annual sage (Salvia viridis). Note the small flowers along the lower half of the stem and the large colorful bracts toward the top.

The blades of leaves of many pitcher plants are flattened and function like those of any other leaves. Some of the leaves of these curious plants, however, are larger and cone-shaped

Figure 7.20 Clary's annual sage (Salvia viridis). Note the small flowers along the lower half of the stem and the large colorful bracts toward the top.

Leaves 123

Cross Sectiuon Flowering Plant

Figure 7.21

(Sarracenia).

Insect-trapping leaves of pitcher plants

Figure 7.21

(Sarracenia).

Insect-trapping leaves of pitcher plants

Merchandisers have been shameless in their wholesale collection of these plants for sale, and pitcher plants may become extinct in the wild. The cobra plant, a pitcher plant restricted to a few swampy areas in California and Oregon, has been placed on official threatened species lists.

Sundews

The tiny plants called sundews (Fig. 7.22) often do not measure more than 2.5 to 5.0 centimeters (1 to 2 inches) in diameter. The roundish to oval leaves are covered with up to 200 upright glandular hairs that look like miniature clubs. There is a clear, glistening drop of sticky fluid containing digestive enzymes at the tip of each hair. As the droplets sparkle in the sun, they may attract insects, which find themselves stuck if they alight. The hairs are exceptionally sensitive to contact, responding to weights of less than one-thousandth of a milligram, and bend inward, surrounding any trapped insect within a few minutes. The digestive enzymes break down the soft parts of the insects, and after digestion has been completed (within a few days), the glandular hairs return to their original positions. If bits of nonliving debris happen to catch in the sticky fluid, the hairs barely respond, showing they can distinguish between protein and something "inedible." Some sundew owners regularly feed their plants tiny bits of hamburger and boiled egg white.

In Portugal, relatives of sundews with less specialized leaves are used in houses as flypaper. In response to contact by living insects, the edges of specialized leaves of similar plants called butterworts rapidly curl over and trap unwary victims.

or vaselike. In some species, these larger pitcher leaves have umbrella-like flaps above the open ends (Fig. 7.21), but the flaps don't prevent a little rain water from accumulating at the bottom. Some Asian pitcher plants are vines with leaves whose long petioles are twisted around branches for support. Their pitchers are formed at the tips of the leaves.

Pitcher leaves have nectar-secreting glands around the rim. The distinctive odor produced by these glands attracts insects, which, while foraging, often fall into the watery fluid at the bottom. If the insects try to climb out, they find the walls highly polished and slippery. In fact, the walls of some pitcher plant leaves are coated with wax, and as the insects struggle up the surface, their feet become coated with the wax, which builds up until the victims seem to have acquired heavy clodlike boots. Most insects never make it up the walls, but even if they do, they still face a formidable barricade of stiff downward-pointing hairs near the rim. Eventually they drown, and their soft parts are digested by bacteria and by enzymes secreted by digestive glands near the bottoms of the leaves.

In North America, the pitcher plants produce their pitchers in erect clusters on the ground, but the mechanisms for trapping insects are similar to those of Asian species. Malaysian tree frogs, which have sticky pads on their feet, are undaunted by pitcher-plant leaves and lay their eggs in them. The eggs contain a chemical that neutralizes the pitcher's digestive enzymes.

Venus s Flytraps

The Venus's flytrap (Fig. 7.23), which has leaves constructed along the lines of an old-fashioned steel trap, is found in nature only in wet areas of North Carolina and South Carolina. The two halves of the blade have the appearance of being hinged along the midrib, with stiff hairlike projections located along their margins. There are three tiny trigger hairs on the inner surface of each half. If two trigger hairs are touched simultaneously or if any one of them is touched twice within a few seconds, the blade halves suddenly snap together, trapping the insect or other small animal. As the organism struggles, the trap closes even more tightly. Digestive enzymes secreted by the leaf break down the soft parts of the insect, which are then absorbed. After digestion has been completed, the trap reopens, ready to repeat the process. Venus's flytrap leaves, like those of sundews, do not normally close for bits of debris that might accidentally fall on the leaf, because nonliving material does not move about and stimulate the trigger hairs.

Bladderworts

Bladderworts (Fig. 7.24), which are found submerged and floating in the shallow water along the margins of lakes and streams, have finely dissected leaves with tiny

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Cross Sectiuon Flowering Plant
Figure 7.22 Sundew (Drosera) leaves.
Cross Sectiuon Flowering Plant
Figure 7.23 AVenus's flytrap plant (Dionaea muscipula).

bladders. The stomach-shaped bladders are between 0.3 and 0.6 centimeter (0.125 to 0.25 inch) in diameter and have a trapdoor over the opening at one end. The trapping of aquatic insects and other small animals takes place through a complex mechanism. Four curled but stiff hairs at one end of the trapdoor act as triggers when an insect touches one of them. The trapdoor springs open, and water rushes into the bladder. The stream of water propels the victim into the trap, and the door snaps shut behind it. The action takes place in less than one-hundredth of a second and makes a distinct popping sound, which can be heard with the aid of a sensitive underwater microphone. The trapped insect eventually dies, is broken down by bacteria, and the breakdown products are absorbed by cells in the walls of the bladder.

Science-fiction writers have contributed to superstitions and beliefs that deep in the tropical jungles there are plants capable of trapping humans and other large animals. No such plants have been proved to exist, however. The largest pitcher plants known hold possibly 1 liter (roughly 1 quart) of fluid in their pitchers, and small frogs have been known to decompose in them, but the trapping of anything larger than a mouse or possibly a small rabbit seems very unlikely.

Small Flowering Plants

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