Guide to organismal biology

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Phylum Coniferophyta - (550 sp. in 50 genera, fr. Gr. conus=cone, ferre=to bear) - conifers

The conifers are the largest and most successful group of living gymnosperms. Many of our familiar forest trees are conifers, including pines, spruces, firs, hemlocks, yews, redwoods and cypress trees. Conifers are the longest lived trees on earth. The current record holder is a bristlecone pine at least 4,900 years old, the oldest living multicellular organism on Earth! Other bristlecone pines may be over 7,000 years old. Conifers are an ancient group, dating back 290 mya. They evolved during the Permian, toward the end of the Paleozoic, at a time when the climate was very cool and dry. Their special water conducting cells, called tracheids, allowed them to thrive in these climates and these same adaptations let them continue to dominate in colder and dryer environments today, such as northern latitudes, mountain slopes, and sandy soils. Because they are superior competitors in such habitats even today, they are the only phylum of gymnosperms to successfully compete with the flowering plants.

Most conifers are evergreens, with the larch and the bald cypress being notable exceptions. Their needle-shaped leaves are also an adaptation to conserve water. Needles usually occur in small bundles, each bundle emerging from a base that is actually a greatly truncated branch. Conifers have tremendous economic importance, as a source of timber and for byproducts such as pitch, tar, turpentine, and amber and other resins. Millions are sold each year as Christmas trees.

Pine Life Cycle

All conifers produce cone shaped strobili, both male cones (often called pollen cones) and female cones (often called seed cones or ovulate cones). Both male and female cones are usually produced on the same tree, but not at the same time, so the trees do not fertilize themselves. Female cones are large and conspicuous, with thick woody scales. Seed cones can persist on the tree for several years after fertilization. Male cones are small and puny looking, and usually don’t last long on the tree. A few species, like junipers and the locally common Podocarpus (front of Richardson), have seeds that are covered with a fleshy coating, and resemble small berries. (not real fruit - Incidentally, all parts of the Podocarpus are poisonous.)

The sporangia produced by the sporophytes are located at the bases of the sporophylls, and collected in the strobilus we call a pine cone. The microspore mother cell in the microsporangia produces the haploid pollen grains. Each scale or sporophyll in the male cone has two microsporangia on its lower surface. Each pollen grain consists of only four cells. When the immature pollen grain finally reaches the seed cone, the megaspore mother cell in the megasporangium produces four haploid megaspores. Three of these megaspores degenerate, and only the fourth germinates into the female gametophyte.

The female gametophyte consists of two or more archegonia, with a single egg in each one. All eggs are usually fertilized. Female cones are a little more complicated than male cones (wouldn’t you know). Each visible scale in the seed cone is really a much reduced lateral branch in itself. So each scale is homologous with the entire male cone. The megasporangium, which is called a nucellus in seed plants, is covered with a layer of protective cells called an integument, which is open at one end. This tiny opening, the micropyle, marks the point where the male pollen tube will grow into the megasporangium. The megasporangium, together with its integument, makes up the ovule. Seeds develop from ovules. Each scale in the seed cone has two ovules on the upper surface of the scale, and so will ultimately bear two seeds side by side.

The pollen grains formed in the microsporangia of pines have tiny wing on either side. (Why? Because they are wind-pollinated? Maybe...but we’ve recently found that it helps them to float up through the micropyle to the egg, like tiny water wings.). The ovulate cones open to receive pollen, then close again to protect the developing embryos.

When pollen grains land on the ovulate cones, they grow a long pollen tube. By the time this tube reaches the archegonia, about 15 months after pollination, the male gametophyte is fully mature. The pollen tube enters through the micropyle. The sperm nucleus divides in two, and the pollen tube discharges two sperm. One sperm nucleus degenerates, the other fertilizes the egg. It takes the female gametophyte about 15 months to mature, and about the same time for the pollen tube of the male gametophyte to reach it.

The seed develops within the megasporangium. The seed is the structure containing the embryonic plant and the stored nutrition to support it. A section of the surface of the scale usually detaches along with the seed, giving the seed a little wing to help disperse it farther from the tree.

Conifer seeds are very complex little structures, containing cells from three generations of the tree. The nutritive tissues inside the seed are actually the haploid body cells of the female gametophyte. The seed also contains the developing diploid sporophyte, the little embryonic conifer. The outer wrapping of the seed, the tough and protective seed coat, is formed from the diploid cells of the parent sporophyte. Pine seeds, along with acorns, are the most important source of plant food for North American wildlife.

Things to Remember

Know the life cycle of the pine.

Ecological, Evolutionary, and Economic Importance

Ephedra is the natural source of the drug ephedrin, which is used to treat hay fever, sinus headaches, and asthma (eg. sudafed tablets).

Zamia floridana is the only cycad native to the U.S., and was used by the Seminoles as a source of food.

Conifers are used for resin, pitch, turpentine, lumber, paper, and Christmas trees.

Pine seeds are a critical source of food for wildlife.

Cycads are important for landscaping, and add nitrogen to the soil for other plants.

Cycad stems are ground for use as sago flour in India, Japan, and other eastern nations.

Ginkgos are used for bonsai, as a source of herbal medicine, and as popular urban shade trees (because of their yellow autumn foliage and their resistance to air pollution).

Consider This

Why do conifers have an adaptive advantage in cool, dry environments?

Conifer seeds are very complex structures, containing cells from three generations of the tree. Can you figure out which tissues come from which generation of the conifer?

Introduction to Angiosperms

Just as Gymnosperms forced non-seed plants into the ecological background, the evolution of Angiosperms, sometime during the Cretaceous, forced gymnosperms into restricted habitats. Wherever the earth was cold or dry, gymnosperms could prevail. But in all other habitats, flowering plants rapidly became the dominant plant life. Their evolutionary origin, however, remains a deep mystery.

Flowering plants are able to survive in a greater variety of habitats than gymnosperms. Flowering plants mature more quickly than gymnosperms, and produce greater numbers of seeds. The woody tissues of angiosperms are also more complex and specialized. Their seeds are enclosed in a fruit for easy dispersal by wind, water, or animals. The leaves of angiosperms are mostly thin, extended blades, with an amazing diversity of shapes, sizes, and types.

The surface of the pollen grain has a complex three-dimensional structure. This structure is unique for each species, like a floral thumbprint. This is one of the ways that female plants can “recognize” pollen grains of the right species. It also means that pollen grains, which are abundant in the fossil record, allow us to reconstruct ancient plant communities, and these communities in turn tells us about ancient climates.

All angiosperms produce flowers, reproductive structures that are formed from four whorls of modified leaves. Most flowers have showy petals to attract pollinators, bribing insects and other animals with nectar, to get them to carry the male gametophyte through the air to another flower. Animal pollination is common in angiosperms, in contrast to the mostly wind-pollinated gymnosperms.

The ovules in angiosperms are encased in an ovary, not exposed on the sporophylls of a strobilus, as they are in gymnosperms. Angiosperm means "covered seed". The ovules develop into seeds, and the wall of the ovary forms a fruit to contain those seeds. Fruits attract animals to disperse the seeds.

Flowers consist of four whorls of modified leaves on a shortened stem: sepals, petals, stamens (an anther atop a slender filament), and one or more carpels. Imagine a broad leaf with sporangia fastened along the edges of the leaf. (Some ferns actually look like this.) Now fold that leave over along the midrib, and you've enclosed the sporangia in a protected chamber. Congratulations! You've just made a carpel. Just as sporophylls are leaves modified to hold spores, carpels are leaves modified to hold seeds.

Usually one or more carpels are fused together to form a stigma (upper surface), a style (long, slender neck), and an ovary (round inner chamber at the bottom) containing one or more ovules. Carpels lie at the heart of every flower. The flower is analogous to the strobilus of pines and more primitive plants, except that only the inner two whorls (stamens and carpels) actually bear sporangia. The base of the flower is called the receptacle, and the tiny stalk that holds it is the pedicel. The life cycle of flowering plants is described in more detail below.


Kingdom Plantae - Angiosperms

Phylum Anthophyta - flowering plants (= Magnoliophyta, Angiospermophyta)

Class Monocotyledonae - monocots (Zea, Lilium)

Class Dicotyledonae - dicots (Helianthus, Tilia)

Flowering Plant Life Cycle

Let’s start with the male plants, which are a little less complicated...Microspores develop in microsporangia in the anthers, at the tip of the stamen. Each anther has four microsporangia. Microspores develops by meiosis from the microspore mother cell. These microspores develop into pollen grains.

Pollen grains are the male gametophytes in flowering plants. Inside the pollen grain, the microspore divides to form two cells, a tube cell and a cell that will act as the sperm. Cross walls break down between each pair of microsporangia, forming two large pollen sacs. These gradually dry out and split open to release the pollen.

Meanwhile, inside the ovary, at the base of the carpel, the ovules, are developing, attached to the wall of the ovary by a short stalk. The megasporangia is covered by an integument, protective tissues that are actually part of the parent sporophyte. The nucellus and integuments together make up the ovule ( ----> seed).

The megaspore mother cell divides by meiosis to produce four haploid megaspores. Three of these megaspores degenerate, and the surviving fourth megaspore divides by mitosis. Each of the daughter nuclei divides again, making four nuclei, and these divide a third time, making a grand total of eight haploid nuclei. This large cell with eight nuclei is the embryo sac. This embryo sac is the female gametophyte in flowering plants.

One nucleus from each group of four migrates to the center. These are called the polar nuclei. The remaining three nuclei of each group migrates to opposite ends of the cell. Cell walls form around each group of three nuclei. The mature female gametophyte thus consists of only seven cells, three at the top, three at the bottom, and a large cell in the middle with two nuclei. One cell of the bottom three cells will act as the egg.

When the pollen grain reaches the stigma of the carpel, it germinates to form a pollen tube. This pollen tube will grow through the neck or style, all the way down to the bottom of the carpel, to a small opening called the micropyle.

The male gametophyte has two cells. One is the tube cell, the other will act as a sperm. As the pollen tube grows closer to the embryo sac, the sperm nucleus divides in two, so the mature male gametophyte has three haploid nuclei.

While the pollen tube is entering the ovule, the two polar nuclei in the female gametophyte fuse together, making one diploid nucleus. The two sperm nuclei enter the embryo sac. One sperm nucleus fuses with the egg nucleus to form a diploid zygote. The other sperm nucleus fuses with the fused polar nuclei to make a triploid cell.

This 3N cell will divide repeatedly to form the endosperm, the stored nutritive material inside the seed. This double fertilization occurs only in angiosperms and in Ephedra, the gnetophytes (though Ephedra doesn’t form endosperm).

The integuments develop into the tough outer seed coat, which will protect the developing embryo from mechanical harm or dessication. Thus the ovule, the integuments and the megasporangium they enclose, develops into the seed. The walls of the ovary then develop into the fruit. All angiosperms produce fruit, although we might not recognize many of these structures as “fruits”. (No such thing as “vegetables”, a convenient way to refer to a combination of fruits and leafy plant parts).


Seeds and Fruits

There is an incredible diversity of flower structure, not only in the number of sepals, petals, stamens, and carpels, but also in the way these modified leaves are attached with respect to the ovary. Linnaeus used these very characteristics to sort out the different related groups of flowering plants in his invention of binomial nomenclature, genus and species. All of these differences can affect the final physical appearance of the fruit. The ovary wall has three layers, each of which can develop into a different part of the fruit.

Simple fruits are fruits that develop from a single ovary. They can be either dry, like grains, nuts and legumes, or fleshy, like apples, tomatoes and cucumbers. Compound fruits develop from a group of ovaries. They can be either multiple fruits or aggregate fruits. In multiple fruits, like the pineapple, the group of ovaries come from separate flowers. Each flower makes a fruit, and these fruit fuse together. In aggregate fruits, like strawberries and blackberries, the fruit develops from a flower with many carpels. Each of these carpels develops as a separate fruitlet, that fuse together to form the compound fruit.

Seeds all bear the plant version of the belly button. They have a crescent-shaped scar called a hilum, where the ovule was attached to the wall of the ovary. Right above the hilum, if you look very carefully, you can also see a little pinprick scar that is a vestige of the micropyle.

Inside the seed, the tiny sporophyte embryo develops. When it is nearly ready to germinate, the seed contains one or two thick embryonic leaves. These seed leaves, or cotyledons, will support the tender baby plant while it establishes its roots and starts to grow its regular leaves.

Most angiosperms, like roses, marigolds, and maple trees, are members of the Class Dicotyledones, the dicots (190,000 sp.). These flowers have seeds with two seed leaves (di - cotyledon). Some angiosperms, like lilies, onions, and corn , are in the Class Monocotyledones, the monocots (65,000 sp.). The seeds of monocots have only one seed leaf (mono - cot..). There are several other differences between these two groups, which are summarized in the chapter on plant structure. There are seed leaves everywhere in Spring, and its impossible to tell what they will become just by looking at them.

Things to Remember

Know the life cycle of flowering plants.

Understand the functions of flowers, seeds, and fruit.

Economic, Ecological, and Evolutionary Importance

Most of our agricultural crops are angiosperms.

Commercial fruits and flowers are multi-billion dollar industries.

Angiosperms are the dominant planetary vegetation.

Consider This

Why are angiosperms better competitors than gymnosperms in most habitats?

The evolutionary innovation of the seed is analogous to the evolution of the amniotic egg in reptiles. Both allowed a large group of organisms to become fully terrestrial. How does the seed give angiosperms an evolutionary advantage over more primitive plants?

The competitive success of angiosperms is partly due to animal pollination, which allowed angiosperms to exist as small scattered populations. The wind pollinated gymnosperms needed large contiguous populations for effective pollination. The coevolution of angiosperms and their pollinators has greatly increased the diversity of angiosperms.

10 - Plant Structure


The transition from small aquatic forms to large terrestrial forms posed a series of major evolutionary problems. Several changes in plant body structure were necessary to meet the new challenges of the terrestrial environment.

The root-shoot system solved the problem of gravity faced by the first truly terrestrial plants. No longer buoyed up by water, land plants have to anchor themselves in the soil with root systems, and develop upright stems to hold their leaves toward the sun. Roots also have to obtain the water and nutrients that aquatic plants are bathed in.

Becoming larger also poses a serious evolutionary problem. Smaller primitive plants could rely on diffusion to move materials in and out of their bodies. Diffusion is too slow, however, to reach the innermost cells of larger organisms, and these internal cells would quickly starve. Larger and more advanced land plants evolved a network of tubes (vascular tissue) to quickly conduct water, nutrients (like nitrogen and phosphorous), and food throughout their bodies.

With over 230,000 species of flowering plants, it is amazing that there are only a few basic patterns of external anatomy that are repeated over and over again. Biologists use these basic patterns (and their endless variations) to help identify and classify organisms. They use a dichotomous key, a guide that presents you with two choices (e.g. leaves simple or compound). Each choice leads to two more choices, until you've successfully identified the organism. Being able to recognize a few of these simple patterns also helps us to appreciate the fundamental unity that underlies the sometimes bewildering diversity of the natural world.

Plant Structure

Plant cells differ in many respects from animal cells. They have chloroplasts, for one thing, and thick cell walls to support their thin cell membranes. The cell walls may be impregnated with lignin for extra rigidity. There are many types of cells commonly found in plants, with a variety of functions.

 1. Growth - meristem cells
 2. Support - parenchyma cells, collenchyma cells, sclerenchyma cells
 3. Transport - xylem cells, phloem cells

Meristem cells are undifferentiated cells that have the ability to divide quickly and repeatedly to build up new tissues of various types. Parenchyma cells are the most common type found throughout the body of the plant They make up the “background tissue” or pith. Collenchyma cells provide support in areas of primary growth. Their walls are sufficiently thickened to provide moderate support for the growing cells. The stringy stuff in celery is made of collenchyma cells. Sclerenchyma cells have a secondary cell wall, impregnated with lignin, and deposited inside the original cell wall. This makes them extra tough, and they are used wherever strong support is needed.

Phloem cells conduct food throughout the plant (ph-loem = f-ood). This conduction can go in either direction, up or down, depending on the concentration gradient of food in the plant. The sieve cells are the conducting elements. These living cells are aided by companion cells, which function to maintain the sieve cells. Xylem cells conduct water and dissolved mineral nutrients throughout the plant. Xylem cells conduct water in one direction, from the roots to the leaves. Xylem cells die soon after differentiating. There are two types of xylem commonly found in plants. The more primitive tracheids, which are characteristics of gymnosperms, and the larger diameter vessels, which are characteristic of angiosperms.

These larger bore vessels are part of the angiosperm’s competitive edge. They are much faster at conducting materials than the thinner tracheids. But, ironically, the more primitive tracheids are one of the main things that have allowed the conifers to still claim dominance over large parts of the planet, the parts that are cold or dry. From the standpoint of the plant, cold and dry and sandy are really all the same thing. All three conditions are major sources of water stress. Sandy is dry, because water quickly percolates down through sandy soils beyond the reach of the roots. Cold means snow and ice, and a tree standing in frozen water might as well be standing in a desert. The narrow needle-shaped leaves of conifers are one adaptation to cold or dry conditions. And the more primitive tracheids also give conifers an edge in cold or dry habitats.

Trees depend on an very thin unbroken column of water, rising from the roots all the way up to the leaves. Water constantly enters through the roots, and constantly leaves through the stomata of the leaves, rising like mercury rises in a thermometer. Whenever water is scarce or hard to get, this column of water often breaks, or cavitates. Cavitation can be deadly, because the plant must quickly reestablish this column of water before it wilts and dies. The larger bore vessels of angiosperms have a hard time coping with cavitation. The narrow tracheids of gymnosperms, however, resist cavitation and can quickly reestablish the flow when it is interrupted. So gymnosperms can outcompete angiosperms in habitats where water is scarce or hard to get, like in the far north, on frozen mountain slopes, or on sandy soils throughout the world. This is why we have mostly hardwoods south of Lake Pontchartrain, where the soils are mostly damp deltaic clays, and mostly pines north of the Lake, where the soils are very sandy.

These different types of plant cells combine into three types of tissues: epidermal tissue, ground tissue, and vascular tissue. Epidermal tissue form the outer layer of the plant body, its “skin”. It includes epidermal cells (“skin” or bark), root hairs, and the guard cells that open and close the stomata. Ground tissue, mostly parenchyma cells, makes up the bulk of the plant. Inside the stem it forms the pith, which functions in support and as a storage site for the sugars made during photosynthesis. Ground tissue on the outer edges of the stem is called the bark (stems) or cortex (roots). This tissue photosynthesizes or stores nutrients. Most root tissues are cortex tissues for food storage. Vascular tissues, the xylem and phloem, function to conduct food, water, and nutrients throughout the plant. Xylem and phloem are found together in vascular bundles, strands of tubes that run side by side.

Despite their superficial diversity, all flowering plants develop in roughly the same way. At the tip, or apex of the plant, both top and bottom, there is an area of very active cell division. These actively dividing cells, extending the shoot and roots, make up the apical meristem. Primary growth is controlled by tissues called meristems (fr. Gr. meri = part of, sta = stem). Primary growth extends the shoot up into the air and the roots down into the ground. Early in development, primary growth is the only type of growth. Many herbaceous annual plants, like lilies and violets, have only primary growth, and never increase much in thickness. But as most plants mature, they begins to thicken. The primary growth (apical meristem) is supplemented by secondary growth (lateral growth) or thickening. And this lateral growth comes from another type of meristematic tissue, the lateral meristem. Whereas the apical meristem occurs at the tips of the plant, the lateral meristem is a thin cylinder of tissue that rises through the plant.

Structure of Stems

Let’s start with the shoot, or stem. At the very top of the stem, and at the very tip of all the growing branches, is an apical meristem. The tender meristem tissue is protected by a terminal bud. Buds are really just greatly shortened stems. The scales of the bud are small modified leaves. These bud scales fall off, after new growth starts at the stem tip or branch tip in the spring. When they fall, they leave behind a characteristic scar, called a bud scale scar, that looks like several circular scars very close together. By counting backwards from the tip of the branch, and counting the number of these bud scale scars, you can actually tell exactly how old a branch is.

New lateral branches emerge from smaller buds called axillary buds, because they are found in the “axil” or armpit of the leaf, just above where the leaf joins the stem. And if you examine any branch, you will see that leaves and branches don’t just pop out at any point along the stem. There are certain points where all such growth occurs, called nodes. The length of stem between these growing points, or nodes, is the internode. So leaves and branches emerge at the nodes.

After the leaves fall off the stem, you can still see a crescent shaped scar, or leaf scar, on the stem where the curved base of the leaf was attached. Vascular bundles extend out from the stem into each and every leaf on the plant. If you look carefully at the leaf scars, you can see a series of tiny pinprick holes. These are the scars of the vascular bundles, or bundle scars.

What do we see when we cut a stem open? The lateral meristem forms the vascular cambium, which develops into new xylem and phloem cells, rising in a cylinder through the stem. Outside the cylinder of the vascular cambium is the cortex. Inside the cylinder is the pith. As the vascular cambium divides and develops into new vascular tissues, it always develops phloem toward the outside of the stem and xylem towards the inside of the stem.

This is very easy to see in lab. Xylem cells are always larger, and stain a dark red. They are always toward the inside of the vascular bundle. Phloem cells are smaller, and stain a light green. They are always found on the outside of the vascular bundle. In dicot stems, which you see here, the vascular bundles are neatly organized as little ovals around the ring of vascular cambium that creates them. In monocots, this vascular cambium is scattered in small strands throughout the pith. As a result, the vascular bundles are scattered throughout the stem, not organized into a ring as they are in dicots. When you look at a monocot stem, the scattered bundles look like little monkey faces. So for monocot, think monkey face.

Vascular tissues are replaced each year. Last year’s phloem is crushed against the bark by the new phloem. Xylem cells grow as a new ring of cells surrounding last year’s xylem cells. Old xylem cells are very stiff, and form what we call the “wood” of the plant. They can also be used as a dumping ground for various compounds in the plant. These annual growth patterns form the growth rings that are easily seen when you cut down a tree. Growth rings each have a broad band of lighter colored wood and a narrow band of darker colored wood. The broad band is the rapid growth of spring, while the narrow darker band is the slower growth of summer.

Stems can be modified in a variety of ways. We’ve already seen one major modification, the rhizome, a horizontal stem that spreads the plant. Stolons, or runners, are another type of modified stem that plants like the strawberry use to spread themselves around. Bulbs, like the onion, are actually a very compressed underground stem. The scales of the onion that we can easily peel back are highly modified leaves. Another type of stem specially modified for food storage is the tuber, the best example of which is the potato. Vines are another form of modified stem. Vines take the strategy of climbing up existing stems, thereby not needing to invest heavily in support structures like a rigid stem.

Things to Remember

Know what growth rings are, and know the difference between spring and summer wood.

Understand the difference between primary and secondary growth. How does this relate to the annual versus perennial growth habit of plants?

Know the position and functions of the apical and lateral meristem tissues in the body of the plant.

How do plant cells differ from animal cells?

Ecological, Evolutionary, and Economic Importance

Important "stem crops" include onions, potatoes, asparagus, and sugar cane.

Structure of Roots

Roots serve not only to hold the plant in the soil, and take up water and dissolved nutrients, but also as a site to store food for the plant. The main body of the root is called the primary root. The lateral or secondary roots develop as lateral extensions from the primary root. Along the outer surface of the root are thousands of root hairs, finger-like extensions of the epidermal cells which handle the actual uptake of water. Unlike the epidermis of the stem, the epidermis of the root has no waxy cuticle to keep water in or out. The outer layer of the root is a thick cortex, used for food storage.

The cylinder of vascular tissue, or stele, runs up through the center of the root. It forms a broad X, very distinctive. The larger cells of the X are the xylem, and the smaller cells nestled in between these arms are the phloem. The outer cells of the ring that encloses the vascular bundles is a special tissue called the endodermis. The endodermis (inner skin) controls the flow of water into the center of the root where the xylem sits. These cells are bordered on every side but one by a thin waxy strip called the Casparian strip. Water and dissolved materials can’t get between or around the endodermal cells. Water must actually pass through them, giving the root a living interface to regulate the influx of water into the plant. (This inner ring of cells, incidentally, is where the lateral roots emerge.)

If we examine the tip of the root, we find the other apical meristem. This meristem is covered by a tough layer of cells called the root cap. (Why?) Just behind the apical meristem is a zone in which cells are growing longer and larger, the zone of elongation. Above this zone is the area of the root where the new cells are starting to differentiate into specialized cells like xylem and phloem. This is the zone of differentiation.

There are several different kinds of roots. Most roots are either tap roots or fibrous roots. Tap roots, like those of carrots or dandelions, are huge primary roots with lots of stored food. Plants like grasses and other monocots, on the other hand, have fibrous roots, in which no one root dominates the rest. Many plants, like English Ivy and cat’s claw vines, have roots that emerge directly from the stem. Such roots are called adventitious roots. A few plants, like corn and mangroves, have large roots that emerge above ground, near the base of the shoot, and help prop up the plant. These roots are called prop roots.

Economic, Ecological, and Evolutionary Importance

Important root crops include carrots, sweet potatoes, turnips, radishes, and beets.

Structure of Leaves

Leaves are structured to make the process of photosynthesis as efficient as possible. Leaves have an upper and lower epidermis, covered by a waxy cuticle. Both surfaces are dotted with numerous stomata, with bean-shaped guard cells that regulate the passage of gases and water vapor to and from the leaf. Most of the stomata are found on the bottom of the leaf.

Between the upper and lower epidermis is a layer of parenchyma cells with many chloroplasts. It is in this “mesophyll” or middle leaf layer that most photosynthesis occurs. Below the upper epidermis is a fairly solid layer of rectangular cells called palisade parenchyma. Below this is a much more open layer of palisade cells, a spongy parenchyma layer, with many air spaces for diffusion of xygen and carbon dioxide.

Leaves consist of a simple flat blade on a stalk. The stalk, or petiole, has a swollen, curved base, where it attaches to the stem. This curved base is called the stipule. Celery is a modified leaf in which the petiole and stipule are very long and fleshy, with a short leaf on the top. A large midrib passes through the center of the leaf, carrying the vascular bundle from the stem out into the tissue of the leaf, sending out numerous side branches, or veins, to reach all parts of the leaf.

Simple leaves consist of a single blade on a single petiole. But many flowering plants have compound leaves, with many leaflets sharing a single petiole. If these leaves are arranged like the fingers on the palm pf your hand, we call them palmately compound. If they are arranged like the barbs on a feather, or “pinna”, we call them pinnately compound. Leaves can be arranged on the stem in one of three ways: they often occur in pairs, opposite one another. Or they may alternate on either side of the stem. Sometimes they emerge in little tufts, or whorls.

Within these basic patterns, leaves vary in a bewildering number of ways. They help botanists to identify plants. Their overall shape, the shape of their bases, the different kinds of leaf margins, or edges, and the different types of hairs on their surface are some of the traits we use to identify different flowering plants.

One of things leaves can tell us at a glance is whether a flowering plant is a monocot or a dicot (the two Classes of flowering plants). The pattern of the veins running through the leaf is a big clue. The veins of monocots run parallel to one another. Just think of a blade of grass. The veins of dicots form a net. Net venation can be either pinnate (oak leaves) or palmate (maple leaves). Let's summarize the external differences between monocots and dicots, a very ancient split in the evolution of flowering plants.

Monocots - one cotyledon (seed leaf), vascular bundles scattered in the pith, flower parts in threes, leaves with parallel venation

Dicots - two cotyledons, vascular bundles in a ring, flower parts in 4’s, 5’s, or multiples, leaves with net venation

Economic, Ecological, and Evolutionary Importance

Important leaf crops include lettuce, cabbage, celery, chicory, and spinach.

Consider This

Gymnosperms like pines are the only major group of plants to compete successfully with angiosperms (flowering plants). What aspect of their internal anatomy has enabled gymnosperms to out-compete angiosperms in habitats that are dry or sandy or cold?

What is the evolutionary strategy of a vine?

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