Angiosperms (Flowering Plants)

Angiosperms, also called flowering plants, are the largest group of plants in the world and are found on every continent including Antarctica. They are usually the dominant plant type in their respective ecosystems, with a notable exception of temperate coniferous forests of the Pacific Northwest and Northern Eurasia. 

In 1898, Charles Darwin wrote that the diversity of angiosperms was an “abominable mystery” and “a most perplexing phenomenon.” The evolutionary mechanisms that led to such an extraordinary diversity of species are still being uncovered. The majority of plant species are angiosperms, and there are as many 400,000 species. A more realistic estimate is 350,000, and uncertainty is due to the estimated amount of species that have yet to be discovered and described by taxonomists.

History and Evolution of Angiosperms 

In order to understand the history of angiosperm evolution, we must study their fossil history. This kind of research is called paleobotany, and many researchers focus entirely on angiosperm fossils. 

Cretaceous Period

Prior to the emergence of angiosperms, the dominant plant type on earth were the conifers, along with other green plants. Gymnosperms arose over 300 million years ago during the Devonian period, based on fossil evidence. They dominated terrestrial plant life for over 200 million years before angiosperms arrived on the scene.

The fossil record indicates angiosperms were present as far back as 145 million years ago, during the early Cretaceous period. By 100 million years ago, angiosperms were already incredibly diverse, determined by pollen analyzed from lake beds. The diversification of angiosperms happened incredibly rapidly over a relatively short period of time, geologically speaking. Since then, they have been the dominant type of land plant. 

 Some of the oldest fossil angiosperms have been found in regions that were once tropical. It is hypothesized that angiosperms originated in warmer regions of the world, and subsequently radiated to both poles.

Angiosperms Compared to Other Plants

How did angiosperms become the dominant vegetation type compared to other plants? One possible explanation is their increased photosynthetic capacity due to intricate leaf venation compared to other plants and their phylogenetic evolution. Additionally, angiosperms typically have smaller genomes than other plants, which allows for cells to be smaller and a higher rate of carbon uptake to occur. Both of the aforementioned features of flowering plants may have allowed them to outcompete neighbors and dominate the landscape.

Angiosperms Compared To Gymnosperms 

Angiosperms and gymnosperms are both seed plants. That is, they reproduce by seeds rather than spores like ferns and mosses do. Another similarity is that they have the ability to produce secondary growth. The key difference between these two plant groups is that angiosperms produce flowers and fruit, with seeds enclosed in ovaries. Gymnosperms produce naked seeds borne in cones.  

Current Ecological Importance of Angiosperms 

Angiosperms are necessary for everyday life, and a primary source of consumer goods. From the food we eat to the air we breathe to the materials our buildings are made of, flowering plants play an important and underappreciated role in sustaining all animals on the planet, including humans.

Food

The most obvious benefit of angiosperms is the food they produce. Flowering plants are necessary to sustain insect populations, as the nectar and pollen they produce are vital food sources for bees, butterflies, and many other insects. Vegetative parts of flowering plants like leaves and roots are equally important for many animals. The fruit plants produced are eaten by birds, mammals, and even fish. Angiosperms are essential to sustain the world’s animals, including humans

The vast majority of the food humans eat comes from flowering plants. Aside from the direct consumption of angiosperms, humans rely on them indirectly by feeding livestock. The following list contains the most important plant families for food production and examples of food plants in each family.

• Poaceae (grass family): Corn, rice, cane sugar, wheat, barley, and all other cereal grains.
• Fabaceae (legume family): Beans, peas, and soy.
• Solanaceae (nightshade family): Potatoes, tomatoes, chili peppers, and eggplant.
• Brassicaceae (mustard family): Broccoli, cabbage, cauliflower, and other common vegetables.
• Rosaceae (rose family): Apples, pears, cherries, strawberries, almonds, and other fruit.
• Cucurbitaceae (cucumber family): Cucumber, squash, gourds, and watermelon.

Medicine

Modern pharmaceuticals would not exist without angiosperms, which contain a diverse suite of secondary compounds. Secondary compounds are not vital for plant growth but are necessary for defense against herbivores and pathogens. Many plants have evolved incredibly complex chemicals to defend themselves, and these very same chemicals have been found to have extraordinary medicinal benefits. Humans have been using plants as medicine for millennia, but only recently have we been able to isolate and synthesize chemicals from medicinal plants.

Some plants are grown and harvested to directly collect chemical compounds, while others have inspired the development of synthetic pharmaceuticals. For example, morphine is derived from the poppy plant Papaver somniferum (Papaveraceae), many alkaloids from nightshade Atropa belladonna (Solanaceae), and phytoestrogens from Angelica sylvestris (Apiaceae). The active compounds in aspirin were originally discovered in the leaves of willow trees in the genus Salix (Salicaceae).

Materials

Many of the materials we use on a day to day basis are made from angiosperm products. Some examples of angiosperm-based textiles are cotton from Gossypium species (Malvaceae), linen from Linum usitatissimum (Linaceae), and hemp from a variety of Cannabis sativa (Cannabaceae). 

 Aside from fibers, angiosperms provide a broad range of building materials, from hardwood lumber to bamboo. Biofuel is another essential flowering plant product, and a more sustainable alternative to fossil fuels like petroleum, coal, and natural gas. A rudimentary form of biofuel is wood, which can be burned to produce heat that is turned into energy. More advanced forms of biofuel include liquids such as ethanol, which is produced from corn and is combined with gasoline.   

Carbon Sequestration

Perhaps the most important service angiosperms provide is carbon sequestration. As plants photosynthesize, they absorb carbon dioxide and convert the carbon into plant material. This act of carbon intake and storage is called carbon sequestration and is an incredibly effective way of removing carbon dioxide, a greenhouse gas, from the atmosphere. Carbon sequestration is necessary in order to mitigate climate change. 

 The places that sequester carbon most efficiently are tropical forests. The Amazon rainforest alone is estimated to absorb as much as 600 million tons of carbon each year. Most of the world’s tropical forests are dominated by angiosperms, which makes them essential for carbon sequestration. 

Structure of Angiosperms

 Vascular Plants

 Angiosperms are one of the three main groups of vascular plants. Vascular plants are defined by having specialized tissues, of which there are two kinds:

• Xylem is responsible for transporting water and minerals. The xylem also produces wood, a structural element found in woody plants. 
• Phloem is responsible for transporting nutrients such as sugars and carbohydrates. 

 Nonvascular plants like bryophytes lack vascular tissues and are thus typically much smaller and more prone to drying out.

Roots

Angiosperm roots are not too different from the roots of gymnosperms. In general, roots are primarily responsible for water and nutrient acquisition as well as structural support. Secondary functions of roots include nutrient storage and vegetative reproduction. Rooting structures are mostly found directly below the surface of the soil, but occasionally can penetrate deep underground, especially in habitats with deep water tables. In the arctic tundra, rooting depths rarely exceed one meter. In contrast, the deepest rooting plant ever recorded was an African flowering plant, Boscia albitrunca (Capparaceae), with a rooting depth of 68 meters. 

Nutrient acquisition is perhaps the most important function of plant roots. Nitrogen, phosphorus, potassium, and an array of micronutrients are essential to plants. These minerals are found primarily in inorganic compounds in the soil. It is up to the roots to absorb minerals and transport them to other organs so plants can photosynthesize. Some roots have symbiotic associations with mycorrhizal fungi and rhizobium bacteria, which make phosphorus and nitrogen acquisition, respectively, much easier.

Stems 

Like the roots, angiosperm stems are not uniquely different from gymnosperms and other vascular plants. In general, stems house vascular tissue and provide structural support. In angiosperms, they vary a great amount. The skinny stem of a basil plant is the same plant part, anatomically speaking, as the trunk of an immense 800-year-old giant sequoia. 

Plant stems can also provide supplemental functions such as photosynthesis, defense, and protection against the environment. Stems often contain chlorophyll, especially in herbaceous plants. Even some large trees have photosynthetic bark, which aids the plant in photosynthesis even when it has dropped its leaves. Spines and thorns are common defensive features that are most frequently found on stems. Woody plants have the ability to produce bark on their stems, which is essential to protect the plant from the environment. Some trees have bark that is several centimeters thick, and can shield vascular tissue from below freezing temperatures and fire.

Leaves

Angiosperm leaves are unique in that they have intricate venation and display a large diversity of shapes and modifications. The main component of a leaf is the blade, which is usually connected to the stem by a petiole. Some leaves have stipules, which is a leaf type structure that often covers the node at which the leaf is connected. 

Angiosperm leaves display a remarkable amount of diversity in their arrangement, size, and shape. There are three leaf arrangements:

• Alternate: one leaf emerging at each node.
• Opposite: two leaves emerging at each node.
• Whorled: more than two leaves emerging at each node.

 In addition, leaves can either be simple or compound. Simple leaves have only one blade connected to the petiole, while compound leaves have two or more leaves connected to the petiole. The figure below illustrates some of the diversity in leaf shapes, which are described in dozens of ways. The broadest distinctions are palmate and pinnate leaves. Palmate leaves resemble a hand with multiple lobes, while pinnate leaves have just one leaf tip at the apex of the blade.

Leaves allow plants to photosynthesize and transpire. The wide, flattened leaf blades of angiosperms maximize solar intake, with the tradeoff of being more prone to drying out due to higher evapotranspiration potential. Leaves contain the majority of a plant’s chlorophyll, which gives them their characteristic green color. They also contain stomata, specialized pores on the surface of a leaf that control gas exchange.

Leaves aren’t always flat and broad, and are often thick and tough to store water. Succulent plants take advantage of this particular modification, which allows them to survive in the most arid of environments. 

Other Specialized Plant Organs

Aside from the three main organs of roots, stems, and leaves, angiosperms produce a variety of specialized organs, many of which are exclusive to flowering plants. Spines, thorns, and spikes are important defensive structures on plants and each is a modification of a preexisting organ. Spines are modified leaves or stipules and are most recognizable in the cactus family (Cactaceae). Thorns are protrusions of the stem, exemplified by the sharp thorns of citrus trees. Spikes are extensions of the epidermis or cortex of a plant and do not contain vascular tissue, unlike spines and thorns. An example of spikes is the sharp branch protrusions on rose bushes.

Another specialized organ is a tendril. Tendrils are another type of leaf modification and can either be simple or branched. Vining and creeping plants rely on tendrils to find and cling on to existing structures, often other plants, in order to grow upward and reach more light without providing their own structural support. 

Carnivorous plants, all of which are angiosperms, have evolved fascinating and complex leaf modifications that allow them to trap insects and other animals they digest to acquire nutrients. Venus flytraps have two paddle-shaped leaves with minute hairs that, when triggered, cause the leaves to close and actively capture prey. Pitcher plants modify their leaves into large, vase-like structures and lure prey inside where they are trapped. 

Bracts are modified leaves that are associated with flowers. Bracts are most often brightly colored and serve the same function as flower petals. For example, the colorful “flowers” on Bougainvillea are actually bracts, which surround multiple small flowers.

Angiosperm Reproduction

The most diagnostic feature of flowering plants is their method of reproduction. Most botanists credit the advent of flowers and fruit as the catalyst for flowering plant diversification.

Flowers

Flowers have complex reproductive organs and display a wide variety of morphology. They may be borne on the end of branches (terminally) or in a leaf axis (axillary). Uncommon floral arrangements include growing directly from the soil (common in parasitic plants), or even on a leaf blade. Flowers can either be solitary or grouped together on a structure called an inflorescence.

Anatomy

Flowers contain a maximum of four parts: sepals, petals, stamens, and carpels. The sepals, collectively called the calyx, protect the flower bud and are usually leafy green structures. Petals are often colorful and meant to attract pollinators. A group of petals is called the corolla. Together, the calyx and corolla make up the perianth. 

 Next are the stamens, which are the male reproductive organs. Stamens consist of a structural part called the filament and pollen-producing anthers at their tip. The innermost part of a flower is the female organ called the pistil. A pistil consists of one or more carpels that contain a stigma, style, and ovary. 

Pollination

In order for an ovary to produce a seed, it must be fertilized. Flowering plants have coevolved with pollinators for millions of years, and it’s often the pollinator that brings the pollen from one flower to another. Insects, birds, and even mammals are usually rewarded with nectar for visiting a flower. In turn, the pollinator inadvertently carries pollination from one flower to the next. Often, pollen is taken from one plant to a different individual of the same species. This act of “cross-pollination” is vital for species to maintain genetic diversity. However, some plants are able to self pollinate.

Fertilization and Embryogenesis

Once a suitable pollen grain from a male anther comes into contact with a female stigma, it germinates and creates a pollen tube that penetrates into the ovary. Then, two sperm cells travel to the ovary. One sperm cell fertilizes the egg creating a zygote, while the other sperm cell fuses with two polar nuclei to form the endosperm. The zygote eventually turns into the embryo, which uses the endosperm as its nutrient source. The embryo and endosperm together constitute a seed. This system using two sperm cells is called double fertilization. 

Fruit

Anatomy

Once one or more ovules are fertilized, the entire floral reproductive structure transforms to form a fruit. A fruit consists of one or more ripened ovaries and any other modified part that forms one cohesive structure. Fruit can either be fleshy or dry and are morphologically diverse. Seeds are usually contained on the inside of a fruit. Surrounding the seed is the pericarp, which consists of an endocarp, mesocarp, and exocarp. The mesocarp is usually the part of the fruit that is eaten.

The two main types of fleshy fruit are drupes and berries. Drupes contain only one seed, which is often quite large (e.g. avocados and peaches). Berries contain multiple seeds. This means tomatoes and cucumbers are botanically considered berries. 

Seed Dispersal

 Fruit have a variety of methods to ensure their seeds are transported to a suitable place. Fleshy fruit most often rely on animals to ingest them and deposit seeds elsewhere. Dry fruit can be dispersed by wind, which we see in dandelions and the samara of maple trees. Other dry fruit have barbs that cling on to passing animals. Some dry fruit are dehiscent, which means they remain on the plant and open to release wind-dispersed seeds. Other unique forms of dispersal include water dispersal (coconuts) and bizarre exploding fruit. Once a seed is dispersed, it begins a new life cycle. 

Groups of Angiosperms:

 The staggering diversity and relatively short evolutionary history of angiosperms have inspired a vast amount of research on their taxonomy and phylogeny. There is a wealth of research on angiosperm phylogeny and so there are multiple systems one can follow. The most widely accepted system is from the Angiosperm Phylogeny Group (APG IV), so we will follow this system as we discuss flowering plant phylogeny.  

The section that follows will discuss different clades, orders, and families of plants. Orders will always end with -ales and families will always end with -aceae. 

Basal Angiosperms

The oldest group of flowering plants are referred to as basal angiosperms. A handful of orders are included within this group and contain a portion of the flowering plants we call dicotyledons, or dicots. 

General features of basal angiosperms include the ability to produce secondary growth. That is, their stems thicken and produce wood underneath vascular tissue. However, it’s important to note that many dicots don’t produce secondary growth and are solely herbaceous plants. Basal angiosperms also have organized vascular tissue. The xylem and phloem are organized on the outside of the stem, right underneath the bark. This group also produces two embryonic leaves, which are called cotyledons. The term “dicot” alludes to the pair (di-) of cotyledons (cot) which are the first leaves produced by a seed after germination.

Amborellales

The order Amborellales is the oldest group of angiosperms, evolutionarily speaking. This group is considered to be the “sister” taxon of all other angiosperms, meaning it diverged before any other angiosperm group did. Because of this, it is an incredibly important group in studying the genomics of flowering plants. 

 The only living member of this group is a single species, Amborella trichopoda. It is probable that Amborellales contained many other species, genera, and families, but today there is only one extant member. Amborella trichopoda is a small, evergreen shrub found only in the cloud forests of New Caledonia, which is home to a great variety of other endemic plants and animals. Interestingly, its vascular system more closely resembles that of a gymnosperm than other angiosperms, likely a relic of its early divergence. In general, Amborella trichopoda provides insight into the evolution of flowering plants as a whole, considering its early divergence. 

A closely related basal angiosperm group to Amborellales is Nymphaeales, which includes water lilies.

Magnoliids

The next oldest group of angiosperms is the Magnoliids, which contains only a few orders. The relationships of Magnoliid orders are primarily based on molecular evidence, and morphological features vary quite a bit within this primitive group. A few important families and notable constituent species in the Magnoliids include:

• Piperaceae (piper family): Black peppercorn.
• Lauraceae (avocado family): Avocado, cinnamon, and bay leaves.
• Magnoliaceae (magnolia family): Magnolias and tulip trees. 

Monocotyledons

Monocotyledons comprise the next most primitive group of flowering plants. This group is also called the monocots and Liliopsida. Monocots, unlike dicots, are strictly defined and comprise a monophyletic group. As the name suggests, monocots produce only one cotyledon which is perhaps the most important difference between this group and other angiosperms.

Unique Features of Monocots

Leaves of monocots typically have parallel venation, with a petiole that has been modified into a sheath that hugs the stem before connecting at a node. Monocot stems are never woody, with a notable exception of palms. Although palm trunks seem to contain wood, the structure is actually composed of many dried leaf sheaths that remain on the trunk. A cross-section of a monocot stem would show disorganized vascular bundles instead of a ring of vascular tissue at the edge.

Families of Monocots

Some monocot families are extremely diverse and of incredible economic importance. Examples (in order of most to least primitive) include:

• Araceae (aroid family): Monstera, philodendron, and peace lily. 
• Liliaceae (lily family): Lilies, tulips, and many more cultivated flowers. 
• Orchidaceae (orchid family): Orchids, which is the second most diverse plant family.
• Arecaceae (palm family): Date palms, oil palms, and coconut.
• Bromeliaceae (bromeliad family): Pineapple, bromeliads, and many air plants. 
• Poaceae (grass family): Corn, rice, wheat, and cereal grains.

Eudicots

The next group of flowering plants is by far the most diverse and includes all other species of angiosperms. Eudicots join the basal angiosperms as the other group that makes up the dicots. This group is a large monophyletic group that is very broadly defined. It includes all lineages that diverged since monocots diverged. 

Common Features of Eudicots

Like the basal angiosperms, eudicots are capable of secondary growth and their stems can produce wood. Again, this feature is not present in all eudicots, as some families are strictly herbaceous. Other features shared with basal angiosperms include organized vascular tissue and two cotyledons. 

Families of Eudicots

Because the eudicots are the most diverse and largest group of flowering plants, it contains a huge number of taxa with a variety of important plants. Here are 30 of the most important families organized in their respective orders, from most primitive to most recently diverged:

• Ranunculales
• Ranunculaceae (buttercup family)
• Papaveraceae (poppy family)
• Vitales
• Vitaceae (grape family)
• Malpighiales
• Euphorbiaceae (spurge family): Euphorbia, cassava, rubber tree, castor oil plant.
• Passifloraceae (passionflower family)
• Salicaceae (willow family): Willows, aspen, poplar, and cottonwood.
• Violaceae (violet family): Violet flowers.
• Fabales
• Fabaceae (legume family): Beans, peas, soy, acacia trees, and many more. 
• Rosales
• Rosaceae (rose family): Blackberries, roses, strawberries, apples, and many other fruit trees.
• Moraceae (fig family): Figs and mulberries. 
• Cucurbitales
• Cucurbitaceae (cucumber family): Cucumbers, squash, and watermelon. 
• Fagales
• Betulaceae (birch family): Birch, alders, and hazelnuts. 
• Fagaceae (oak family): Oaks and beeches. 
• Myrtales
• Myrtaceae (myrtle family): myrtle, clove, guava, and eucalyptus. 
• Melastomataceae (melastome family): An extremely diverse family found mostly in the tropics.
• Sapindales
• Anacardiaceae (mango family): Poison ivy, mango, cashew, and sumac.
• Sapindaceae (soapberry family): Chestnut, maples, and lychee.
• Rutaceae (citrus family): Lemons, lime, and oranges. 
• Malvales
• Malvaceae (mallow family): Cotton, hibiscus, and cacao.
• Caryophyllales
• Cactaceae (cactus family)
• Ericales
• Ericaceae (heather family): Blueberry, cranberry, and huckleberry.
• Primulaceae (primrose family): Many common garden plants.
• Solanales
• Solanaceae (nightshade family): Tomato, eggplant, chili pepper, and potato.
• Gentianales
• Rubiaceae (madder family): Coffee and various ornamental plants.
• Lamiales
• Lamiaceae (mint family): Mint, basil, sage, rosemary, and thyme. 
• Plantaginaceae (plantain family)
• Bignoniaceae (bignonia family): Calabash, and a great variety of tropical trees and vines. 
• Asterales
• Asteraceae (daisy family): Daisy, sunflower, lettuce, chrysanthemum, and many other ornamental flowers. This is the most diverse plant family. 
• Apiales
• Apiaceae (carrot family): Carrot, celery, and parsley. 
• Araliaceae (aralia family): English ivy, umbrella tree, and ginseng. 

Conclusion

Angiosperms are the most diverse, and perhaps the most important group of plants on earth. Their unique method of reproduction allowed them to dominate the globe and rapidly diversify, leading to a vast array of food and medicinal plants. Protecting existing forests that are dominated by angiosperms is of the utmost importance in order to preserve their mysteries waiting to be discovered.