The cycle of life
Summary
Angiosperms and gymnosperms have five main life stages: seed (dormant), juvenile (vegetative growth), mature (reproductive growth), senescence and death. Annuals complete their lifecycle in a single growing season. Biennials live for two growing seasons, flowering in the second. Perennials live for three or more seasons and may be woody or herbaceous. Monocarpic plants flower only once and then die. Polycarpic plants flower repeatedly. Ephemeral plants complete their entire lifecycle rapidly to take advantage of specific situations.
All plants, like all animals, experience different stages as they go through life, from being “born”, through youth to adulthood, old age, and death. As they pass through these stages, they grow, develop, reproduce, and die. Two of the main four groups of plants (angiosperms and gymnosperms) follow broadly the same life cycle stages; the other two (pteridophytes and bryophytes) have a very different lifecycle. We will focus on angiosperms/gymnosperms in this article.
Main life cycle stages of a flowering plant
Lifecycle stages
The main stages of a plant’s life cycle are straightforward: seeds germinate; the plant grows; it bears flowers (angiosperms only), which are pollinated; it sets seed, which are dispersed to start the cycle anew; it ceases to grow and reproduce; then it dies.
Lifecycle of spore-based plants
Pteridophytes and bryophytes reproduce through spores, not through seed. Their lifecycle is based on “alternation of generations”. The first generation, the “sporophyte” releases spores. These do not grow into mature plants, instead growing into tiny plantlets called “gametophytes”. These in turn produce both sperm and eggs. If water is present, the sperm will then travel to a nearby egg and fertilise it. This fertilised egg will then develop into a new sporophyte. Note that spore-based plants need water to reproduce.
Seed
Seeds can be thought of as dormant plants, waiting for the right conditions to break dormancy and germinate. A seed contains all that is necessary for an embryonic plant to develop, surrounded by a coat that protects the embryo from the external environment until the time is right.
Plants have developed a wide range of methods to ensure that seeds only germinate when the chance of survival is best. For example, some require a long period of cold (a winter) followed by warmer conditions (spring). Others have hard seed coats that need to be worn down by harsh conditions (or by being eaten by animals). Some even need to be exposed to extreme heat, allowing those plants to recover quickly in the aftermath of a forest fire.
Juvenile
Once a seed germinates, the plant enters the juvenile stage of its life. From this point on, until maturity, the plant will grow at the most rapid pace of its entire life. Growth in the juvenile stage is “vegetative”: roots, shoots and leaves, but no flowers or fruit, and no seed development. Some plants’ juvenile stage can be over in a matter of weeks or months; others can take many years (such as Wisteria floribunda, which can be juvenile for 20 years or more).
Moving beyond the juvenile phase requires the plant to develop to a size where it is large enough to generate enough sugars (through photosynthesis, so the plant also needs enough leaves) and energy (through respiration) to be able to grow and sustain flowers (angiosperms only) and set seed. Furthermore, as with the seed stage, some plants need exposure to particular conditions, typically a period of cold weather, before they will progress to maturity.
Maturity
When a plant has achieved a large enough size (and met any other conditions required by that plant), its growth pattern shifts, with its resources being dedicated increasingly to reproduction. Vegetative growth slows, flower buds form and swell (or seed cones develop). Pollen is transferred from the male parts of the plant to the female parts of another plant (a process called “pollination”), and seeds begin to develop. In the case of angiosperms, fruit begin to set, encasing the seeds in a protective cover.
Once seed has been produced and begins to mature, the plant will stop producing flowers, and it will enter the next stage of its life. This can be delayed for many plants by deadheading; removing the spent flower heads before seeds can form and thereby encouraging additional flowering.
Senescence
A plant that has successfully produced seed has completed the most critical part of its lifecycle; the plant no longer needs to generate the same levels of energy to produce flowers and fruit. As a result, the plant slows its metabolism, reducing photosynthesis and respiration, and ceasing further growth. Since some parts of the plant are not needed any more (such as leaves for photosynthesis), the plant may harvest the tissues for their nutrients, transferring these to other parts of the plant where they can still serve a purpose. This is the reason leaves change colour and fall in the autumn; once the nutrients are removed, the substances that remain (chemicals called “anthocyanins”) provide the colour.
Plants that live for more than one year experience these three main lifecycle stages (juvenility, maturity and senescence) more than once. How this manifests depends on the strategy the plant has evolved (see below about perennial plants).
Death
The final stage of life is its end. At some point, the plant ceases to photosynthesise and to respire. With no energy being produced, its cells die, and the process of decay and decomposition begins. Dead plant tissues are gradually decomposed through the action of small creatures, fungi and bacteria. As they break down, they become “organic matter”, and are gradually incorporated into the soil.
Lifespan and lifecycle
All plants go through the same lifecycle stages, but the time span over which these occur can vary significantly. At the one end are plants such as Cardamine hirsuta, which completes its entire lifecycle in just 4 to 6 weeks. At the other, individual specimens of Pinus longaeva can live for close to 5,000 years. The lifespan of plants is another way of classifying them.
Annual plants
Some plants complete their lifecycle in a single growing season. These plants (known as “annuals”) represent approximately 6% of all plant species and 15% of herbaceous (non-woody) plants, including many plants with edible parts. Annual plants contribute to around 80% of worldwide food consumptions (statistics from Wikipedia).
An annual lifecycle may represent an evolutionary survival strategy for situations in which adult plants are under extreme stress; such as when the climate is harsh in summer, or is highly variable. Annual plants need not devote resources to developing extensive root systems, and can instead focus on seed production; in particular on producing seeds that can persist in the soil until the environment is suitable for the plant. By maintaining a soil “seed bank” like this, annual plants can ensure their survival in challenging circumstances.
Biennial plants
An alternative strategy adopted by some plants is to produce vegetative structures (roots, stems and leaves) in one growing season; to put these into a dormant state for the winter; then to flower and set seed in a subsequent season, before senescence and death. Such “biennial” plants are often described as having a lifespan of two growing seasons. However, if conditions are not right, a biennial may not be able to grow sufficient vegetative structures to support flowering, and may have one or more further years of such growth before flowering and dying.
Biennial plants tend to occur naturally in densely planted areas with frequently disturbed soil, such as woodland clearings. Their seeds may lie dormant in the soil until such time as disturbance creates the conditions for germination (for example, by exposing the seeds to light). At the same time, such disturbance creates gaps in the otherwise dense planting. By devoting resources to rapid growth of vegetative structures (rather than flowers), biennial plants can potentially outcompete annuals to make use of such gaps, establishing themselves and shading out competitors at ground level.
A biennial lifecycle occupies a relatively small niche, accounting for around 1%-1.5% of all plant species.
Perennial plants
The vast majority of plants take three years or more to complete their lifecycle. These are the “perennial” plants. Some of these plants enter dormancy in cold weather, harvesting the nutrients from their leaves and storing them elsewhere in the plant. The majority of these “deciduous” plants drop their leaves, though a few (such as Fagus sylvatica) retain the dead leaves on their stems until new growth begins in spring. Other perennials retain their leaves throughout the year, and are said to be “evergreen”.
Perennial plants are commonly divided into two groups based on their strategies for surviving cold weather in winter: the “woody” perennials and the “herbaceous” perennials.
Woody perennials are those whose stems become rigid as they mature. Each year, as well as growing bigger (through stem lengthening and branching, and the development of new leaves, so called “primary” growth), their stems grow thicker (“secondary growth”) and become woody. The older parts of the plant no longer produce leaves or flowers, and can be thought of as permanently senescent. The younger parts may be in different lifecycle stages; for example, some plants flower on the current season’s growth and others on older growth; in the former group, the new growth is mature, whereas in the latter group, new growth remains juvenile. Woody perennials may be deciduous or evergreen.
Herbaceous perennials are perennials that are not woody. The majority of herbaceous perennials are deciduous and die back to their roots each winter, growing back from the roots in the spring when warm weather returns. Once they have flowered, their above ground tissues become senescent, and the nutrients that these hold are harvested by the plant, and stored in the roots; following this, the above ground parts undergo cell death and begin to decay. There are also evergreen herbaceous perennials, often (but not always - the common daisy, Bellis perennis, is an evergreen herbaceous perennial) native to warmer climates where winter damage to soft tissues is less of a risk.
Woody perennials are often further divided into trees, shrubs and sub-shrubs; categories that are not clearly defined and often overlap. Trees are generally woody perennials that have a single stem (though some are multi-stemmed) and bear lateral branches at some distance from the ground (though some branches may be lower down). Shrubs have multiple stems, which branch from close to the ground. Sub-shrubs are woody only at their base, with new growth being herbaceous, and the tips dying back each year in winter. The term sub-shrub is also sometimes used to mean a small or prostrate shrub.
Other lifestyle factors
In addition to the length of their lifespan, plants’ lifecycle can vary in other ways.
Monocarpic and polycarpic plants
Plants can be divided into two groups, based on how many times they flower. “Monocarpic” plants flower and set fruit only once in their lives. Once they have flowered, their lifecycle is complete and they die. Other plants flower repeatedly, sometimes for many years; these are the “polycarpic” plants.
The majority of monocarpic plants are annuals or biennials. However, perennial monocarpic plants do exist. Agave americana, the “century plant”, can live for over 30 years before flowering once and dying. Some bamboos live for 20 or more years (in some cases up to 100 years), before again flowering once and dying.
The terms monocarpic and polycarpic strictly refer to how many times a plant flowers from its apical meristem (see this article). A plant may reproduce itself (for example by suckering at the base), with the new plants flowering again. Although it may appear that the plant is flowering repeatedly without dying back, it is still considered monocarpic. Similarly, some plants may see only a part of the plant flower and die back, with new growth then flowering again. Once more, these are considered monocarpic.
Ephemeral plants
Some plants have evolved strategies that involve only a very short period of active growth to take advantage of specific situations. These are the “ephemeral plants”, and they fall into four groups.
Spring ephemerals grow under the canopies of deciduous trees. Before these trees leaf out, the forest floor is bathed in sunlight, which turns to shade as new leaves form. Spring ephemerals appear early in spring, bloom quickly, produce seed, then die back to underground structures (roots, bulbs and rhizomes) for the remainder of the year. As their full lifecycle is several years long, these plants are perennial, but only appear for a very short time in the growing year.
Desert ephemerals are adapted to grow extremely quickly when water becomes available in deserts. For the majority of the time, deserts are dry and barren, without sufficient water to sustain plants. However, rain does fall for short periods, and when this happens, desert environments can suddenly bloom prolifically. These are the desert ephemerals. Most survive the dry periods as seeds and are annuals; some die back to underground structures and are perennial.
At the other extreme, mud flat ephemerals grow rapidly when water levels recede. Some rivers and lakes rise in rainy seasons or after snow melt, before falling back in dry weather. Mud flat ephemerals take advantage of these short, drier periods to grow and set seed quickly, leaving their seeds buried in the mud until the next low water period.
Finally, weedy ephemerals have evolved to reproduce rapidly, allowing multiple lifecycles to be completed in a single year. This allows them to take advantage of soil disturbance, spreading rapidly from one disturbed area to another, whether cause by human activity or by that of animals.
Some examples
Annuals
Helianthus annuus (sunflower)
Nigella damascena (love-in-a-mist)
Mysotis sylvatica (forget-me-not)
Biennials
Digitalis purpurea (foxglove)
Hesperis matronalis
Woody perennials
Buxus sempervirens (boxwood)
Taxus baccata (yew)
Lavandula angustifollia (lavender)
Herbaceous perennials
Echinacea purpurea (coneflower)
Armeria maritima (sea thrift) - evergreen
Monocarpic plants
Agave americana (century plant)
Daucus carota (carrot)
Polycarpic plants
Malus x domestica (eating apple)
Choisya ternata (Mexican orange blossom)
Spring ephemerals
Galanthus nivalis (snowdrop)
Hyacinthoides non-scripta (English bluebell)
Desert ephemerals
Arabidopsis thaliana
Xylorhiza tortifolia (Mojave aster)
Mud flat ephemerals
Cyperus fuscus (brown flatsedge)
Weedy ephemerals
Cardamine hirsuta (hairy bittercress)
Chenopodium album (fat hen)