Almost every plant consists of one or several stems. Almost all stems bear leaves, attached to regions called nodes. All plants produce at least two types of leaves: foliage leaves and bud scales. Just above each attachment point is an axillary bud, some of which develop into branches whereas others develop into flowers, but many axillary buds remain dormant. Almost all plants have roots. In almost all plants, roots are confined to one end of the stem. Roots, stems, and leaves are the vegetative organs of a plant (they are responsible for obtaining energy and materials for growth and survival, but they do not carry out sexual reproduction). Conifers make seed cones and pollen cones, angiosperms make flowers. Each flower is a reproductive shoot; that is, each flower has a short stem with leaflike structures attached to it. Sepals protect the rest of the flower as it develops, and petals attract pollinators after the sepals spread apart allowing a flower to open. Above the petals are stamens that produce pollen, followed by carpels (often called pistils). Anthers is the part of a stamen that produces pollen. Carpels consist of three parts: a stigma, a style, and an ovary. Within the ovary is one or several ovules. Stamens and carpels are essential because each pollen grain contains two sperm cells and each ovule contains an egg cell. Insects, birds, and wind carry pollen from stamens to carpels, usually from the stamens of one plant to the carpels of a different plant of the same species. After a sperm cell fertilizes an egg cell, an embryo and the surrounding tissues grow to be a seed while at the same time some carpel tissues develop into a fruit. In angiosperms, all seeds develop inside a fruit. In many flowers, one or two of these organs are extremely underdeveloped or even absent completely and in some flowers the various organs are fused to each other whereas in other flowers all organs remain separate and distinct; however, each and every type of flower is just a modification of one basic plan. The internal organization of plants is even simpler and more uniform than the external form. While young, every part of a plant – every root, stem, leaf, petal, and all the rest – consists of an external epidermis, internal ground tissues, and vascular tissues. Plants have two distinctly different vascular tissues. Xylem transports water and minerals upward from roots to all aerial parts, especially leaves and flowers. Phloem carries sugars and various organic compounds as well as minerals and water throughout the plant. Xylem and phloem almost always occur together in a small number of vascular bundles. No plant ever puts all its water, sugars, and minerals into just one large vascular bundle (as opposed to a human aorta). The description of plant structure to this point refers to young plants as well as to young growth at the ends of branches and roots. This is called the primary plant body, and all the tissues and organs are produced by cells located at the very tips of stems and roots. Herbs are plants that have only this primary body. Woody plants are defined as those that produce a secondary body inside their primary body. Woody plants are those that produce wood (secondary xylem) and bark (of which occurs secondary phloem). Other tissues that comprise most plants are cortex, pith, mesophyll, and cork.
All organisms must take in energy and raw materials from their environment; there are no exceptions whatsoever. Plants take in energy through several separate processes. Energy is provided by sunlight and is captured by the process of photosynthesis. The bulk of a plant’s body is composed of organic molecules that can all be traced back to the reaction of carbon dioxide and water during photosynthesis. Plants obtain the needed water through their roots, and carbon dioxide is absorbed from air through the epidermis. Minerals are a fourth raw material plants need, and these, too, are absorbed by roots. Photosynthesis is the central metabolism by which plants acquire energy (the exceptions are parasitic plants that absorb organic material from a host plant). Plants are green because they contain the pigment chlorophyll, a molecule that both captures the energy of light and then passes that energy to other chemicals, transforming them. Through several metabolic steps, the captured light energy forces carbon dioxide to react with water, which yields oxygen and carbohydrate. The oxygen diffuses out of leaves as a waste product; the carbohydrate is used as the starting point for all metabolic pathways. Photosynthesis only occurs when light, chlorophyll, and carbon dioxide are present. The light energy captured by photosynthesis must be moved from leaves out to other organs that do not photosynthesize, and this transport is carried out by phloem. Most plants store energy by polymerizing glucose molecules together into starch, which is more compact and stable than sugar. Respiration is another type of energy metabolism and it occurs in all organisms, with no exceptions whatsoever. Respiration breaks down glucose and transfers its energy to ATP. During photosynthesis, light energy is captured as carbon dioxide combines with water, producing oxygen and glucose. Respiration is more or less the reverse. Oxygen and glucose react, producing carbon dioxide and water, with the energy being transferred to ATP rather than being given off as light. Plants use minerals in much the same way we do. Plants lose minerals as they shed their leaves, withered flowers, and fruit. Starting with just glucose, minerals, and water, plants synthesize every single molecule they need for all aspects of their life.
The greatest source of information for any individual is its parents: the DNA contained in the sperm cell and egg that initiated the life of the individual. In plants, DNA is stored in the nucleus of each cell, as well as in plastids and mitochondria, which are also present in all living plant cells. A plant, just like any other living organism, also uses information from the environment (such as temperature, sunlight, gravity, and touch). Various parts of a plant itself create information (such as hormones).
There are several large, recognizable groups of plants such as mosses, ferns, conifers, and angiosperms. Mosses, liverworts, and hornworts are often grouped together and called bryophytes. They all lack vascular tissues and seeds and all are haploid. All other groups of plants have vascular tissue and are diploid. The more advanced seed plants are divided into two groups, those with naked seeds (gymnosperms) and those with “clothed seeds” (angiosperms, or flowering plants). The carpels of angiosperms are always sealed, and the ovules are never visible without cutting the young fruit open. Angiosperms have more complex wood and leaves, as well as several modifications that improve photosynthesis. Angiosperms also cooperate with animals that pollinate their flowers and distribute their seeds, whereas conifers never rely on animals for pollination, and only a few cooperate with animals for seed dispersal. The term gymnosperms is not an accurate one because gymnosperms is not a clade. Plants are a clade: true plants originated from green algae only once. It is not known, but cautiously assumed, that seeds evolved only once and thus seed plants are a clade. The early seed plants gave rise to these clades: cycads, gnetophytes, Gingko, conifers, and angiosperms. At present, we use the terms basal angiosperms for those angiosperms that diverged early; monocots for the clade of lilies, palms, agaves, orchids, and their relatives; and eudicots for all the rest of the angiosperms.
Thallophytes are plants that have a body that is not composed of roots stems, and leaves, and which does not have typical plant tissues. All plants have multicellular bodies; there are no exceptions. Plants that lack vascular tissues are nonvascular plants. All other plants have vascular tissues and so are vascular plants, also known as tracheophytes. All plants produce spores; there are no exceptions. The term “spore-bearing plant” usually refers to those plants that never make seeds, which are mosses, liverworts, hornworts, lycophytes, and ferns. All other plants make seeds as well as spores. A common term for “spore-bearing plant” is cryptogam. Spermatophytes is the technical term for seed plants.
Plants live in a complex, dynamic, and changing world. Plants, animals, and all other organisms interact with each other. Direct interaction includes neutralism (neither organism benefits or is harmed), mutualism (both organisms benefit), predation (one organism benefits by harming another), commensalism (one organism benefits, whereas the other is neither harmed nor helped), amensalism (one organism harms another without receiving any benefit itself), and competition (both organisms harm each other).
—June 2021
—July 2023
Lance Jones 