In the most basic and fundamental terms, what is it about a plant that makes it a plant and not an animal? What is the "essence of plantness"? The theme of the essay that follows is that the fundamental difference in nutritional mode of plants and animals is the principal factor that has shaped the evolution-ary development of these two large groups of organisms and has resulted in the differences we now observe between them. (An earlier version of this essay has been published in The American Biology Teacher, vol. 52, pp. 354-7, Sept. 1990.)

Autotrophs and Heterotrophs
In order to live and grow, all organisms require a source of carbon and a source of energy in addition to mineral elements and water. Plants are autotrophs or "self-feeders". They synthesize their own food using carbon dioxide from the atmosphere as a source of carbon and sunlight as a source of energy. These two resources along with the minerals and often water as well, are present in the environment at very low concentrations.

In order to collect these diffuse resources needed for growth, plants produce and expose a great deal of surface area to the environment. The high surface area to volume ratio found in plants is one of their most characteristic features. It is through this extensive surface area, literally spread out into the environment, that plants absorb the diffuse resources needed for growth. The more surface area produced, the greater the absorbing capacity of the plant. The leaves absorb sunlight energy and carbon dioxide. The young roots and root hairs are less obvious, but also expose a tremendous amount of surface area to the environment through which water and minerals are absorbed. These resources are converted into organic molecules through photosynthesis and subsequent metabolism and in that state represent concentrations of carbon, energy and minerals. Plants may therefore be characterized as "collectors and concentrators".

Animals are heterotrophs or "other-feeders". They cannot make their own food and must rely on preformed, concentrated sources of carbon, energy and minerals (i.e. "food"), consisting of plants or organisms that have eaten plants. Animals are motile and much more compact than plants, having a much lower surface area to volume ratio. Being compact and motile, animals are able to seek out food and water in their environment. By "using" the food (i.e. respiration), animals return carbon, energy and minerals back to the environment in an unconcentrated form: atmospheric carbon dioxide, heat energy and mineral elements no longer incorporated in organic molecules. Animals may therefore be characterized as "scatterers".

The collector-concentrators (plants) and scatterers (animals) therefore constitute a recycling system in the earth's ecosystem. It has been suggested that if multicellular life forms are discovered on other planets, they will undoubtedly exhibit these two basic life styles of collectors and scatterers.

There are, of course, a number of qualifications to be added to this simplistic view. One very important point is that everything in the ecosystem, except energy, is recycled. Energy reaches the earth as sunlight energy. A small fraction (1% or less) is captured in photosynthesis and transformed into chemical energy which is eventually released through respiration in the form of heat energy which leaves the earth as infra-red radiation (heat energy). A second point is that bacteria and fungi are also very important in the "scattering" process. In fact, these heterotrophs are more crucial than animals to the completion of the decom-position process started by the animals. A third point is that oxygen is also important in this recycling system. Oxygen released by plants as a by-product of photosynthesis is required by heterotrophs and autotrophs for respiration.

Plant Growth and Development
Most, if not all, other basic characteristics of plants are related to their collector-concentrator life strategy. Growth in plants occurs from localized areas, called meristems, which remain embryonic throughout the life of the plant. Plant growth therefore continues indefinitely and is said to be indeterminate. Embryonic tissue in animals occurs throughout the organism (contributing to generalized growth), but ceases to be embryonic once the animal is mature. Growth in animals (as well as growth in certain plants organs such as leaves, flowers, and fruits) is characterized as determinate. Also, plants have far greater ability to regenerate tissues and even whole organs than most animals.

All plants have apical meristems at the tips of the stems and the tips of the roots. Meristems contain the equivalent of animal embryonic stem cells, which you've heard so much about in the news. Primary tissues produced by these apical meristems contribute to increasing the length of the stems and roots. Leaves are also produced as a product of the shoot apical meristems. This primary growth may be characterized as "reaching" growth and is responsible for producing the large amount of surface area that characterizes plants. The adaptive significance of indeterminate growth, localized at the stem tips and root tips is that the plant continually (or more accurately, seasonally) produces fresh surface area (leaves and root hairs) for absorbing those diffuse resources. Because leaves and root hairs are expendable structures with a limited life span, their replacement through indeterminate growth of apical meristems is essential to the survival of the plant.

Cell enlargement and especially cell elongation are associated with "reaching" primary growth. It would be expensive (in terms of resources) to generate this reaching growth by producing many cells full of cytoplasm or by producing fewer large cells full of cytoplasm. It is much less expensive for a small cell to enlarge by accumulating water. To avoid diluting the cytoplasm, the water must be contained in a bag, the vacuole. Plant cells typically contain a large central vacuole whereas animal cells do not. The absence of large vacuoles in animal cells correlates with the lack of reaching growth in animals.

Woody plants with aboveground parts that live for many years need additional support if they are to remain erect. The solution to this architectural problem is to produce additional tissue to strengthen the stem and roots. A lateral meristem (another localized growing point) called the vascular cambium produces cells that increase the diameter of stems and roots. Cell elongation is not associated with this "supporting" secondary growth.

Plants are supported by cell walls that surround each individual cell rather than by internal skeletons (as in vertebrates) or exoskeletons (as in arthropods). The cell wall is another feature of plant cells which animals lack. Young plant tissues are supported in part by a "hydraulic skeleton" consisting of water pressure in the vacuole being exerted against the restraint of the primary cell wall. As the plant tissues mature, some cells develop stronger, thicker (secondary) cell walls that provide support for the plant even in the absence of vacuole pressure. Although secondary cell walls provide more support than primary cell walls, they do not allow for cell division and elongation as do primary cell walls. The plant is thus faced with a trade-off and must restrict reaching growth to the younger tissues near the apical meristems. (Cell division, but not cell elongation, occurs in the vascular cambium.) Another feature restricting growth in plants to localized growing points is the fact that to achieve any supporting strength for the organism, the cell walls of adjacent cells must adhere tightly to one another. Generalized growth (as found in animals) would require adding new cells in the midst of existing tissue, all the cells of which have already established firm attachments. This pattern of growth would necessarily result in a weaker structure in an organism in which cell walls surround every cell.

It also seems likely that the presence of the cell wall is associated with the collector-concentrator life strategy. The generation of a high surface area to volume ratio depends on the production of thin and/or skinny structures. It is apparently more efficient and practical (in an evolutionary sense) to provide each cell with its own support in the form of a cell wall than to provide some sort of endoskeleton or exoskeleton for each of these extensions of the plant body. It is also imperative that the skeletal structure not interfere with the absorption of those resources. The primary cell wall is freely permeable to anything dissolved in water, although the secondary cell wall is rather impermeable.

Indeterminate growth from localized meristems (including axillary buds) provides plants with much greater developmental flexibility than is found in animals and has many important consequences for plants. Most organs in an animal are established in the embryo and therefore animals are unable to replace lost organs. Although plants are stationary with a limited capacity (mostly chemicals or spines) to protect themselves from predators, they are able to replace missing organs (e.g. leaves) readily from apical meristems. The origin of the germ line (those cells that produce gametes) in plants also reflects this flexibility. In animals the germ line is established and set aside in the embryo. Plants lack a distinct germ line -- gametes are produced from cells in the ovules of flowers at many different places on the plant. The differentiation of flowers from vegetative meristems frequently is influenced by environmental cues. Developmental flexibility thus allows a plant to reproduce sexually under the most appropriate conditions and to sustain significant damage without losing its reproductive capacity.

Developmental flexibility also extends to differentiated cells in plants. Plant cells which are not highly differentiated (e.g. parenchyma cells) are usually capable of returning to the meristematic state under the appropriate conditions and giving rise to an entire plant -- a phenomenon called totipotency. This is not the case for animals cells whose developmental fate is determined early and is usually irreversible. Totipotency is associated with the very common occurrence of asexual reproduction in plants (it is very rare in higher animals) and the horticultural practice of making cuttings of desirable plants. It is also essential for plants' remarkable ability to regenerate lost parts, unlike most large animals. Regeneration is also key to plants' ability to survive and reproduce as sessile, sedentary organisms. The field of plant biotechnology depends heavily on totipotency; once a plant cell in tissue culture has been transformed through genetic engineering, it can be induced to differentiate and grow into a mature transgenic plant carrying the new gene(s).

Like all living things, plants are capable of responding to their environment. Being unable to respond through motility as animals do, plants can only respond through growth -- growth made possible by the developmental flexibility of localized, indeterminate meristems. In many ways these growth responses in plants substitute for motility responses in animals. The flower is a growth response that functions to bring sex cells together, either by dispersing pollen into the wind or by attracting pollinators. The seed can also be thought of as a growth response. Seeds are unique structures in the sense that development is arrested after development has begun; no comparable stage is found in animals. The seed is a miniature, dormant plant that can be passively, but widely, dispersed in the environment. The offspring of animals disperse themselves. The seed also provides a survival function because seeds can survive seasons (a dry summer or a cold winter) which can be fatal to a growing plant. Many animals hibernate during the unfavorable season. The fruit is a growth response that aids in seed dispersal. Examples include "wings" on maple and elm fruits, hairs on dandelion fruits, edible fruits, fruits that float such as the coconut and fruits that contain stickers which attach to passing animals. Other plant growth responses include phototropism (the growth of a plant towards the light) and gravitropism (the shoot grows up and the root grows down).

Gas Exchange
All living organisms must engage in selective exchange of materials with their environment and this exchange must take place across a wet cell surface. In animals these wet surfaces are located deep within the body (e.g. lungs, intestines, kidneys), and various circulatory systems function to maximize the contact between living and non-living components of the exchange process. With respect to gas exchange in vertebrates, air is moved in and out of the lungs by bulk flow (breathing) and the gasses (O2 and CO2) cross the cell membranes by diffusion. In plants the wet cell surfaces are both external (root hairs) and internal (photosynthetic cells in the leaf). Since leaves are thin, the wet cell surfaces responsible for gas exchange are near the surface of the leaf and there is no need to move air into the leaf by bulk flow, an impossibility in any event due to the lack of muscles and nerves in plants. Carbon dioxide enters the leaf by diffusion down a diffusion gradient which is maintained by the assimilation of carbon dioxide in photosynthesis. At the same time, water is diffusing out of the leaf down a diffusion gradient maintained by an immense volume of atmosphere at less than 100% relative humidity.

Water loss represents a mixed blessing for the plant. On the one hand, evaporation of water from the leaves (transpiration) creates the tension (suction) that pulls water and minerals up a plant through the xylem. Transpirational water loss also cools the leaf. (A thin structure such as a leaf also radiates heat energy effectively -- another feature that minimizes overheating in direct sunlight.) On the other hand, water loss can be a serious problem as the soil becomes dry. To reduce water loss the cells on the surface of the shoot (the epidermis) have modified outer cell walls containing a waxy material. This cuticle greatly reduces unwanted water loss, but also prevents gas exchange. To allow for gas exchange, there are holes (called stomata) in the epider-mis. Two modified epidermal cells, called guard cells, surround each stoma and control the size of the opening. The control mechanism is such that when the plant is experiencing a water shortage, the guard cells automatically close the stomata, greatly reducing the rate of water loss. Of course, carbon dioxide no longer diffuses into the leaf when the stomata close. The plant must therefore perform a balancing act, continually reaching a compromise between accumulating carbon for growth and not losing so much water that life is threatened.

Plants use a tremendous amount of water, only a few percent of which is used in photosynthesis; the rest is lost in transpiration. Plants typically require approximately 600 pounds of water (about 75 gal) to produce one pound of dry organic matter. Plants with C4 and CAM photosynthesis require approximately 300 and 50 pounds of water, respectively, to produce that same pound of dry organic matter. On the other hand, transpiration is crucial in plants' ability to take up water and minerals from
the soil. It's a key part of the water transport system.

The Evolution of Plants
Land plants descended from green algae, which share many common characteristics with plants including their photosynthetic machinery and gene sequences. Unicellular autotrophic algae are well adapted to the collector-concentrator life strategy by virtue of the fact that smaller objects have more surface area per unit volume than larger objects with the same shape. With the evolution of multicellularity and the transition to terrestrial habitats, surface area became a liability due to the potential for evapora-tive water loss. Early land plants were leafless stems with a relatively low surface area to volume ratio. With the evolution of leaves, made possible by the cuticle, the stoma-guard cell apparatus and vascular tissue, plants once again possessed the large surface area to volume ratio required for efficient collecting and concentrating.

Plants as Ecosystem Components
In a broader, ecological context, plants are the producers. As a result of their collecting and concentrating activities, plants become concentrated sources of reduced carbon, energy and minerals. Thus, plants provide food for heterotrophs (consumers) and are a vital part of mineral cycles as well as the carbon, hydrogen and oxygen cycles in the earth's biosphere. Plants are also involved in the water cycle. A great deal of water is transferred from the soil to the atmosphere as a result of transpiration from plant leaves. The cooling effect of evaporation of water is the reason it is cooler in the shade of a tree than it would be in the shade of a similar, but non-living, structure. Plants also help build soil structure both physically (by root growth) and chemically (by adding organic matter). The importance of plants to ecosystems is demonstrated by the fact that we identify characteristic ecosystem types (biomes) by the plants growing there (e.g. deserts, grasslands, tropical rain forests). Have you thanked a green plant today?

Are Plants 'Inferior'?
As animals, we identify much more immediately with other animals than with plants. Plants do not move around, they do not eat or drink and they do not respond the way animals do (i.e. in an obvious way) to anything in their environment. It is almost as if plants are less alive than animals. We "kill" or "butcher" an animal, but we "pull up" weeds, "harvest" or "pick" fruits and vegetables or "cut down" trees -- words that do not suggest we are ending a life. (Interestingly, we "break" rather than "kill" an egg, even though it is animal life.) If we value animal life more highly than plant life, it follows that we consider plants to be inferior -- after all, they don't even have brains! Still, plants most definitely do sense their environment. They are sentient beings. They just respond differently than animals.

Once again we must consider life styles. An animal needs a central nervous system and a brain to coordinate its movements as it seeks out concentrated sources of food and water, shelter, and a mate. Plants are all spread out in the environment (leaves, stems and roots) and it is simply impractical to get up and move around in that condition. In other words, what would a plant do with a brain if it had one? Animals have brains and move around, not because they are superior, but because their life style requires it. Plants lack brains and are stationary, not because they are inferior, but because their nutritional mode requires a sedentary life form and does not require a nervous system. Plants are just as well adapted to their life style as animals are adapted to theirs. If we feel animals are superior, it is only because we are animal chauvinists.

Addendum - Other Organisms

Fungi
Fungi such as the common brewers yeast and mushrooms are heterotrophic organisms placed in a third kingdom of eukaryotic organisms. Like animals, fungi are consumers of concentrated sources of carbon, energy and minerals, but the approach is different. Animals are ingestive heterotrophs; they actively seek out their food, ingest it (often physically breaking it down in the process) and then digest it to small molecules which are absorbed. The absorption process has been internalized; a long intestine with many tiny folds provides the required surface area. Fungi are absorptive heterotrophs. Because they are nonmotile, they cannot seek out and ingest food they way an animal does, but fungi can and do produce spores which are passively distributed to new sources of food. The basic fungus body is a single tubular filament (hypha) which grows throughout the food substrate. Digestive enzymes are secreted into the environment and the products are absorbed. Thus, absorption is externalized. This life strategy has resulted in the evolution of the primitive, but extensive and invasive, fungal mycelium (a mass of hyphae) which exposes a great deal of surface area to the substrate. The only highly differentiated fungal structures are the reproductive bodies (e.g. mushrooms) which represent only a fraction, albeit the most obvious fraction, of the fungus. By sequencing DNA, molecular biologists have firmly established that fungi and animals share a common ancestor. In other words, though fungi share superficial features with plants, they are more closely related to animals.

Protists
The Protista is the fourth kingdom of eukaryotic organisms and includes a very mixed assemblage of mostly unicellular creatures.

The algae are a very diverse group of photoautotrophs which includes single celled forms floating in lakes and in the sea (phytoplankton) or living on rocks and mud in shallow water. Seaweeds, including the kelps which reach 100 ft or more in length, are also algae. Most of the discussion above on the collector-concentrator life strategy of plants also applies to the algae. Recall that smaller objects have larger surface area to volume ratios than larger objects of a similar shape. There is therefore little need for single celled algae to produce extensions to increase surface area. Seaweeds are usually very thin and/or produce leaf-like extensions to increase surface area. It's also worth noting that these days the green algae tend to be grouped with plants, as plants.

Protozoa are single celled animals (e.g. Paramecium and amoebas) with a life style much like that of larger animals. Many protozoa are able to supplement their ingestive nutrition with absorptive uptake of dissolved food. Most botanists do not include single celled algae among the protozoa, although many zoologists do.

Archaea and Bacteria
The archaea and bacteria all have a prokaryotic type of cell organization; most are heterotrophic and all lack a true nucleus containing their DNA. They also lack the degree of cellular organization and compartmentation characteristic of plants, animals, fungi and protists. Being very small single cells with a high surface area to volume ratio, they function much like fungi except that they are not capable of penetrating solid substrates as easily as fungi do. Many are motile, however, and can seek out more concentrated sources of food within an aqueous medium. Some bacteria are chemoautotrophs. Others are photoautotrophs, but the photosynthetic process is different in that some substance other than water is the electron (hydrogen) donor and oxygen is not released. The cyanobacteria or "blue-green algae" are specialized photoautotrophic bacteria which possess oxygen-evolving photosynthesis virtually identical to that of plants. The incredible metabolic diversity of bacteria is one of their most interesting characteristics. It's also humbling to note that from a cell number and mass perspective, there are a lot more of them than us! BY FAR!!!