The essential characteristics of plants

The following is intended as a very concise summary of the characteristics of plants that distinguish them from animals and other organisms.  It is provided as a guide to students, instructors, and the botanically curious who want to grasp the big picture of plant life.  My book, "Plant Life: a Brief History," expands on these themes in an evolutionary context, exploring how and why plants are the way they are.

1.       Plants are photosynthetic.  Plants are primarily oxygenic photoautotrophs, i.e. they conduct photosynthesis in which oxygen is released as a byproduct.  They share this fundamental metabolism with cyanobacteria, various organisms referred to as algae, and even a few animals.  Photosynthesis in plants and algae occurs in chloroplasts, which are descendants of cyanobacteria captured through primary endosymbiosis. 

2.       Plants are multicellular, primarily terrestrial organisms descended from green algae.  The formal Plant Kingdom (clade Embryophyta) is descended from the green algal group Charophyta, and consists of complex, multicellular, mostly terrestrial plants in which early embryonic growth is protected and nurtured in special chambers on the parent plant. 

3.       Plant growth is indeterminate and adapted to gather diffuse resources.  The resources required for photosynthetic life, including light, carbon dioxide, water, minerals, tend to be distributed diffusely, requiring broad, antenna-like systems, above and below ground, to gather those resources.  The diverse forms of plant architecture reflect different strategies for optimizing those resource gathering systems.   Most plants have no fixed size or shape, though some have a well-defined lifespan.  They expand indefinitely, adding new photosynthetic and absorptive organs throughout their life. 

4.       Shoots consist of simple repeated units exhibiting serial homology.   A shoot is a young section of stem with leaves or other derived organs produced serially through growth at its tip.  Leaves are usually determinate structures of fixed size, shape, and lifespan, and are attached at points along the stem called nodes.  Nodes may be separated by sections of stem called internodes.   An axillary bud, from which a branch shoot may arise, is situated in the axil (basal angle) of each leaf.   Leaves may be modified into bracts, tendrils, spines, or floral organs through modification of a common fundamental development plan (i.e. these structures are serially homologous).

5.       New tissues and organs are formed at meristems.   Growth in plants is localized in specialized tissues called meristems.  The meristems for primary growth (apical meristems) are located at the tips of shoots and roots, and in plants with elongate stems, cell division and expansion may continue for some time within the internodes.  Branching in stems is achieved by the growth of axillary buds, and in roots by the formation of new meristems deep within root tissues.   The stems and roots of many plants expand in thickness through cylindrical secondary meristems that produce wood and bark.

6.       Plants are hydrostatic systems.  Plant cells have rigid cell walls made primarily of cellulose, which limit cell expansion caused by osmosis.  This results in turgor pressure, which plays a part in cell growth, tissue expansion, movement of plant parts, maintenance of organ rigidity, and transport of food, water, and minerals.  The cell walls are outside of the living protoplasts, and along with intercellular spaces and hollow conducting cells, collectively create a non-living matrix (apoplast) through which water may move from one part of the plant to another.  Cytoplasmic strands (plasmodesmata) connect the living protoplasts of plant cells, resulting in a second network (symplast) through which fluids including dissolved sugar and other organics, may also move from one part of the plant to another, often through active manipulation of osmotic forces.   A large central vacuole contains water and other stored substances, and helps regulate cytoplasmic water and solute content. 

7.       Plants have complex reproductive cycles involving alternation of generations.  As non-motile organisms, multicellular plants are dependent on external factors, such as wind, water, or animals, to complete sexual reproduction and disperse their genetic progeny to distant locations.  In the simplest land plants, sperm cells swim to eggs through water, but dispersal to new locales is a separate process achieved by airborne spores.  As the long-distance dispersal of spores and the short-distance travel of gametes have very different requirements, plants typically alternate between two distinct multicellular bodies, or generations.  A diploid spore-producing body (sporophyte) produces spores through meiosis, and is typically tall so as to launch the spores for effective dispersal by air currents.  Spores germinate into haploid gamete-producing bodies (gametophytes), which remain small and close to the ground, so that sperm cells can be released into films of surface water and need travel only a short distance to unite with an egg on a genetically different plant.   In seed plants, sperm-producing gametophytes are tiny and remain within specialized spores (pollen grains) that are carried by wind or animals to the embryonic seeds (ovules) where the egg-producing gametophytes are located. 

8.       Plants defend themselves without moving.  To protect themselves against predation by herbivores, plant organs produce layers of fibrous or stony tissues, cover themselves with superficial hairs or scales, arm themselves with piercing spines, thorns or prickles, produce repellent or toxic chemicals, or provide food and shelter for animals that actively defend the plants.   Some plants invest little in defense, and instead rely on rapid replacement growth.