Plant

I

Introduction

Plant, any member of the plant kingdom, comprising about 260,000 known species of mosses, liverworts, ferns, herbaceous and woody plants, bushes, vines, trees, and various other forms that mantle the Earth and are also found in its waters. Plants range in size and complexity from small, nonvascular (without vein-like structures) mosses, which depend on direct contact with surface water, to giant sequoia trees, the largest living organisms, which can draw water and minerals through their vascular systems to heights of more than 100 m (330 ft).

Only a tiny percentage of plant species are directly used by human beings for food, shelter, fibre, and drugs. At the head of the list are rice, wheat, maize, legumes, cotton, conifers, and tobacco, on which whole economies and nations depend. Of even greater importance to human beings are the indirect benefits reaped from the entire plant kingdom, which for more than 3 billion years has been carrying out photosynthesis. Plants have laid down the fossil fuels that provide power for industrial society, and throughout their long history plants have supplied sufficient oxygen to the atmosphere to support the evolution of higher animals. Today the world's biomass is composed overwhelmingly of plants, which not only underpin all food webs but also modify climates and create and hold down soil, making what would otherwise be stony, sandy masses habitable for life.

II

Differentiation from Other Kingdoms

Plants are multicellular green organisms; their cells contain eukaryotic (with nuclei) protoplasm held within more or less rigid cell walls composed primarily of cellulose. The most important characteristic of plants is their ability to photosynthesize—that is, to make their own food by converting light energy into chemical energy—a process carried out in the green, chlorophyll-containing plastids (cellular organelles) called chloroplasts. A few plants have lost their chlorophyll and have become saprophytic or parasitic—that is, they absorb their food from dead or living organic matter, respectively—but details of their structure show that they are evolved plant forms.

The animal kingdom is also multicellular and eukaryotic, but its members differ from the plants in deriving nutrition from other organic matter; by ingesting food rather than absorbing it, as in the fungi; by lacking rigid cell walls; and, usually, by having sensory capabilities and being motile (able to move), at least at some stage. See Classification.

Fungi, also eukaryotic and long considered members of the plant kingdom, have now been placed in a separate kingdom because they lack chlorophyll and plastids, and because their rigid cell walls contain chitin rather than cellulose. Fungi do not manufacture their own food; instead they absorb it from dead or living organic matter.

The various groups of algae were also formerly placed in the plant kingdom because many are eukaryotic and because most have rigid cell walls and carry out photosynthesis. Nevertheless, because of the variety of pigment types, cell wall types, and different forms and structures found in the algae, they are now recognized as part of two separate kingdoms, containing a diversity of plant-like and other organisms that are not necessarily closely related. One of the divisions (phyla) of algae—comprising the green algae—is believed to have given rise to the plant kingdom, because its chlorophylls, cell walls, and other details of cellular structure are similar to those of plants.

III

Grouping Plants According to Evolutionary Relationships

In the evolution of plants it is now thought that single-celled green plants migrated from the sea to freshwater where they evolved into multicellular organisms, and then probably invaded the land several times before one plant lineage survived and eventually diversified into all known land plants. The common ancestor was likely to be closely related to tiny green algae plants known as coleochaetes, which still live in some of the world’s pristine freshwaters. Further, preliminary results from a major study using cladistics—systematic classification based on evolutionary relationships—has indicated that green plants, red plants, and brown plants evolved from three different varieties of one-celled, plant-like organisms and should therefore be grouped into separate kingdoms. Green plants would still comprise the largest kingdom of trees, shrubs, grasses, ferns, mosses, and flowering plants (see Angiosperms), whereas brown and red plants have survived mostly as species of seaweed and microscopic algae called diatoms. Recent research has also proposed that the first flowering plants that developed 142 million years ago were closely related to any of three existing species: Amborella, which grows only in New Caledonia and produces small, cream-coloured flowers[qv]; Nymphaea, or water lilies; and Austrobaileya, a plant native to Australia.

IV

Plant Phyla

The many species of organisms in the plant kingdom are divided into several divisions (the botanical equivalent of phyla) totalling about 260,000 described species. The bryophytes are a diverse assemblage of three classes of nonvascular plants, with about 16,000 species, that includes the mosses, liverworts, and hornworts. Bryophytes lack a well-developed vascular system for the internal conduction of water and nutrients. The familiar leafy plant of bryophytes is the sexual, or gamete-producing (the gametophyte), generation of the life cycle of these organisms. The spore-producing generation (the sporophyte) of bryophytes is wholly or partially dependent on the gametophyte. Because of the lack of a vascular system and because the gametes require a film of water for dispersal, bryophytes are generally small plants that tend to occur in moist conditions, although some attain large size under favourable circumstances and others (usually very small) are adapted to desert life.

The other divisions are collectively termed vascular plants or tracheophytes. Vascular tissue is internal conducting tissue for the transport of water, minerals, and food. There are two types of vascular tissue: xylem, which conducts water and minerals from the ground to stems and leaves, and phloem, which conducts food produced in the leaves to the stems, roots, and storage and reproductive organs. Besides the presence of vascular tissue, tracheophytes contrast with bryophytes in that tracheophyte leafy plants are the asexual, or spore-producing, generation of their life cycle. In the evolution of tracheophytes, the spore-producing generation became much larger and more complex and is independent of the gamete-producing generation which became reduced. In seed-bearing plants (gymnosperms and angiosperms) the gametophyte generation is no longer free-living but is contained in the sporophyte tissue. The sporophyte embryo is contained in a seed which is dispersed from the plant. This ability to evolve larger and more diverse sporophytes while reducing the vulnerable gametophyte, together with the ability of the vascular system to lift water, freed tracheophytes from direct dependence on surface water. They were thus able to dominate all the terrestrial habitats of the Earth, except the higher arctic zones, and to provide food and shelter for its diverse animal inhabitants.