The rise of land-dwelling animals paralleled the rise of plants, which have always been the basis for animal life. Fossil plants are a valuable source of information regarding such phenomena as changes in climate, ancient geography, and the evolution of life itself.
The earliest fossil plants are represented by a phylum called the thallophytes. The geological record of the thallophytes is incomplete. Of seven large groups, only a few are represented by fossils.
Although some records from the Paleozoic era have been found, the earliest identifiable specimens found are from the Jurassic period. The dearth of fossils from this group of plants can be attributed to their minute size and the fragile nature of their remains.
The thallophytes are in the most primitive plants, lacking roots, stems, leaves, and conducting cells. The simplest thallophytes are in the subphylum of autophytic thallophytes, which include blue-green bacteria (or cyanobacteria, formerly known as blue-green algae), diatoms, and algae.
All these plants produce chlorophyll. Cyanobacteria are unicellular organisms occurring in colonies held together by a jellylike material. Diatoms are one-celled plants enclosed in a wall consisting of two overlapping valves.
The next class, called simply algae, consists of several different types of seaweed, such as chara, or stone wort, which secretes lime with which it encrusts its leaves and is responsible for many freshwater limestones of the past. Many fossils that have been described as algae were actually molds of burrows or tracks of animals.
The second subphylum of the thallophytes is called the heterophytic thallophytes. These plants are distinguished by the absence of chlorophyll; as in animals, their principal source of energy is organic. The heterophytic thallophytes are subdivided into three classes.
Bacteria, one-celled plants without definite nuclei, are the chief agents of the decomposition of organic matter; without bacteria, more prehistoric plant and animal remains would have been preserved. The next class, slime fungi, are sticky masses enclosing many nuclei but without cell walls. Slime fungi have never been found as fossils.
The final class is the fungi, which are composed essentially of a branching mass of threads called the mycelium, which penetrate the cell walls of their “host”—plant or animal—and live upon its substance. Lichens aremade up of a fungus and an alga living together in symbiosis. Fossil lichens have been recognizedonly fromvery recent formations.
The most distinct advance of the bryophytes over the thallophytes is in their method of reproduction. The spores produced by these plants germinate by sending out a mass of green threads, the protonema.
The simplest bryophytes are the liverworts. The mosses, which are more abundant today than the liverworts, possess leaves consisting of many small chlorophyll-bearing cells.
Because the ancient members of the bryophyte group were more delicate than the modern forms, they have been preserved only under exceptional conditions, such as those provided by the silicified peat beds at Rhynie, Scotland, which contain fossils from the Devonian period.
Pteridophytes arewell represented by the ferns, which have existed from the Devonian period.Another class of pteridophytes is the horsetails (Equisetales), which also have existed from the Devonian to the present.
The third class of pteridophytes is the club mosses, which are largely creeping, many-branched plants with numerous tiny, mosslike leaves spirally arranged on the stem. The final class of pteridophytes, Sphenophyllales, consisted of slender plants with jointed stems and leaves in whorls. These climbing plants are known from the Devonian to the Permian periods.
The fourth phylum, the spermatophytes, are distinguished by the production of seeds, although the lower groups have the same alternation of the vegetative (asexual) and reproductive (sexual) generations as is seen in the pteridophytes. The chief distinguishing characteristics of the spermatophytes are the formation of a pollen tube and the production of seeds.
The first class, the gymnosperms, are typified by the pines, mostly evergreens. Members of one order of gymnosperms, Cycadofilicales, were fernlike in habit but were not actually ferns.
Because the leaf and stem remained practically unchanged, it is very easy to mistake the early seed plants for ferns. One of the most familiar of the fernlike fronds of the Pennsylvanian coal deposits is Neuropteris, which had large, compound leaflets.
The stem in most forms was thick and short and covered with an armor of leaf bases. It represents an advance over previous plants in that it had a true flower because both male and female organs were borne on the same axis and were arranged in the manner of later flowering plants.
Thus Cycadales is an intermediary in the line of development of the angiosperms (flowering plants) from their fern ancestors. This order formed the dominant vegetation of the Mesozoic, ranging from the Triassic into the Lower Cretaceous.
The next order of gymnosperms, Cordaitales,is an extinct group of tall, slender trees that thrived throughout the world from the Devonian to the Permian period. The leaves of these trees were swordlike and distinguished by their parallel veins and great size, reaching up to 1 meter.
The Cordaitales were the dominant members of the gymnosperm forests during the Devonian period. The fourth order of gymnosperms, Ginkgoales, resembles the conifers in general appearance. The leaves, however, are fanlike and are shed each year. Like the cycads and ferns, the male cells are motile in fertilization.
The order Coniferales includes mostly evergreen trees and shrubs, with needles or scale like leaves and with male and female cones. Derived from Cordaitales of the Paleozoic, Coniferales possesses fewer primitive characters than Ginkgoales.
The yews, which are comparatively modern, have fruit with a single seed surrounded by a scarlet, fleshy envelope. Another family, Pinaceae, having cones with woody or membranous scales, are represented by Araucaria,which is very common in the Petrified Forest in eastern Arizona.
The Abietae, one of the more common families of evergreens, includes pines, cedars, and hemlocks dating back to the Lower Cretaceous. One of the most extraordinary members of the conifers was the family Taxodiaceae, which includes the genus Sequoia, represented today only by the redwood and the Sequoia gigantea, which grow in California.
These species’ twigs, cones, and seeds were abundant in the Lower Cretaceous of North America. Finally, the family Cupressaceae includes the junipers and is known from the Jurassic.
The second class of spermatophytes is the angiosperms, commonly known as flowering plants. The angiosperms contain the plants of the highest rank. This group comprises well over one-half of all known living species of plants. The typical flower is composed of an outer bud-covering portion, the stamens, and the pistil.
When the wind or an insect brings the pollen into contact with the pistil, the pollen is held in place by a sugary solution. After the pollen penetrates an ovule, the nucleus divides several times. This fusion is called fertilization. The embryo, consisting of a stem with seedling leaves, is called a seed.
Both subclasses of the angiosperms first appeared in the upper part of the Lower Cretaceous. Dicotyledoneae (the dicotyledones, or dicots) comprises a primitive subclass that begins with two seedling leaves that are usually netted-veined.
The stem is usually thicker below than above, with the vascular bundles arranged to form a cylinder enclosing a pith center. As growth proceeds, new cylinders are formed. The last of the dicots to appear was the sassafras tree, flourishing throughout North America and Europe since the Lower Cretaceous.
The second subclass, Monocotyledoneae (monocotyledones, or monocots), descended from the dicots. These plants are distinguished by the fact that they begin with a single leaflet, or cotyledon.
The veins of the leaves are parallel, the stemis cylindrical, and the roots are fibrous. This subclass is represented by the grasses and grains. Fossils from this subclass date back to the upper part of the Lower Cretaceous of eastern North America. The fossil record of the palm goes back to the mid-Cretaceous.
Evolution of Plants
The evolution of plants is the story of their struggle to adapt themselves to land. One of the changes necessary in the development of land flora was the change from a cellular structure to a vascular one, which opened up possibilities for increase in size and laid the foundation for the trees. In order to adapt to land, plants also had to develop a resistance to the dehydrating quality of the air.
The earliest plants, the thallophytes, were closely tied to water. One of the first examples of flora adapting to land were the freshwater algae. The change from a cellular to a vascular structure led to the development of roots; the pteridophytes were the first plants to take this step.
The mosses and ferns adapted to land but still required rain or dew for the union of the gametes. It is only the spermatophytes that developed a device that freed them from the necessity of external water for fertilization to occur. This ability permitted the spermatophytes to proliferate throughout the earth.