Fossil plants

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.

Thallophytes

The earliest fossil plants are represented by a polyphyletic group usually called the thallophytes, but sometimes also called Thallophyta, Thallophyto, or Thallobionta. The geological record of the thallophytes is incomplete, and the taxonomy and nomenclature surrounding fossil plants remain controversial. Of the large groups, only a few are represented by fossils. Although doubtful records from the Paleozoic era have been found, the earliest identifiable specimens are found in the Jurassic period. The death of fossils from this group of plants can be attributed to their minute size and the fragile nature of their remains. The thallophytes are among the most primitive plants, lacking roots, stems, leaves, and conducting cells. The simplest thallophytes are autophytic thallophytes, including blue-green algae diatoms and algae. These plants produce chlorophyll. Blue-green algae are made up of a class of unicellular plants that occur in colonies held together by a jellylike material. Related to green and brown algae were the diatoms, one-celled plants enclosed in a wall consisting of two overlapping valves. The next class, simply called algae, consists of several types of seaweed, such as chara or stonewort, 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 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. Fungi are also rarely preserved as fossils, although mycelial threads of a fungus have been detected under the bark of trees from the Pennsylvanian coal beds. Lichens are a type of fungi made up of a fungus and an algae living together in symbiosis. Fossil lichens have been recognized only from very recent formations.

Bryophytes and Pteridophytes

The next phylum to emerge, the bryophytes, exhibits a distinct advance over the thallophytes. Bryophytes adapted more successfully to the terrestrial environment. They were able to take water and other necessary substances from the soil by means of rootlike hairs called rhizoids. 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.

The pteridophytes were much more advanced than the bryophytes. While the structure of the bryophytes was primarily cellular, that of the fern plant is vascular. Unlike the bryophytes, the pteridophytes originate from the fertilized egg and produce spores. Pteridophytes are well represented by the ferns, which have existed from the Devonian period. Another class of pteridophytes is the horsetails (Equisetales), which have existed from the Devonian to the present. Equisetales include the calamites, trees of the Permian period, with lance-shaped leaves attaining heights of thirty meters, and annularia, a smaller plant with a stem of five to eight centimeters in diameter, which was abundant in the Pennsylvanian period.

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. One member of this class, Lepidodendron, was a lofty tree of the later Paleozoic, appearing in the Lower Devonian and dying out in the Permian. The leaves were arranged in a convex outline to form a “leaf cushion.” The stigmaria consisted of spreading, rootlike underground stems and were common in the Pennsylvanian period. Sigillaria was a tree that resembled Lepidodendron in general appearance except that its leaf cushions were hexagonal in outline.

An order 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 and are represented by sphenophyllum, a small branching plant with slender, ribbed stems.

Spermatophytes: Gymnosperms

Another group, 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, is typified by the pines, mostly evergreens. One order of gymnosperms, Cycadofilicales, were fernlike in habit but were not ferns. Because the leaf and stem remained practically unchanged, it is easy to mistake the early seed plants for ferns. One of the most familiar fernlike fronds of the Pennsylvanian coal deposits is Neuropteris, which has large, compound leaflets. Pecopteris had fronds like those of Neuropteris, but the leaflets were attached to the stalk by their whole width. Lyginodendron also had stems varying in diameter from three millimeters to four centimeters. Its leaves were very large, divided, and fernlike in appearance.

The second order of gymnosperms, Cycadales, or the cycads, is an extinct family of trees or shrubs. In most forms, the stem was thick and short and covered with an armor of leaf bases. The method of fertilization in the cycads was also primitive. The male cells were motile, swimming actively to the ovule after the pollen tube rupture. By its lateral branching and hair-like scales covering the leaf bases, Cycadales also recall the ferns. Nevertheless, it represents an advance over previous plants in that it had a true flower because male and female organs were borne on the same axis and arranged like later flowering plants. Thus, Cycadales is an intermediary in developing the angiosperms 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 worldwide from the Devonian to the Permian period. The leaves of these trees were sword-like and distinguished by their parallel veins and great size, reaching up to one meter. The stem resembles the conifers except for the large pith, which recalls the cycads. The reproductive organs of Cordaites were small male and female catkins. The Cordaitales were the dominant members of the gymnosperm forests during the Devonian period.

The fourth order of gymnosperms, Ginkgoales, is composed of one species—Ginkgo biloba, or the ginko tree, which resembles conifers in appearance. The leaves, which have remained largely the same since the Mesozoic period, are fanlike and shed each year. Like the cycads and ferns, the male cells are motile in fertilization. Their fossil record dates to the early Permian period, though fossilized leaves are more commonly found than bark. 

The order of Coniferales includes mostly evergreen trees and shrubs with needle or scale-like leaves and with male and female cones. Derived from the Cordaitales of the Paleozoic, Coniferales possess fewer primitive characteristics 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. Taxodiaceae includes the genus Sequoia, represented today only by the redwood and the Sequoia gigantea. The trunk attains a height of more than ninety meters and a diameter of more than 9 meters. Sequoia's twigs, cones, and seeds were abundant in the Lower Cretaceous of North America. Finally, the family Cupresseae includes the junipers and is known from the Jurassic.

Spermatophytes: Angiosperms

The second class of spermatophytes is the angiosperms. The angiosperms contain the plants of the highest rank. This group comprises more than half of all known living species of plants. The members of the angiosperms are commonly known as the flowering plants. The typical flower is composed of an outer bud-covering portion, the stamens, and the pistil. Each part of the flower is a specially modified leaf. 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 is 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. One of the most primitive dicotyledones is the American tulip tree, which appeared first in the Upper Cretaceous. Sassafras is the last representative, flourishing throughout North America and Europe since the Lower Cretaceous. The poplar is known to have lived during the Lower Cretaceous period in Virginia and Greenland.

The second subclass, Monocotyledoneae, descended from the dicotyledones and is distinguished because the plant begins with a single leaflet, or cotyledon. The veins of the leaves are parallel, the stem is cylindrical, and the roots are fibrous. This subclass, represented by the grasses and grains, is especially important to humans today. 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.

Spermatophytes: A Success Story

The evolution of plants is the story of their struggle to adapt 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 an increase in size and laid the foundation for the trees. 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. Freshwater algae was one of the first examples of flora adapting to land. 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.

Principal Terms

catkin: a dense cluster of scale-like flowers

Mesozoic era: the era that began 245 million years ago and ended 66 million years ago; it includes three periods—the Triassic, the Jurassic, and the Cretaceous

motile: having the ability to move spontaneously

Paleozoic era: the era that began about 543 million years ago and ended 245 million years ago; it includes six periods—the Cambrian, the Ordovician, the Silurian, the Devonian, the Carboniferous, and the Permian

phylum: one of the broad categories used in the classification of organisms

pistil: the female organ of flowers, which receives the pollen

pollen: the male cells of fertilization

stamen: the male organ of flowers, which bears the pollen

symbiosis: a condition in which organisms live together in close association, especially when the relationship is mutually beneficial

vascular: possessing a series of vessels that form a conducting apparatus for food and sap

Bibliography

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