Conceptual Overview

Plants possess unique properties that distinguish them from other types of living things. We are primarily going to focus on the properties of seed plants, particularly flowering plants, in the study of plant anatomy--i.e. the microscopic study of cells, tissues and organs. As such, we must come to the recognition that while it is possible to make general statements about their distinguishing characteristics, there are from time to time exceptions that do not fit the rules. Nevertheless, the rules are usually accurate and the collective set of rules certainly state notable properties.

To begin with, plants have a unique set of pigments with which they are associated. Among the pigments, the light-trapping chlorophylls (e.g. chlorophyll-a and chlorophyll-b) are typically wide-spread in foliar structures and young stems. The ability of such green-pigmented plants to trap light and to utilize it in the production of carbohydrates (simple sugars in particular), makes them photo-autotrophic. That is, they are capable of synthesizing their own food source in the presence of light, requiring only water, minerals, and air from their natural surroundings to survive. This property separates the green plants from heterotrophs, which require an external source of carbon-based food materials for survival.

As will be evident in the succeeding units, plant cells possess a non-living but often biologically active cell wall that encloses the protoplasmic cell contents. That cell wall may be simple, thin, complex, thick, or with other unique properties and associated components. In all cases, it possesses cellulose as a building structure. Cellulose, in turn, is composed of simple sugars (glucose) in a unique organization. While cellulose is the most characteristic polymeric substance comprising cell walls, there are also a variety of unique compounds found only in plants which are incorporated into cell walls to greater or lesser extent. The wall is also a site of active cell secretion that frequently contains enzymes of living cells as well as strengthening polymers in living and non-living cells.

Plants are typically separated from animals by possessing two rather distinct phases to their life-cycle, the sporophyte phase which is diploid and leads to the formation of spores by meiosis, and the gametophyte phase which is haploid and leads to the formation of gametes by mitotic activity. These two distinct phases of the life-cycle are referred to as an alternation of generations, and its expression is not found in other living organisms. The alternation of generations takes many forms in different taxonomic groups of plants, but in the seed plants the dominant phase (and the most visible) is the sporophyte phase. Throughout the life of the plant there is continuous growth. In seed plants this takes place in zones called meristems, primarily apical or lateral. Continuous indeterminate growth is also a characteristic of plants, as animal forms of life have a set size and form of development that, once reached, is not exceeded. The apical (shoot and root) meristems in plants give rise to primary growth, while the lateral meristems (vascular cambium and cork cambium) develop secondary growth. The detailed and unique features of these meristems and their derivatives will be pursued in succeeding units of study.

While being an approximation, seed plants are typically divided into the gymnosperms and the angiosperms. In both of these groups of seed plants the gametophyte (gamete-producing) generation has been much reduced from that in non-seed plants. (In bryophytes, the vegetative plant is homologous with the pollen grain or embryo sac of seed plants.) Ovule parts (specifically, integuments) develop into a seed coat, and food reserves are deposited in the endosperm or cotyledons of the embryo. Because of the protective coat, seeds may survive cold and drought, as well as journeys by water, wind, or animal coats, which may disperse the plant population. Exceptionally durable seeds of Lotus sp., identified by carbon-14 dating as 7,000+ years old have been germinated and grown. The Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta are the four living divisions of gymnosperms. They appear to have evolved earlier than other seed plants--the angiosperms (flowering plants), and they do not have ovaries to protect the developing seed. Today, there remain 720 known gymnosperm species in existence.

Conifers (Coniferophyta) reproduce via a woody structure called a cone. Conifers are gymnosperms, most species of which are typically evergreen and keep their leaves all year. However, such gymnosperms as larch, tamarac and bald cypress are deciduous. Retention of foliage enables them to adapt to warmer, sunny winter days, as well as allowing them to take advantage of early spring sunshine when deciduous trees are just putting out their new leaves. Because conifers do not need to produce all of the photosynthetic needles in one season, there is a considerable energetic savings in not being deciduous. These characteristics help conifers live at higher latitudes (more northern in Canada, for example) and higher elevations where the growing season is shorter. The needle-shaped leaves are also resistant to drought. Conifer woods contain tracheids to conduct water. Tracheids are long tapered cells with overlapping ends. They are believed to be ancestors of elongated cells that became modified for water transport at the center of the stem. In an evolutionary sense, tracheids are also thought to have given rise to the shorter and wider cells called vessel elements, which combine with fibers, tracheids and parenchyma cells to make up xylem, or water-conducting tissue in angiosperms.

Traditionally, the flowering plants have been divided into two major classes (or subphyla): the Magnoliopsida (dicots) and the Liliopsida (monocots). Many people take this separation into two classes for granted because it appears obvious, but botanists have not always recognized these as the two fundamental groups of angiosperms. Theophrastus (c. 370 BC) is credited with first recognizing differences between the two groups and his classification of plants was further based upon overall growth form, e.g. trees, herbs, vinesùa system which endured until the 1600's, and still has practical application.

The existing ambiguity in the definitions of monocots and dicots is a real phenomenon resulting from a shared ancestry of the two groups. It is now believed that some dicots are more closely related to monocots than to the other dicots, and that the angiosperms do not all fit neatly into two groups. In other words, the dicots include a basic paraphyletic group from which the monocots evolved. The major differences between the two classes can be pointed out as in the table below; but it is important to point out that no single feature can be used to definitively classify the flowering plants.

The Earth is considered by most scientific estimates to be approximately 4.55 billion years old. However, the first land plant fossils may have appeared in the Silurian, about 440 million years ago. They probably originated from green algae. The first vascular plants appeared in the Lower and Middle Devonian (408-387 million years ago). These plants were constructed on a central axis with lateral branches devoted to maximizing photosynthetic activity and showing a dichotomous pattern of branching. The first seed plant fossils have been found in the Upper Devonian, and date to about 374 million years ago.

Subunits:		Feature:
The Uniqueness of Plants		Video: Anatomical Research of Fossil Plants
Geological & Evolutionary History
The Basic Plant Body
Groups of Land Plants
Structural Size Relationships