Know major trends of specialization in the flowering plants. Know the functions of a herbarium and the techniques of preparing herbarium specimens.
flowering plant once saved my life in an uncon-
Aventional way. While in Washington's Olympic National Park, I was conducting field research on a group of relatively small herbaceous plants known as bleeding hearts (Fig. 23.1). At one point, I found myself on a slope of loose shale above a steep cliff. Suddenly, the shale started to move, and before I could do anything about it, I was rapidly sliding on my back, feet first, directly toward the edge of the cliff. There were no bushes, trees, or anything stable I could cling to or grab that would at least slow my journey to doom in an avalanche of shale. Just a few feet before I was to hurtle over the cliff, however, I cannoned into a large clump of bleeding hearts that somehow had rooted securely enough beneath the shale surface to stop my speedy plunge. Obviously, I lived to tell the tale, and it won't surprise you to know that ever since the experience, I have had a special place in my garden for bleeding hearts!
The Pacific bleeding hearts that saved my life are only one of more than 250,000 known species of flowering plants that comprise, by far, the largest and most diverse of the phyla of the Plant Kingdom. These flowering plants are called angiosperms.
The term angiosperm is derived from two Greek words: angeion, meaning "vessel," and sperma, meaning "seed." The "vessel" is the carpel, which is like an inrolled leaf with seeds along its margins. A green pea pod, for example, is a carpel that resembles a leaf that has folded over and fused at the margins, enclosing the attached seeds. Many flowers have pistils (see Chapter 8) composed of either a single carpel or two more united carpels. A seed develops from an ovule within a carpel and is part of an ovary that becomes a fruit. Although the angiosperms generally have organs and tissues similar to those of the gymnosperms, the enclosed ovules and seeds of the angiosperms distinguish them from gymnosperms, which have exposed ovules and seeds.
All angiosperms are presently considered to be in the Phylum Magnoliophyta (previously known as the Antho-phyta). Other classifications of flowering plants are discussed in Chapter 16. Phylum Magnoliophyta is divided into two large classes, the Magnoliopsida (dicots) and the Liliopsida (monocots). Table 8.1 on page 135 lists the distinguishing features of members of the two classes.
Since Darwin's Origin of Species appeared in 1859, there have been two major theories concerning the origin of angiosperms. The older and now more or less disregarded theory was held by the German botanist Adolph Engler and his followers. It suggested that flowering plants evolved from conifers and that primitive flowers are similar in structure to the strobili of conifers. Such flowers are inconspicuous and in clusters, such as those of the grasses, oaks, willows, and cattails.
Most contemporary botanists, however, point to anatomical, chemical, and fossil evidence, along with cladistic interpretations, that indicate angiosperms evolved independently from the pteridosperms (seed ferns—discussed in Chapter 22). They also hypothesize that a flower is really a modified stem bearing modified leaves. The most primitive flower is thought to be one with a long receptacle and many spirally arranged flower parts that are separate and not differentiated into sepals and petals. In addition, the stamens and carpels are flattened and numerous. Such flowers are found among relatives of magnolias and buttercups, leading many modern botanists to postulate that all present-day flowering plants are derived from a primitive stock with such characteristics. A further discussion of primitive and advanced characteristics follows later in the chapter.
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