The Plant Kingdom

The kingdom Plantae is monophyletic—all plants descend from a single common ancestor and form a branch of the evolutionary tree of life. The shared derived trait, or synapo-morphy, of the plant kingdom is development from embryos protected by tissues of the parent plant. For this reason, plants are sometimes referred to as embryophytes. Plants retain the derived features that they share with green algae: the use of chlorophylls a and b and the use of starch as a photo-synthetic storage product. Both plants and green algae have cellulose in their cell walls.

There are other ways to define "plant" and "plant kingdom" and still come out with a monophyletic group (clade). For example, combining plants as defined above with a group of green algae called the charophytes results in a mono-phyletic plant kingdom with several shared derived traits, including the retention of the egg in the parent body. The addition of the chlorophytes (the remainder of the green algae) to the group just described gives another monophyletic group, with synapomorphies including the possession of chlorophyll b, that can be called a plant kingdom. There are no hard-and-fast criteria for defining a kingdom (or any other taxonomic rank), so these definitions of the plant kingdom are all valid.

In this book, we choose to use the first definition given above, in which the kingdom Plantae comprises only the em-bryophytes (Figure 29.1). Some botanists refer to a group consisting of the Plantae plus the green algae as the "green plant kingdom," to the red algae as the "red plant kingdom," and to the stramenopiles as the "brown plant kingdom."

Ancestral organism

Three Clades Plant Kingdom

Plantae

29.1 What Is a Plant? There are three ways to define a plant kingdom, depending on which clade is chosen. In this book, we use the most restrictive definition: plants as embryophytes. Here, the two green algal clades are not considered plants.

"Brown plants"

"Red plants"

>- "Green plants"

Plantae

29.1 What Is a Plant? There are three ways to define a plant kingdom, depending on which clade is chosen. In this book, we use the most restrictive definition: plants as embryophytes. Here, the two green algal clades are not considered plants.

There are ten surviving phyla of plants

The surviving members of the kingdom Plantae fall naturally into ten phyla (Table 29.1). All members of seven of those phyla possess well-developed vascular systems that transport materials throughout the plant body. We call these seven phyla, collectively, the tracheophytes because they all possess conducting cells called tracheids. The tracheophytes constitute a clade.

The remaining three phyla (liverworts, hornworts, and mosses), which lack tracheids, were once considered classes of a single larger phylum. In this book we use the term non-tracheophytes to refer collectively to these three phyla. The nontracheophytes are sometimes collectively called bryo-phytes, but in this text we reserve that term for their most familiar members, the mosses. Collectively, the nontracheo-phytes are not a monophyletic group. They are the three basal clades of the plant kingdom.

Life cycles of plants feature alternation of generations

A universal feature of the life cycles of plants is the alternation of generations. Recall from Chapter 28 that alternation of generations has two hallmarks:

► The life cycle includes both multicellular diploid individuals and multicellular haploid individuals.

► Gametes are produced by mitosis, not by meiosis. Meiosis produces spores that develop into multicellular haploid individuals.

If we begin looking at the plant life cycle at a single-cell stage—the diploid zygote—then the first phase of the cycle

29.1 Classification of Plantsa

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