Algal ancestors

This phylogenetic diagram represents a hypothesis for the evolutionary relationships between plants and green algae. The earliest plants were nonvascular. These plants evolved into more-complex forms of plants, the vascular plants. For updates on phylogenetic diagrams, visit go.hrw.com. Enter the keyword HM6 Phylo.

Notice in Table 28-1 that vascular plants can be further divided into two groups, seedless plants and seed plants. Seedless plants include the phylum of ferns and three phyla made up of plants closely associated with ferns. Seed plants—plants that produce seeds for reproduction—include four phyla of gymnosperms (JIM-noh-SPUHRMZ) and one phylum of angiosperms (AN-jee-oh-SPUHRMZ). Gymnosperms, which include pine trees, are seed plants that produce seeds that are not enclosed in fruits. Angiosperms, also known as flowering plants, are seed plants that produce seeds within a protective fruit. Examples are apple and orange trees.

The Fossil Record of Plants

Figure 28-2 shows the possible origin of major plant groups. Much of what is now known about plant phylogeny comes from the fossil record. The fossil record is incomplete, but scientists hypothesize that plants evolved from algal ancestors. The strongest evidence lies in the similarities between modern green algae and plants. Both have the same photosynthetic pigments—chlorophylls a and b, both store energy as starch, and both have cell walls made of cellulose.

co Connection

Reforestation Efforts

The process of replacing trees that have died or been cut down is called reforestation. Natural reforestation occurs when seeds grow into new seedlings. Throughout the world, many governments and private landowners do not replace trees after land has been cleared to produce timber, build roads, and construct buildings.

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