12.5–12.7: The advent of the seed opened new worlds to plants.

Shepherd’s purse (Capsella rubella) seed and capsule.

As we’ve seen, the evolutionary development of vascular tissue allowed plants to grow larger. Vascular plants could colonize areas where the soil at the surface of the ground was not always wet, because their roots could penetrate the soil to reach water and nutrients. The next big innovation in plant evolution was the seed, an embryonic plant with its own supply of water and nutrients encased within a protective coating (FIGURE 12-13).

Figure 12.13: What is a seed? A seed is a package that contains a multicellular embryo and a store of carbohydrate, the endosperm, to fuel its initial growth. This makes many seeds desirable as food sources.

Unlike spores, which are single cells that contain only DNA, RNA, and a few proteins, seeds contain both a multicellular embryo and a store of nutrients, mostly starch. This nutritive tissue—called endosperm in flowering plants and trees—can fuel the seed’s initial growth. A seedling draws energy from the endosperm while it extends its leaves upward to begin photosynthesis and its roots downward to reach water and nutrients in the soil. There are two modern groups of seed-producing plants: gymnosperms (including pines, firs, and redwoods) and angiosperms (all flowering plants and trees).

Like plants that do not produce seeds, seed plants have a life stage, called the gametophyte, which produces haploid gametes (sperm and egg). This is analogous to humans’ production of haploid gametes. Because the gametophyte is much smaller than the sporophyte in seed plants, these plants are considered sporophyte dominant. Pollen grains and ovules are the male and female gametophytes, respectively, of seed plants. In brief (we expand on this later in the chapter), a haploid female gamete (egg) forms inside the ovule. When a pollen grain lands near the ovule, it produces a tube that grows into the ovule. Sperm from the pollen grain move through the pollen tube into the ovule and fertilize the egg. The external layer of the ovule then forms the seed coat.

Another of the challenges that plants face is dispersing their seeds. The seed stage is the only opportunity most plants have to scatter their offspring, and seeds and seed pods have many ways to do this. These range from the forceful send-off of exploding seed pods, to seeds that hitch a ride in or on passing animals, to those that are so small and light that they can float in water or air (such as dandelion seeds)—not to mention the use of fruits in seed dispersal. As we’ll see later in the chapter, many methods for dispersing seeds have evolved.