12.11–12.12: Plants and animals have a love-hate relationship.

Fruits such as these peaches are structures that contain fertilized seeds.

Leaving home is an inevitable part of growing up. But as difficult as it may be, imagine you had to move away but didn’t have a car or moving van—or even legs. How would you do it? This is yet another issue facing plants. Ingenious methods have evolved for transporting the male gametes to the female sex organs for fertilization, but that still results in all the fertilized eggs, and all the new offspring they develop into, living right on the female plant—or in nearly the same location, if they manage to fall off the parent plant. This is a problem because parent and offspring would end up competing for the same light, space, soil nutrients, and other resources.

The solution is the fruit, a structure that aids in dispersing seeds—the reproductive packets made up of the embryo, some food reserves, and a hard coat. The fruit usually develops from the ovary (and sometimes from nearby tissue), right around the seeds, which develop from the ovules. This should cause you to look at a field of flowers differently. Given that the ovary is part of a flower, after pollination and fertilization, that flower will turn into a fruit. Another way of looking at it is that every fruit you eat was once a flower.

Pea pods, sunflower seeds, corn kernels, and hazelnuts are a few examples of fruits. These are “dry fruits,” and the seeds they contain are transported by wind, water, or animals (FIGURE 12-25). Many angiosperms, though, make fleshy fruits that consist of the ovary plus some additional parts of the flower. For example, the core of an apple is the ovary and the flesh of the apple is derived from adjacent parts of the flower. Blueberries, watermelons, oranges, tomatoes, and peaches are other examples of fleshy fruits.

Figure 12.25: Methods of dispersing seeds.

The fleshy part of a fruit is often larger than the ovary, and an angiosperm invests a lot of energy in producing fleshy fruits. The payoff comes when an animal eats the fruit and then defecates the seeds at a location far from the parent plant. In other words, fleshy fruit is the bait that some flowering plants use to get animals to disperse their seeds.

When you look at fruits, you can see several characteristics that help this system work.

• Fruits are colorful—red is the most common color, followed by yellow and orange. All of these colors contrast with green foliage and make fruits conspicuous.
• Fruits typically taste good—plants pour sugars into fruits, and their sweetness appeals to many animals.
• Fruit is good for animals, serving as a significant source of sugar, enabling animals to produce ATP for their needs. Additionally, fruit has other nutritional value; many birds incorporate the red and yellow colors of fruits into their own color patterns, and colorful male birds are appealing to female birds. Thus, male birds that eat fruit are likely to produce a lot of offspring who also eat fruit—a good deal for the plant.

Of course, there’s one more requirement to make fruit a successful way for a plant to disperse its seeds: the seeds must survive passage through the animal’s digestive tract and be able to germinate when they come out. That’s not a problem—seeds are fully viable when they emerge. It’s easy enough to test this yourself. The next time you eat fresh tomatoes, check the toilet about 24 hours later. The seeds that have passed through your digestive system are still intact. If you want to test their viability, that’s easy—recover a few, rinse them off, and put them on a wet paper towel in a sealed plastic bag. If you want to do a controlled experiment, you can take some seeds directly from a tomato and put them in another bag. Put both bags in a warm, dark place, and check them in a couple of days. You’ll find that both sets of seeds have sprouted. (Or, you can just take our word for it.)

Seeds not only survive being eaten by animals—some seeds will not germinate unless they are eaten. For some plants, the pulp that surrounds the seeds in the fruit inhibits germination, and it must be digested away before the seed will sprout. The seeds of other plants have chemicals in the seed coats that must be removed by the acidity of an animal’s stomach before the seeds will germinate. And there’s another plus to this system: the seeds are deposited with a bit of manure that provides nutrients for the seedlings.