Like all animal species, plant species must spread their offspring to suitable areas
where they can grow and pass on their parent’s genes. Young animals generally spread (1) ______
by walking or flying. Because plants don’t have that ability, they may somehow (2) ______
hitchhike. Some plant seeds scatter by blowing in the wind or floating on water.
Many other plant species, though, trick an animal into carrying their seeds. How do
they do They enclose them within a tasty fruit and advertise the fruit’s ripeness by
its color or smell. The hungry animal collects and swallows the fruit, walks or
flies off, but later spits out the seeds somewhere far from its parent tree. Seeds (3) ______
can thereby be carried thousands of miles. It may surprise you to learn that plant (4) ______
seeds can resist digestion. In fact, some seeds actually require passage through an
animal’s body before they can grow. Wild strawberries offer a good example of
hitchhiking tactic. When strawberry seeds are still young and not yet ready to be (5) ______
planted, the surrounding fruit is green, sour, and hard. When the seeds final mature, (6) ______
the berries turn red, sweet, and tender. The change in the berries’ color serves as
a signal to birds which then eat the strawberries, fly off, and eventually spit
out the seeds. Naturally, strawberry plants doesn’t set out with a conscious intent (7) ______
of attracting birds only when their seeds were ready to be dispersed away. Nor (8) ______
did birds set out with the intent of planting strawberries. Rather, strawberry plants
evolved through natural selection. The sweeter and reder the final strawberry, the (9) ______
more birds spread its ripe seeds; the greener and more sour the young strawberry,
the birds destroyed the seeds by eating berries before the seeds were ready. (10) ______