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  3. Like all animal species, plant species must spread their offspring to suitable areas where they can grow and pass on their paren

Like all animal species, plant species must spread their offspring to suitable areas where they can grow and pass on their paren

admin2011-02-11  37
问题    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 by 【M1】______
walking or flying. Because plants don’t have that ability, they may somehow hitchhike.  【M2】______
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 some-【M3】______
where far from its parent tree. Seeds can thereby be carried thousands of mi]es. It may  【M4】______
surprise you to learn that plant 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 【M5】______
ready to be planted, the surrounding fruit is green, sour, and hard. When the seeds final 【M6】______
mature, 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 of attracting birds only 【M7】______
when their seeds were ready to be dispersed away. Nor did birds set out with the intent of   【M8】______
planting strawberries. Rather, strawberry plants evolved through natural selection. The
sweeter and ruder the final strawberry, the more birds spread its ripe seeds; the greener and 【M9】______
more sour the young strawberry, the birds destroyed the seeds by eating berries before the   【M10】______
seeds were ready.
【M7】
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