Answer:
These plants have adapted chemical measures to protect themselves.
Step-by-step explanation:
Milkweed:
It has ability to protect itself. They produce sandpapery leaves (toxins) that can stop human heart. For insects they produce a poison which glue their mouth of insects.
Tobacco:
Its leaves emit a chemical. When hornworm caterpillars eat these leaves then that chemical mixed with their spit and they doom into their own spit.
Poison Ivy:
This plant has a skin irritant and use this irritant to protect itself from caterpillars and slugs.
Hence, these plants use chemicals for their defense.