How do sieve tube member (stm) cells differ from tracheids? stm cells function as living cells; tracheids function as dead cells stm cells lack a companion cell; tracheids have a companion cell stm cells occur in angiosperms; tracheids only occur in gymnosperms stm cells have secondary cell walls; tracheids?

1. in what ways have non-native species been beneficial? 2. what are some of the hypotheses for the collapse of honeybee colonies? 3. distinguish between regional and functional extinction. 4. why is it difficult to measure background extinction rates? 5. what three factors are most important in causing extinction rates to climb?