Which of the following is essential for the study and identification of living organisms?

In what aspects do the organisms of the biosphere exhibit diversity?

$Nicotiana$ $sylvestris$ flowers only during long days,while $N.$ $tobacum$ flowers only during short days. If raised in the laboratory under different photoperiods,they can be induced to flower at the same time and can be cross-fertilized to produce self-fertile offspring.
What is the best reason for considering $N.$ $sylvestris$ and $N.$ $tobacum$ to be separate species?

Into how many groups did Theophrastus classify plants?

Give a scientific reason: Knowledge of classification is very useful.

What is the full form of $ICBN$?