These are pieces of stems placed under suitable condoms which will produce new plants please help

Suitable environment is the need of plants to grow.

Explanation:

Stems also grows into some plants because many plant grow without roots such as rose plant. if only the stem is sown in the soil and provides suitable environment so they will grow and produces leads as well as roots and grow as a full plant so that's why the pieces of stems placed under suitable conditions which will produce new plants.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products