Examination of your slide will show that a few nuclei do not appear next to a cell wall

The onion root tips are very famous for observing the distinct phases of mitosis as the chromosomes are very large and get very dark when stained.  

The epidermal cells in the onion take place cell division (cytokinesis) and mitosis (nuclear division). The interphase cell usually possesses one or number of nuclei, thus, a few nuclei are not witnessed adjacent to a cell wall.  

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