Fill in the blanks with appropriate word/words.
$(a)$ $A$ phase of growth which is maximum and fastest is $.......$ .
$(b)$ Apical dominance as expressed in dicotyledonous plants is due to the presence of more $.......$ in the apical bud than in the lateral ones.
$(c)$ In addition to auxin,a $.........$ must be supplied to culture medium to obtain a good callus in plant tissue culture.
$(d)$ $.........$ of a vegetative plants are the sites of photoperiodic perception.