The tip of a plant is used for tissue culture as it is:
rich in undifferentiated cell mass
Tips of plants are generally used for tissue culture as they are rich in a group of undifferentiated cells that are capable of rapid division. These cells divide when transferred to a nutrient medium to give rise to an unorganised mass of cells called the callus.
They do not posses sclerenchyma cells as they are dead cells that do not divide.