In all cases the dimensions of the leaf are enlarged by interstitial growth of its parts.
According to Wulfert [60] the primitive germ-cells of Gonothyraea can be distinguished soon after the fixation of the planula, appearing amongst the interstitial cells of the ectoderm.
In some corals the septa are solid imperforate plates of calcite, and their peripheral ends are either firmly welded together, or are united by interstitial pieces so as to form imperforate theca.
The risk of interstitial condensation in the roof construction is avoided.
It is also used in the treatment of interstitial cystitis.