Fritz Prinsler has the most eye-catching veining. Average warmth, minimum of 55 F. in winter. Keep compost moist at all times but never waterlogged. Reduce watering in winter. Use soft, tepid water.
Mist leaves frequently.
Take stem cuttings in spring. The cause is dryness at the roots, even a short period of drying out can cause serious leaf loss. Cold air is another common cause of defoliation. Other possibilities are too much sun or draughts. Surround the pot with moist peat and mist the leaves regularly.