What plants have no leaves?

Bryophytes have no roots, leaves or stems. Moss and liverworts belong to this group. They are flowerless plants that grow in clumps.

Is there an explanation why moss or other plants increases in numbers even they have no leaves?

Mosses grow slow. They grow through a combination of cell growth and cell division. The key to plant growth is meristem, a type of plant tissue consisting of undifferentiated cells that can continue to divide. Also, they still have stems and roots for them to grow.