Do all plants come from seeds?

Not every plant grows from a seed. In fact, there are a numerous ways in which plants can reproduce! We'll share more on that, in a moment, but first let's outline a few meaningful terms so we may clearly understand the various ways in which we can grow new plants:

Grafting: a 'horticultural' technique whereby plants are joined to grow together. The upper part of the combined plant is called the scion while the lower part is called the rootstock.
Horticulture: the science and art of growing plants.
Rhizome: a plant stem, like that of bamboo, capable of producing the shoot and root systems of a new plant. 
Spore: a hearty reproductive cell produced by nonflowering plants, fungi, and algae.
Tubers: a  thickened part of an underground stem of a plant, such as the potato, with buds from which new plant shoots can grow.

With these in mind, let's go on! Some plants, like ferns and mosses, grow from spores, where as others use asexual vegetative reproduction, growing new plants from rhizomes or tubers. We can also use techniques like grafting or take cuttings to make new plants.