When you think of plants, you probably assume that all plants reproduce by seed dispersal. Believe it or not, there are plants that do not have any seeds. They either produce seed-like objects, called spores, or they produce asexually. Seedless plants have other atypical properties as well, such as the fact that some lack a system of retaining and transporting water.
Seeds vs. Spores
The main difference between seed plants and seedless plants is their methods of reproduction. As their names imply, seed plants produce seeds using sexual reproduction, and seedless plants do not. Seedless plants often rely on spores, which are usually unicellular and haploid (containing one set of chromosomes), and contain little or no added stored food to sustain them. Spores often lay dormant until conditions are favorable, at which point they undergo mitosis to become diploid. Seeds, on the other hand, are produced as diploid (containing two sets of chromosomes).
Although both seedless plants and seed plants both usually produce sexually, some seedless plants can reproduce asexually. This method of reproduction is less common, and it occurs when plant material (e.g., leaves) fall off the parent plant and regenerate into new plants on their own. Seed plants never reproduce asexually, as they always create a new generation of plants through the joining of two gametes (i.e., egg and sperm).
Vascular and Non-vascular Plants
The term "vascular" refers to the vascular tissue in a plant. This tissue allows the plant to retain water for long period of time, as well as to move the water around to different parts of plants. All seed plants are vascular, whereas seedless plants can be vascular or non-vascular. Seedless vascular plants, which include ferns and horsetails, have true roots, stems, and leaves. Non-vascular plants, such as mosses, liverworts, and hornworts, do not. In order for non-vascular plants to survive, they need to live in moist places because they cannot retain water on their own.
Most plants that people see on a regular basis are seed plants. These include those that obviously contain seeds, such as apple trees and berry bushes, as well as those that create less obvious seeds, such as dandelions and pine trees. The most common seedless plants are mosses and ferns, but liverworts, hornworts, quillworts and horsetails are also seedless.