Talk about the sub-phylum of gymnosperms.

AKA naked seeds, points to one of the principle characteristics of plants belonging to this sub-phylum, that their ovule and seeds are exposed on the surface of modified leaves known as sporophylls. Ovule is not enclosed in an ovary. The seed is formed from the ovule but there is no fruit. They lack specialized water-conducting vessels of flowering plants but have tracheids. They include four classes.