310 



PLANT MORPHOLOGY 



garnet ophyte does not form antheridia, but gives rise directly to two 



sperms or their eciuivalent. Swimming sperms, universal throughout the 



bryophytes and pteridophytes, occur only in two orders of living gymno- 



sperms, being replaced in all other spermatophytes by nonmotile male 



cells. In all seed plants the sperms or male cells, as the case may be, reach 



the egg through the agency of pollen tubes, within which they develop. 



Following fertilization, the embryo develops inside the ovule, which 



becomes a seed. 



1. GYMNOSPERMAE 



The gymnosperms, numbering only about 700 living species, are an 

 ancient group with a long geologic history. All the evidence points to 



Fig. 258. Diagram showing the geologic distribution and the relationships of the major 

 groups of vascular plants. 



their origin from the ferns. They were abundant during the Paleozoic, 

 while during the greater part of the Mesozoic they became the dominant 

 group of land plants (Fig. 258). All existing gymnosperms are woody 

 plants and most of them are trees. Gymnosperm means "naked seed," 

 the seeds being borne not in a closed vessel, as they are in angiosperms, 



