THE MAIN LINES OF PLANT EVOLUTION 



that the ferns were derived from such psilophytes. Like all tracheophytes 

 except the psilophytes, the plant is differentiated into true roots, stem, and 

 leaves. The stem is generally subterranean, and so is a rhizome, and small 

 roots arise all along its length. The leaves are large fronds, and these are 

 the only aerial parts of the plant. The sporangia occur in clusters called 

 sori on the undersurfaces of the leaves. The spores, when released, germi- 

 nate on moist ground to form monoecious gametophytes, which are al- 

 ways small, thallus-like plants. The sperm of one plant swim to the 

 archegonia of another, and there fertilization takes place. A new sporo- 

 phyte then develops from the zygote. 



Although the ferns most familiar in temperate latitudes are all of mod- 

 erate size, in the tropics there are tree ferns in which the stem forms an 

 erect trunk as high as seventy-five feet. The leaves form a palm-like cluster 

 at the top. The most primitive order of ferns, the Coenopteridales, devel- 

 oped great forests of fern trees during the Carboniferous period. These, 

 together with the giant club mosses and horsetails (erroneously called 

 fern allies) formed the dominant vegetation of the times, and their re- 

 mains have come down to the present as coal beds. Like the psilophytes, 

 these primitive ferns bore the sporangia at the tips of the branches. Like 

 so many other ancient groups, these giant ferns became extinct during 

 the Permian, but not before giving rise to the three modern orders of 

 ferns. 



Gymnosperms. The second class of the subphylum Pteropsida is the 

 class Gymnospermae, the dominant members of which are the conifers, 

 but their allies include the extinct seed ferns, the cycads or sago palms, 

 the maidenhair trees, and the very aberrant order Gnetales. The class may 

 have arisen in the Pennsylvanian period from some group of ferns via the 

 seed ferns. The seed ferns were long classified with the typical ferns, but 

 accumulating data have shown that their affinities are closer to the gym- 

 nosperms. The basic datum is the fact that they did reproduce by seeds, 

 an innovation of the greatest importance. The seed ferns formed an im- 

 portant part of the Carboniferous forests, but they later declined in impor- 

 tance and finally became extinct during the Jurassic period, but not 

 without leaving more advanced gymnosperms as descendants. The most 

 successful of these is the order Coniferales, including all of the well- 

 known evergreens. These reached their greatest development in the Meso- 

 zoic Era, and have been declining since. Three other orders have survived 

 to the present time, however. These are the Cycadales, or sago palms, 

 with nine living genera; the Ginkgoales, represented only by a single 

 species. Ginkgo biloba, which has survived only because it has for cen- 

 turies been cultivated in Chinese temple gardens; and the Gnetales, in- 

 cluding three genera of very aberrant plants, of which the most important 

 is perhaps Ephedra from which the drug ephedrine is obtained. 



Unlike the lower tracheophytes, gymnosperms are generally large, 

 woody trees, although some are shrubs. Most are evergreens. But their 

 most important characters are concerned with the reproductive cycle. At 

 the outset, the gymnosperms produce two types of spores, megaspores 

 which develop into female gametophytes, and microspores or pollen 



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