THE MAIN LINES OF PLANT EVOLUTION 



are known from the earlv Cambrian, and it mav well be that even the 

 Devonian psilophytes are late remnants of much more ancient stages in 

 the origin of the higher plants. 



The living Psilotales (Figure 38), Psilotum and Tmesipferus, are quite 

 similar to Psilophijfon. In nature, they are confined to tropical and sub- 

 tropical regions, but they can be grown in greenhouses anywhere. The 

 major morphological difference between the living and extinct species is 

 that the sporangia are located in the axils of the "leaves" of the living 

 plants, whereas they were located at the tips of branches in the extinct 

 species. Another point of considerable interest relates to the alternation 

 of generations. Only the sporophytes of the psilophytes have been pre- 

 served in the fossil record. But the gametophytes of the modern genera 

 are known. The gametophyte looks very much like a fragment of rhizome, 

 bearing numerous archegonia and antheridia which are rather similar to 

 those of the bryophytes. These may give an indication of the character of 

 the gametophyte of the extinct psilophytes, but there is no assurance that 

 they do. In any event, the Psilotales are regarded as the little-changed 

 descendants of the ancient psilophytes. 



Subphylum Lycopsida. The Lycopsida, comprising the club mosses, 

 appear to have been derived from psilophytes in the Devonian period. All of 

 the living lycopsids (there are only four genera) are small plants, generally 

 less than a foot high, but this has not always been true, for during the 

 Carboniferous period ( Mississippian and Pennsylvanian combined ) there 

 were giant club mosses as tall as 135 feet. These were the dominant plants 

 of the time, and their fossil remains are a major part of the coal beds. But 

 these giant club mosses became extinct in the Permian period, perhaps 

 as a result of inability to adapt to the severe Permian climate, for this was 

 a time of extensive glaciation. 



The club mosses (Figure 39) are advanced over the psilophytes in 

 many respects. The differentiation into root, stem, and leaves is complete. 

 Their leaves, which are small and spirally arranged, are supplied with 

 vascular tissue. The sporangia are enclosed in specialized leaves called 

 sporophylls, clusters of which occur at the tips of branches. Such clusters 

 are called cones or strobili. The life cycle of Lycopodium, one of the living 

 genera, is quite simple, and perhaps not very different from that of the 

 psilophytes. The spores are all alike, and are scattered by the wind. Those 

 which fall on favorable ground germinate to form small, subterranean, 

 thallus-like gametophytes. These are monoecious. Sperm swim to the 

 archegonia to fertilize the eggs. The zygote then produces a young sporo- 

 phyte which is at first a parasite upon the gametophyte. But a root is 

 soon formed and the gametophyte decays. In another living genus, Selagi- 

 nella, the life cycle is modified in a fashion quite suggestive of the seed 

 plants. There are two types of spores, megaspores which are contained in 

 sporophylls in the lower part of the strobilus, and microspores which are 

 contained in the upper sporophylls. While still contained within the spo- 

 rangia, these megaspores and microspores germinate to produce female 

 and male gametophytes respectively. The gametophytes are thus parasites 

 upon the sporophyte, although the female gametophyte may contain some 



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