THE MAIN LINES OF PLANT EVOLUTION 



gonia in the female, develop. In some species, the sexes are separate 

 (dioecious), while in other species both male and female shoots may 

 occur on the same plant (monoecious). In either case, the sperm are 

 released and swim to the archegonia, where fertilization occurs. The em- 

 bryo now develops within the archegonium, and remains attached there 

 by a foot when it grows to maturity. In addition to the foot, the mature 

 sporophyte consists of a stalk and a terminal capsule which contains the 

 spore mother cells. Here the meiotic divisions occur, and the spores are 

 released to start the cycle over again. The gametophyte, consisting of the 

 haploid protonema, rhizoids, shoots, and leaf-like structures is much larger 

 than the sporophyte, and is independent because of the possession of 

 chlorophyll. The sporophyte, consisting of foot, stalk, and capsule, is 

 smaller and dependent on the gametophyte because it is lacking in chloro- 

 phyll, or nearly so. In neither the gametophyte nor the sporophyte are 

 there any specialized vascular (conducting) tissues, although a limited 

 amount of conduction of foods and water does occur. Nor are there any 

 fibrous tissues. Lack of these elements ( vascular and fibrous tissues ) is 

 probably the most important single factor which has prevented the de- 

 velopment of large plants in this phylum. 



Phylogeny of the Bryophytes. In view of the simplicity of the bryo 

 phytes, and in view of the many characters which they share with the 

 green algae, it is highly probable that the bryophytes were derived from 

 an ancestor among the green algae, most probably a tide-flat dweller for 

 which terrestrial adaptations would already be advantageous. One group 

 of bryophytes, the horn worts, suggests relationship to higher plants. 

 While the gametophyte is a small, simple thallus, the sporophyte is larger 

 and quite differentiated, much the dominant phase of the hornwort. It 

 is well supplied with chlorophyll, but still depends upon the gametophyte 

 for absorption of water and minerals from the substrate. The capsule has 

 a central axis of elongate cells which suggests vascular tissue. The epi- 

 dermis has stomata like those on the leaves of vascular plants. With such 

 an array of traits suggesting those of vascular plants, it is difficult to 

 avoid the inference that modern hornworts may be but little changed 

 from an ancestral stock by which bryophytes gave rise to vascular plants. 

 Nonetheless, this inference appears to be misleading, for vascular plants 

 appear in the fossil record in the Silurian, and the bryophytes appear more 

 than 100,000,000 years later. Obviously, if these data are correct, bryo- 

 phytes cannot be ancestral to tracheophytes. And so the weight of opinion 

 among botanists now favors direct origin of the vascular plants, the phy- 

 lum Tracheophyta, from a chlorophytan ancestor. 



ORIGIN OF THE VASCULAR PLANTS 



The phylum Tracheophyta is an ancient and much varied group, which 

 includes the dominant plants of today. It is subdivided into four subphyla, 

 the subphylum Psilopsida, including the most primitive of vascular plants, 

 of which all but two genera are extinct; the subphylum Lycopsida, or 

 club mosses; the subphylum Sphenopsida, or horsetails; and the subphy- 



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