342 BOTANY: PRINCIPLES AND PROBLEMS 



given spore will generally produce either a strictly male or a 

 strictly female gametophyte. Attached to each spore are four 

 thread-hke structures, the elaters (Fig. 214), which coil tightly 

 around it when moist but expand when dry, and thus aid in spore 

 dispersal. 



The gametophyte (Fig. 216) is an irregular thallus which 

 develops chlorophyll, shows no tendency toward the subterranean 

 habit of the lycopod prothallus, and is provided with long, 

 branching lobes. The two sexes are usually on separate plants 

 and the gametophytes are therefore said to be dioecious. The 

 sexual organs resemble in general those of other pteridophytes. 

 The antheridia occur at the tips of the lobes and the archegonia 

 at their bases. Each sperm has a tuft of cilia. In embryonic 

 development no suspensor is formed, and the whole process 

 considerably resembles that found among the ferns. 



The Equisetineae, like the Lycopodineae, were represented 

 in earlier periods of the earth's history by large, tree-like species 

 with well developed cambium and secondary wood. Many of 

 these were also heterosporous. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



769. Which of the three groups of pteridophytes is the most ancient, 

 in your opinion? Why? 



770. What other groups of plants already described (aside from the 

 pteridophytes) were once probably very abundant but are now repre- 

 sented by only a comparatively small number of species? 



771. Why is asexual reproduction so common among the pterido- 

 phytes? 



772. Do you think that heterospory has arisen more than once in the 

 evolution of the plant kingdom? Explain. 



773. Which do you think is more primitive among pteridophytes, 

 thin- walled sporangia or thick- walled ones? Why? 



774. Can you suggest a reason for the fact that ferns are now so 

 much more abundant than club mosses or horsetails? 



775. Why are there no tree-ferns in temperate climates? 



776. How does the trunk of a tree-fern differ from that of an ordinary 

 tree? 



777. Why does not the trunk of a tree-fern make good lumber? 



