CHAPTER XXVII 

 HETEROSPORY 



338. Heterospory.* Heterospory arose in the pteridophytes 

 with the establishment of two sizes of spores, called megaspores 

 (large spores) and microspores (small spores). Heterospory and 

 the independence of the sporophy te are the chief contributions 

 of the pteridophytes to the progress of plant evolution. The 

 establishment of megaspores and microspores was merely the 

 beginning of a number of far-reaching developments in plants, 

 all of which are really parts of the general principle of hetero- 

 spory. They all reach their highest degrees of specialization in 

 the seed plants, as will be described in the next chapter and 

 summarized in Chapter xxix, but most of them are clearly 

 illustrated in the pteridophytes. 



These developments resulting from heterospory are : 



1. The gametophytes became differentiated in sex so that 

 the megaspore always develops a female gametophyte and the 

 microspore a male gametophyte. 



2. The sporangia assumed two forms : megasporangia de- 

 roted to the production of megaspores and microsporangia 

 devoted to the production of microspores, as illustrated by Mar- 

 silia, Isoetes, Selaginella, and, as will appear in the next chapter, 

 the seed plants. 



3. The spore leaves, or sporophylls, were differentiated into 

 megasporophylls and microsporophylls which develop, respec- 

 tively, megasporangia and microsporangia, as illustrated by 

 Isoetes, Selaginella, and the seed plants; the sporophylls of 

 Marsilia bear both forms of sporangia. 



* To THE INSTRUCTOR : In a brief course or with immature students this 

 chapter should be omitted. 



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