292 EVOLUTION OF PLANTS 



there is a gradual reduction of the gametophyte. This 

 in the lower forms is long lived and much like the 

 simpler liverworts in its structure, and bears both arche- 

 gonia and antheridia. Other forms develop male and 

 female gametophytes from similar spores, and, finally, 

 heterospory has arisen in several groups of Pterido- 

 phytes. In these, two sorts of spores are produced 

 which on germination give rise respectively to exceed- 

 ingly reduced male or female plants. Heterospory is 

 found in several groups of living ferns, and in one 

 genus, Selaginella, among the club-mosses. It is evi- 

 dent from a study of fossil Pteridophytes that it was 

 also developed in the Equisetineae. In Selaginella the 

 germination of the spores begins within the sporangium, 

 which sometimes falls away with the contained spores. 



The permanent retention of the spores within the 

 sporangium until the germination is complete, and the 

 thickening of the sporangium-wall as a protection to 

 the included gametophyte and embryo, the whole finally 

 becoming detached from the sporophyte, is the origin 

 of the seed of the higher plants, which is therefore only 

 a further development of the macrosporangium of the 

 heterosporous Pteridophytes. 



In the seed plants, or flowering plants, the reduction 

 of the gametophyte reaches its extreme, but there is no 

 absolute break between Pteridophytes and Spermato- 

 phytes. The retention of the germinating macrospore 

 within the sporangium has necessitated a different 

 method of fertilization, hence the development of the 

 pollen-tube. The lower Spermatophytes, especially the 

 Cycads, while developing a pollen-tube from the ger- 

 minating microspore, nevertheless produce spermato- 



