Jan., 1905.] Heterosporous Pteridophyte. 255 



THE LIFE CYCLE OF A HETEROSPOROUS PTERIDOPHYTE. 



John H Schaffner. 



The Heterosporous Pteridophytes represent the highest stage 

 of development in the second or intermediate series of plants. 

 The term heterosporous is applied to plants in which there are 

 two kinds of nonsexual spores, large and small, called respectively 

 megaspores and microspores. This peculiar spore condition is 

 also present in the seed plants. The megaspore always gives rise 

 to a female individual and the microspore to a male. In the 

 lower Pteridophytes there is only one kind of nonsexual spores 

 and they are, therefore, called homosporous. The Homosporous 

 Pteridophytes should be kept distinct from the Heterosporous, 

 since the development of heterospory represents one of the 

 advancing waves of evolution which brought about profound 

 changes in the character of the vegetation of the earth. The 

 elimination of chance environment in the development of uni- 

 sexual individuals by predetermining the sex in the spore and the 

 reduction in the size of the gametophytes appear to have been 

 necessary conditions in the evolution of the seed plants. 



The living species of Heterosporous Pteridophytes fall into six 

 genera, the remnants of former extensive and dominant groups. 

 The total number of species is about 635, somewhat more than 

 the living Gymnosperms wJiich constitute the next higher sub- 

 kingdom. Because of their position as the lowest of heterospor- 

 ous plants a thorough knowledge of their morphology and life 

 history is necessary to a proper understanding of the structures 

 and processes found in the G^'mnosperms and Angiosperms. 

 Nevertheless, much of their life history is still obscure and vari- 

 ous statements rest on conjecture or imperfectly established facts. 



The living Heterosporous Pteridophytes are small herbaceous 

 perennials or rarely annual plants, usually not more than a few 

 inches high. They are quite insignificant when compared with 

 their ancient relatives some of which developed into great trees. 

 The six living genera are named as follows: Azolla, Salvinia, 

 Marsilea, Pilularia, Isoetes, and Selaginella. Selaginella is, how- 

 ever, a very complex genus. Azolla and Salvinia are small float- 

 ing plants; Marsilea and Pilularia are creeping geophilous forms 

 usually in wet places; Isoetes, known popularly as quillwort, 

 grows in wet or- swampy ground and has a short, upright, usually 

 simple rhizome with grass-like leaves ; Selaginella has very small 

 leaves on branching herbaceous stems which grow either hori- 

 zontal or erect in wet ground or sometimes in dry places subject 

 to periodical moisture. These plants are usuallv placed in the 

 classes and orders of the Homosporous Pteridophytes, but since 

 they represent a very important advance in the plant kingdom. 



