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 which 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 Gymnosperms 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, 
grow's in wet or swamp}^ 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 usually placed in the 
classes and orders of the Homosporous Pteridophytes, but since 
they represent a very important advance in the plant kingdom. 
