March, 1906.] Life Cycle of a Homosyorous Pteridophyte. 485 
There is considerable similarity in the life cycles of the three 
classes. The general account given below of the life history of a 
leptosporangiate fern will hold good for any of our common 
species of Adiantum, Asplenium, or Dryopteris, but other groups 
may show important differences in details. In no subkingdom 
is the antithetic alternation of generations more clearly marked 
and each generation lives independently for a part of its life. 
The sporophyte or nonsexual generation is the conspicuous plant 
although the gametophyte is usually of some size and easily 
distinguishable except in the Ophioglossales and some Lyco- 
podiales where it is entirely subterranean. 
The sporophyte of our common ferns has a horizontal rhizome 
and compound leaves which commonly form a rosette above 
ground. The stem consists of a general ground tissue contain- 
ing closed concentric fibro-vascular bundles. The stem and root 
tips have definite apical cells. In the Ophioglossales the 
bundles are Ofjen and arranged as in the higher plants, form- 
ing a ring of wood and central pith. There is also a definite 
cambium layer outside of the xylem cylinder. 
The younger leaves of the ferns are sterile but later rosettes 
of spore-bearing leaves are produced. The rosette of sporophylls 
corresponds to the fertile or spore-bearing parts of a flower in 
the higher plants. In some of the ly copods there are also simple 
zones of spore-bearing leaves alternating with the zones of 
sterile foliage leaves, the growth of the stem not being stopped 
when the sporophylls are developed. But in other lycopods and 
in the horsetails the sporophylls are arranged in closely crowded 
cones which terminate the branches, their growth in length 
being permanently checked. In these groups, therefore, we 
have true primitive flowens — modified and specialized spore- 
bearing shoots. The three essentials of a flower are ( 1 ) a 
stopping of the growth of the floral axis, ( 2 ) a shortening of the 
floral axis and consequent crowding of the floral organs, and ( 3 ) 
a modification of the spore-bearing leaves into specialized 
sporophylls. 
The sporangia are produced in clusters called sori, often verv 
numerous. Each sporangium produces a number of cells which 
become free and more or less spherical in the sporangial cavitv. 
These cells are called sporocytes. Each sporocyte divides twice, 
producing a tetrad of cells. These four cells finally separate and 
give rise to four nonsexual spores. During the first division in 
the formation of the spore-tetrad the number of chromosomes 
in the nucleus is reduced one-half, or from a 2x number to an .r 
number. The v number of chromosomes is continued through 
the entire subsequent history of the following gametophyte 
generation. The sporangia are stalked and are provided with a 
