March, 1906.] Life Cycle of a Homosporous Pteridophyte. 48 S 



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 sporophvte 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 open 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 vounger 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 lycopods 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 flowers — 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 cavity. 

 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 .v 

 number. The x number of chromosomes is continued through 

 the entire subsequent history of the following gametophyte 

 generation. The sporangia are stalked and are provided with a 



