REPRODUCTION AND DEVELOPMENT 255 



Life Cycle of Fern. Alteration of generations, or metagenesis, 

 is modified in the ferns, the phylum Pteridophyta (Fig. 174). The 

 large fern plant with which we are familiar is a sporophyte, a 

 spore-bearing generation. If one examines the leaves of a fern at 

 certain seasons, some of them will be found to bear on their under 

 surfaces crescentic shaped brown bodies, known as sori. In these 

 are the sporangia. When mature the capsules of the sporangia open 

 and release spores. In the ground these spores develop into small, 

 heart-shaped, flat green plants known as prothalli. In a prothallus 

 there develop both archegonia and antheridia, the sperm from the 

 antheridia fertilizing the ova in the archegonia. Development of the 

 fertilized eggs occurs in the archegonia and results in the large 

 green fern, the sporophyte generation. Thus in the Pteridophyta 

 the GAMETOPHYTE generation has been reduced to an obscure struc- 

 ture, but is not done away with completely; asexual generation is 

 necessary in the life cycle. 



Life Cycle of Flowering Plant. In the spermatophytes, the 

 flowering plants, the gametophyte generation has been still further 

 reduced (Fig. 175). The ordinary flowering plant is the sporophvte 

 generation. The flower is a complex of modified leaves, which 

 gives rise to two types of spores; one, the megaspore, develops in 

 the base, receptacle, of the flower and there gives rise to a 

 gametophyte generation, which produces only an ovum. This 

 MEGAGAMETOPHYTE consists usually of not more than eight cells, one 

 of which is the egg cell. Rising from the centre of the flower is a 

 shaft, the pistil, consisting of a group of tubes that spread out at 

 the upper end to form the stigma. Each of these tubes leads into a 

 separate chamber containing a megagametophyte in the base of the 

 flower. Around the base of the pistil are attached a series of modi- 

 fied leaves known as the stamens. At the tip of each stamen is an 

 ANTHER, which is the origin of another type of spore, the micro- 

 spore, capable of producing only a microgametophyte generation 

 that develops only the male cells or sperms. The ripened micro- 



