252 ELEMENTS OF BIOLOGY 



some features at least of the broad outlines of Plant Biology. In 

 previous pages emphasis has been placed on the distinctive char- 

 acters of the metabolism of plants and animals and on some of the 

 features of organization that are common to the two groups. 

 Metagenesis constitutes another character that both exhibit to a 

 greater or lesser extent. Three of the four phyla of plants, the 

 Bryophyta or mosses, the Pteridophyta or ferns, and the Spermato- 

 phyta or flowering plants, pass through sexual and asexual genera- 

 tions alternately in their life cycles. Only the Thallophyta, the 

 single-cell and exceedingly simple plants, have single life histories, 

 reproduction in these forms being either sexual, asexual, or both, 

 without regular sequence. So before examining the phenomena of 

 metagenesis in animals we may with profit familiarize ourselves 

 with the chief features of plant-life cycles. 



Life Cycle of Moss. As indicated above, in the Bryophyta, the 

 mosses and their allies, the life cycle comprises two phases. The 

 green leafy moss plant with which we are all familiar is a sexual 

 generation, that is, produces gametes or sex cells (Fig. 173). In the 

 tip of such a green plant is found a vase-shaped structure, the 

 ARCHEGONiuM, in which is the ovum. In nearby tips are sperm- 

 producing organs, the antheridia, club-shaped structures that pro- 

 duce considerable numbers of sperms. In moisture, that is dew or 

 rain, the active sperms swim down the neck of the vase-shaped 

 archigonium; there fertilization takes place. The fertilized egg re- 

 mains in its original position and develops there, growing into a 

 long spike, which contains no chlorophyll but draws its subsistence 

 from the green plant in the fashion of a parasite. This is a new 

 asexual generation of the moss. A spindle-shaped sporangium, in 

 which great numbers of single cells termed spores are formed, 

 develops on the upper end of this spike. When the sporangium 

 ripens the spores are released and, falling on the ground, develop 

 as independent green moss plants that produce only ova and sperm, 

 never spores. Spores, then, are single cells capable of developing 



