176 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



hidden by leaves, a cluster of flask-shaped structures called archegonia, 

 at the bottom of each of which is a single rather large egg cell. 



Fertilization of the egg can take place only when the antheridia and 

 archegonia are wet from rain or dew. In such an event the sperm 

 cells ooze out in a mucilaginous substance secreted from the walls of 

 the antheridium and pass in drops of water to the necks of the 

 flask-shaped archegonia. Here they are chemically attracted by a 

 substance exuded from the inside of the archegonium and swim down 

 the tubular neck until one meets the egg cell, when fertilization takes 

 place. The gametophytic phase of the moss is the haploid stage of the 

 chromosomes, fertilization of the egg restoring the diploid number 

 characteristic of the sporophyte. This generation begins with the 

 cell division which follows the fertilization of the egg in the archego- 

 nium and results in the growth of a tiny stalk, bearing at its upper end 

 a capsule, that in the adult sporophyte is filled with asexual spores. 

 During the formation of the spores within the capsule, the formative 

 tissues produce a number of large, rounded spore mother cells, from 

 each of which by nuclear divisions tetrads, or groups of four spores, 

 are formed. During this tetrad formation, a reduction division 

 takes place so that the spores contain only the haploid number of 

 chromosomes. 



The moss capsule is quite a complex structure with a cap, or oper- 

 culum, that covers an urn-shaped affair bearing at its upper end a 

 circle of teethlike structures collectively called the peristome. As the 

 sporophyte ripens it dries up and the numerous ripe spores are scat- 

 tered by the action of the peristome teeth, the latter being very 

 hygroscopic, or sensitive to moisture. When the weather is humid or 

 wet, the teeth of the peristome curl up and when dry they straighten 

 out, thus expelling the spores, which may then be scattered by the 

 wind. The germinating spore does not grow directly into a leafy 

 plant, but first forms a protonema or algalike filament from which 

 upright stalks later arise, while rhizoids grow downwards from it, 

 thus forming again the moss plant. This life cycle with its alterna- 

 tion of gametophytic and sporophytic stages is characteristic of the 

 life cycle of mosses and liverworts, as well as the higher group of the 

 ferns (Filicinae). 



In the flowering plants (Angiospermae), one finds an almost com- 

 plete suppression of the gametophytic generation, the sex cells or 

 gametes being produced in modified leaflike parts of the flower. The 

 floral parts — sepals, petals, stamens, and carpels — are thought of 



