102 FOUNDATIONS OF BIOLOGY 



leaves which are of very simple construction compared with 

 those of the Flowering Plant we have studied. True roots are 

 not present, but their function is performed by filamentous 

 outgrowths called RHIZOIDS. At the top of the leafy moss 

 plant, inconspicuous reproductive organs are developed. 

 Some species bear both antheridia and archegonia on the 

 same plant, while others have only one type. The leafy moss 

 plant is thus a sexual individual, or gametophyte. When the 

 reproductive organs are mature, sperm escape from the 

 antheridia and, swimming about in moisture which has col- 

 lected on the leaves, are attracted to the archegonia contain- 

 ing the eggs, apparently by a chemical substance secreted 

 within these organs. A single sperm which has made its way 

 down into an archegonium, fuses with the egg to form a 

 zygote. The fertilized egg retains its position in the arche- 

 gonium and germinates. The result is a rod-shaped embryo 

 which grows not only upward through the archegonium and 

 so out into the world, but also downward into the tissues of 

 the gametophyte, from which it secures practically all of its 

 food materials. 



The essentially parasitic nature of the new individual 

 renders the development of leaves superfluous, so it consists 

 of a simple upright stalk at the top of which reproductive 

 organs are borne. These are sporangia and accordingly the 

 individual is a sporophyte. The ripe spores are liberated and, 

 falling to the ground, each forms a filamentous outgrowth 

 called a PROTONEMA. Soon a bud arises on the protonema 

 which develops into a leafy moss plant. 



A common Moss thus exhibits in its life history an alterna- 

 tion of sexual and non-sexual generations. The leafy moss 

 plant, with antheridia and archegonia, produces gametes 

 and is the gametophyte. The leafless generation, which 

 develops from the fertilized egg in the archegonium, produces 



