PHYLOGENY 



from drying. The plants are always oogamous, that is, the egg is a large, 

 nonmotile cell which must be sought by the sperm. The sex organs of 

 both sexes include a jacket layer of sterile, protective cells. All of the 

 aerial parts of the plant are covered by a waxy cuticle which protects the 

 plant against drying. Finally, alternation of generations is well developed, 

 with the sporophyte being substantially a parasite upon the gametophyte 

 in the bryophytes, for the former is always attached to the latter, and it 

 contains inconsequential amounts of chlorophyll, if any. 



Most of these characters, other than retention of the embryo in the 

 maternal sex organs, are already known in the algae, although no alga 

 has all of them in combination. Other bryophyte characters tie these 

 plants in with the algae more closely. Like the algae, the bryophyte body 

 is a thallus, a rather simple cell mass which is not differentiated into roots, 

 stems, and leaves. Structures which resemble these parts are present, but 

 the same may be said of many of the algae. The gametophyte is larger 

 than the sporophyte, as in most algae, and photosynthesis is largely re- 

 stricted to the gametophyte. 



Life Cycle of a Moss. A typical moss may serve as an example ( Figure 

 36). If a spore (the agent of dispersal) falls on suitable ground, it germi- 

 nates to form a network of threads not unlike a filamentous alga and is 



o 



called the protonema. The protonema sends fine, root-like rhizoids into 

 the substrate for attachment, and also for procurement of water and salts. 

 Buds on the protonema develop into shoots bearing leaf-like structures. 

 At the tips of these shoots, the sex organs, antheridia in the male or arche- 



FiGUKE 36. The Life Cycle of a Moss. 1. Mature gametophyte with a sporophyte (2) 

 growing upon it and discharging its spores. 3. The eap of tlie spore capstde. 4. l^ipe 

 spores. 5. terminating spores. 6. ^'oung ganietopliyte with protonema. 7. Portions of 

 male and lenialc gametophytes showing antlieridia and archegonia. 8. Antlieridinm (hs- 

 charging sjierm (9). 10. Areliegonium willi egg. 11, 12, and 13. Developmental stages 

 of the sporopliyte. (From Pauli, "The VVorkl of Life," Houghton Mifflin Co., 1949.) 



112 



