61? - Heredity and Evolution 



ANTHERIDIUM- 



/ARCHEGONIUM 



Fig. 31-19. Psilotum, one of three surviving kinds of psilops'dan plants. Such 

 specimens, which are only 2 to 3 feet tall, are sometimes exhibited by botanical 

 gardens as "living fossils." Note that the sporophyte (left): (1) has a horizontal 

 underground stem without any roots; (2) has very small scalelike "leaves"; (3) dis- 

 plays a primitive dichotomous manner of branching; and (4) bears a number of 

 three-lobed sporangia in the axils of some of the leaves. The gametophyte (right) 

 is a semimicroscopic thallus that possesses both archegonia and antheridia. (From 

 The Plant World.) 



nize the land almost as soon as the Bryo- 

 phyta. They are, in fact, the most rudi- 

 mentary of all Tracheophyta. The vascular 

 tissues are very primitive, but the dominance 

 of the sporophyte generation is unmistak- 

 able. Typically there is a horizontal under- 

 ground stem (called a rhizome) from which 

 a number of repeatedly forking cylindrical 

 green branches extend up vertically into the 

 air. True roots are lacking and "leaves," 

 when present, take the form of very small 



"scales" (Fig. 31-19). None of the species ap- 

 pears to have achieved a height of more than 

 about two feet. Probably the Psilopsida arose 

 from a green algal stock. This is indicated by 

 the forking form of the body, the lowly de- 

 gree of cellular differentiation, and the pro- 

 duction of flagellated sperm, which must 

 swim through the water to reach the eggs. 

 The Lycopsida (Subphylum 2: Phylum Tracheo- 

 phyta). This primitive group of tracheophvtes 

 also reached a climax in earlier times. In the 



