104 



FOUNDATIONS OF BIOLOGY 



a plate of chlorophyll-bearing cells with rhizoids attaching 

 it to the ground. On its lower surface are developed repro- 

 ductive organs, antheridia and archegonia, which form gam- 

 etes. The prothallus therefore is a gametophyte. (Fig. 54.) 



Sperm are liberated from the antheridia and swim in the 

 moisture from dew or rain to the archegonia. A single sperm 



FIG. 54. The life history of a common Fern, chiefly Aspidium. a, the entire 

 sporophyte, X A; b, portion of a leaf showing groups of sporangia (sori), X i; c, a 

 sorus showing sporangia, X 10; d, a sporangium, X 50; e, & single spore, X 100; 

 /, ventral view, X 3 and g, a median section, X 6 of a prothallus showing rhizoids, anthe- 

 ridia, and archegonia; h, antheridium liberating sperm, X 120; t, single sperm still at- 

 tached to a remnant of 'mother cell', X 300; j, open archegonium with sperm passing 

 down to egg, X 120; k, young sporophyte developing from zygote. (After Ganong.) 



works its way down an archegonium and fuses with the egg 

 to form a zygote. Then the zygote, which remains in the 

 archegonium, proceeds to divide and forms at first a small 

 plant, with stem and leaf which grows upward and root 

 which seeks the soil. During the process of root and shoot 

 development the plant retains its attachment to the parent 



