322 
PLANT STUDIES 
and archegonia appear, so that it is evidently a gameto- 
phyte. This gametophyte escapes ordinary attention, as it 
is usually very small, and lies prostrate upon the substra- 
tum. It has received the name prothallium or prothallus, 
so that when the term prothallium is used the gametophyte 
of Pteridophytes is generally referred to ; j ust as when the 
term sporogonium is used the sporophyte of the Bryophytes 
is referred to. Within an archegonium borne upon this little 
prothallium an oospore is formed. When the oospore ger- 
FIG. 293. Prothallium of a common fern (Aspidmm): A, ventral surface, showing 
rhizoids (rh), antheridia (an), and archegonia (ar) ; B, ventral surface of an older 
gametophyte, showing rhizoids (rh) and young sporophyte with root (w) and leaf 
(b). After SCHENCK. 
minates it develops the large leafy plant ordinarily spoken 
of as "the fern," with its subterranean stem, from which 
roots descend, and from which large branching leaves rise 
above the surface of the ground (Fig. 293, B). It is in 
this complex body that the vascular system appears. No 
sex organs are developed upon it, but the leaves bear numer- 
ous sporangia full of asexual spores. This complex vascular 
plant, therefore, is a sporophyte, and corresponds in this 
life history to the sporogonium of the Bryophytes. This 
