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PLANT STRUCTURES 



and archegonia appear, so that it is evidently a gameto- 

 pliyte. 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. 111. Prothallmm of a common fern (Aspidium): 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 SCUENCK. 



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. Ill, 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 



