GROUPS OF PLANTS. 



59 



ment from the prothallus (Fig. 35, pr) . This latter organ is, how- 

 ever, only a temporary provision, for as soon as the root grows 

 out and penetrates the soil, it dies off and the sporophyte thus 

 becomes independent. The stems are frequently more or less 

 condensed and lie prostrate in the soil, developing foots from the 

 under surface and leaves from the sides and upper surfaces. The 

 leaves which constitute the conspicuous part of the ordinary ferns 

 consist of a stalk and lamina or blade on which are borne the spor- 

 angia (Figs. 277; 36, A). The sporangia usually occur on the 

 under surface of the leaf in groups or clusters known as sori 

 (Fig. 36, ^). The sori are of characteristic shape and in certain 



Fig. 35. The brake fern (Pierts). A, differentiation of cells in germinating oospores; 

 B, later stage showing development of embryo: pr, prothallus; f, foot embedded in the 

 archegonium (aw); w, root; s, young stem; b, young leaf. A, after Kienitz Gerloff; B, 

 after Hofmeister. 



species are covered by a plate called the indusium (Fig. 36, 

 B) which rises from the epidermis. In some species the entire 

 leaf becomes a spore-bearing organ, and is then known as a 

 SPOROPHYLL (Figs. 36, 37, 38), to distinguish it from the foliage 

 leaves. The sporangia develop a row of cells around the margin 

 constituting what is known as the annulus (Fig. 36, n). The 

 form of the annulus determines the manner of dehiscence of the 

 sporangia, which occurs on drying. The spores are ejected with 

 considerable force (Fig. 36, D). They (Fig. 36, E; Fig. 39) 

 are either bilateral or tetrahedral and require a short period to 

 elapse before they germinate. They retain their vitality for a long 

 time except those which are green, i.e., contain chlorophyll. The 



