PRINCIPAL GROUPS OF PLAXTS. 89 



The oospore which is held in the venter of the archegonium is 

 not a resting spore, but germinates immediately and early differen- 

 tiates into the several organs ( Fig. 52 ) . These arise independently 

 and include a stem-bud (Fig. 52, s) ; a first leaf or cotyledon 

 (Fig. 52, b), so called because it does not arise out of the stem as 

 the later leaves do; a first or primary root (Fig. 52, w) ; and 

 a foot or haustorial organ (Fig. 52, /) whereby it obtains nutri- 

 ment from the prothallus (Fig. 52, 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 con- 



FiG. 52. The brake fern (,Pteris). 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. 



densed and lie prostrate in the soil, developing roots 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. 53 to 55). The sporangia usually occur on the 

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

 (Fig. 53, A). The sori are of characteristic shape and in certain 

 species are covered by a plate called the indlsium (Fig. 53, B) 

 which rises from the epidermis. In some species the entire leaf 

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

 PHYLL (Figs. 54, 55), to distinguish it from the foliage leaves. 

 The sporangia develop a row of cells around the margin consti- 



