PTERIDOPHYTA 



255 



consists of a rounded, cushion-like base of colorless tissue that gives rise to 

 numerous upright lobes of green tissue (Fig. 213A). These are irregular, 

 thin, and plate-like. Rhizoids are abundantly produced on the lower 

 surface. The base has a marginal meristem. Both kinds of sex organs 



Fig. 212. Early development of the sporangium of Equisetum arvense. A, longitudinal 

 section of young sporangiophore with two sporangium initials, X200; B, longitudinal 

 section of young .strobilus, X12; C, a sporangiophore of same, showing first division of 

 initials, X200; D, longitudinal section of one-half of sporangiophore, showing early dif- 

 ferentiation of sporogenous tissue, X200; E, longitudinal .section of slightly older strobilus 

 than shown above, X8; F, a sporangium of same, showing further development of sporog- 

 enous tissue and differentiation of tapetum, X 200. 



are borne on the basal portion of the same prothallium but are seldom 

 present at the same time. The archegonia usually appear before the 

 antheridia. When closely crowded, the prothallia are small and bear 

 only antheridia. For this reason, they were once erroneously regarded 

 as dioecious. The development of the antheridium resembles that of the 

 lycopods (Fig. 21ZB-E). The superficial initial divides by a transverse 

 wall, the outer segment producing the sterile jacket and the inner one the 

 spermatogenous tissue. The sperms are large, coiled, and multiciliate. 



