PTERIDOPHYTA 



237 



archegonia generally do not appear until the megaspore is shed. In some 

 species the megaspore is retained until fertilization has occurred, or even 

 until the embryo has appeared. Here the male gametophytes are carried 

 to the megasporophylls by wind or gravity and there they liberate their 

 sperms. In such cases it is apparent that a condition closely approaching 

 seed formation is reached. 



Embryo. The embryo of SelagineUa resembles that of Lycopodium in a 

 general way, but shows certain important differences and some variation 





A 



Fig. 194. Female gametophyte of SelagineUa apoda. A, section of megaspore containing 

 young gametophyte in free-nuclear stage; B, section of megaspore with mature gameto- 

 phyte, consisting of a large nutritive cell and small-celled tissue in which an archegonium 

 has developed. {After Lyon.) 



among the different species (Fig. 195). Commonly the first division of 

 the fertilized egg, which is transverse, separates an outer suspensor cell 

 from an inner embryonal cell, but the suspensor usually becomes more 

 highly developed than in Lycopodium. No quadrant stage is formed. 

 Instead the embryonal cell produces three cells — a terminal one, which 

 forms the stem, and two lateral ones, each of which gives rise to a leaf. 

 One of the leaf segments produces the foot and later the primary root. In 

 some species the foot and root, as well as the suspensor, are derived from 

 the upper cell that arises from the first division of the fertilized egg. 



Summary. The stem of SelagineUa is elongated and branched, the 

 leaves numerous and small. Each leaf bears a ligule. The vascular sys- 

 tem of the stem is typically an exarch protostele with amphicribral organ- 

 ization ; sometimes more than one stele is present. There is no secondaiy 

 thickening. All species have a definite strobilus and are heterosporous. 

 The microsporangia produce many microspores, the megasporangia four 

 megaspores. Dehiscence takes place by means of a vertical slit. The 

 male gametophyte, developed entirely inside the microspore, consists of 



