170 PLANT STRUCTURES 



having the heterosporous-eusporangiate combination, which 

 is a feature of the Seed-plants. 



The embryo is also peculiar, and is so suggestive of the 

 embryo of the Monocotyledons (see § 114) among Seed- 

 plants that some regard it as possibly representing the 

 ancestral forms of that group of Spermatophytes. The 

 peculiarity lies in the fact that at one end of the axis of the 

 embryo is a root, and at the other the first leaf (cotyledon), 

 while the stem tip rises as a lateral outgrowth. This is 

 exactly the distinctive feature of the embryo of Monocoty- 

 ledons. 



The greatest obstacle in the way of associating these 

 quillworts with the Club-mosses is the fact that their sperms 

 are of the large and spirally coiled multiciliate type which 

 belongs to Filicales and Equisetales (Fig. 144), and not at 

 all the small biciliate type which characterizes the Club- 

 mosses (Fig. 140). To sum up, the short unbranched stem 

 with comparatively few large leaves, and the coiled multi- 

 ciliate sperm, suggest Filicales ; while the solitary spo- 

 rangia and the heterosporous-eusporangiate character sug- 

 gest Selaginella. 



