238 



PLANT MORPHOLOGY 



only one prothaUial cell and one antheridium, the latter producing many 

 small bic-iliate sperms. The female gametophyte, with relatively exten- 

 sive vegetative tissue, develops largely within the megaspore, the protrud- 

 ing portion forming several archegonia. These are of an advanced type 



Fig. 195. Development of the embryo of Selaginella martensii. A, first division of ferti- 

 lized egg into suspensor and embryonal cells; B, embryonal cell divided into three cells; C, 

 differentiation of stem and leaf primordia; D, later stage, the stem and leaf primordia 

 developing by an apical cell; E, later stage, showing differentiation of foot and root; F, 

 older embryo; sus, suspensor; s, stem tip, If, leaf; I, ligiile; r, root tip; /, foot. (After 

 Pfeffer.) 



with only one neck canal cell. The embryo has a suspensor. Selaginella 

 is related to Lycopodium, differing from it chiefly in being heterosporous. 



3. Lepidodendrales 



The Lepidodendrales are a Paleozoic group of lycopods. They ranged 

 from the Devonian through the Permian but made their greatest display 

 during the Upper Carboniferous, when they were one of the dominant 

 plant groups. They were chiefly tree-like and often reached a height of 

 30 m. or more. The two most prominent genera were Leyidodendron and 



