30 PAL^EONTOLOGICAL BOTANY. 



four liygrometric club-shaped filaments called elaters (Figs. 20 



Fk. 13. 



Fi'4. 14. 



Fiir. 16. 



Fi?. 15. 



Fi-. 17. 



Fig. 13. Scalariform vessels taken from a Tree-fern. They are 

 marked with bars like the steps of a ladder, hence their name. The 

 membrane occasionally disappears, so that the walls are made up of 

 fibres only at some parts. 



Fig. 14. Sporangia of a Fern, supported on stalks, p, each of 

 which ends in an elastic cellular ring, s, partially surrounding the 

 spore-case, and opening it when mature. 



Fig. 15. Lycoiwdium clavatum, a common Club-moss. The leafy 

 branch, I, ends in a stalk bearing two spikes of fructification, /. 



Fig. 16. A kidney-shaped 2-valved case, containing small spores 

 (microspores) of Lycopodium. 



Fig. 17. Two-valved case, containing large spores (macrospores) of 

 Sela^inella. 



