THE PTERIDOPHYTES 441 
- showing meshes that are surrounded by polygonal ridges with — 
straight sides. In nature, these spores, falling out of the ripe 
spore cases, germinate on moist soil, each producing a fleshy 
prothallus which is submerged in its lower portion and exposed 
above ground in its upper part. This structure has antheridia 
. Fic. 331.—Selaginella Martensii. a, vegetative branch; 6, portion of the stem, 
bearing cones (x); c, longitudinal section of a cone, showing microsporangia (mic. 
sp.) in the axils of microsporophylls, and megasporangia in the axils of megasporo- 
phylls; ¢, microsporangium with microsporophyll; ¢, microspores; f, portion of wall 
of sporangium, greatly magnified; g, megaspore; h, microsporangium opened, and 
most of the microspores scattered; i, megasporangium, with megasporophyll; , 
same, opened, showing the four megaspores. (Gager.) : 
and archegonia imbedded in its upper end and shows elongations 
of its epidermal cells as rhizoids in its tuberous, sunken, lower part 
which absorb nutriment from the soil. The mature antheridia 
burst open during wet weather and liberate biciliate antherozoids. 
Each mature archegonium produces an ovum in its basal cavity. 
Antherozoids (sperms) swim to the mouth of the. archegonium, 
pass down its canal and gather about the ovum. 
