EXPLANATION OF THE FIGURES. 485 



FIG. 



4. Mother-cell, whose primary nucleus is no longer visible ; the four newly- 



formed nuclei lie in one plane, X 300. 

 4*. The same specimen treated with watery tincture of iodine. The action of 

 the tincture has separated the four nuclei to some extent from one 

 another. 



5. Mother-cell which has already divided into four special-mother-cells, 



x 300. 



6. View from above of a half-ripe spore, X 400. 



7. Set of four special-mother-cells, each of which contains a similar half-ripe 



spore, X 50. 

 7 b . A similar special-mother-cel!, together with its enclosed spore, isolated by 

 gentle pressure with the covering glass, x 400. 



8. A young large spore with abnormally developed exosporium, x 400. 



9. Mother-cell of small spores, divided into four special-mother-cells, in each 



of which a spore has originated, x 300. 



10. Set of four special-mother-cells, x 400. 



11. Set of two vigorous special-mother-cells, and one abortive one, x 400. 



12. Cell with abnormally thick walls from a young capsule, X 300. 



Eigs. 13 — 17- Selaginella helvetica. 



13. Small spore, five months after sowing. In the internal cavity a large 



number of small globular cells has been formed, x 400. 



14. A similar spore subjected to gentle pressure. Some of the above cells have 



escaped. 



15. A similar spore two weeks later, lightly pressed. Each of the escaped 



cellules now exhibits a very delicate spiral spermatozon. 



16. Large spore shortly after sowing ; in longitudinal section, X 200. 



17. Prothallium seen from above, six weeks after sowing, x 300. 



Figs. 18 — 23. Selaginella Martensi. 



18. Inner membrane of a large spore just taken from the capsule, removed out 



of the exosporium, and viewed perpendicularly to the surface of the 

 rudiment of the prothallium which is attached to its inner side, x 400. 



19. Prothallium six months after sowing, in longitudinal section, x 200. 

 19*. One of the archegonia of this specimen. 



20. Young germ-plant isolated, and cut through longitudinally, x 200. 



20*. The spore, from the interior of which this germ-plant was taken. The 

 distended prothallium enclosing the somewhat developed embryo, pro- 

 jects far out of the fissures of the apex of the spore-membrane, x ]5. 



21. An unfolded germ-plant, drawn out of the spore, X 3. 



21*. The prothallium in which it originated, taken out from the exosporium, 

 X o. 



22. Germ-plant whose first leaves have been removed (the stipules of the latter 



are remaining), together with the prothallium freed from the outer spore- 

 membrane, X 30. 



23. A germinating spore, in whose prothallium (which projects from the outer 



spore-membrane) two germ-plants have originated, x 5. 



