EXPLANATION OF THE FIGURES. 477 



PLATE XLV. 



SALVINIA NATANS. 

 FIG. 



1. Longitudinal section of a large spore at the commencement of germination, 



X 30. 



2. A fragment of the exosporium of a microspore, in longitudinal section, 



X 300. Three layers are distinguishable. A thiii inner layer, a thicker 

 middle layer, and a very thick, apparently cellular, outer layer. 



3. A very young prothallium, detached, with a fragment of the inner spore- 



membrane adhering to it, x 200. 



4. The same, the longitudinal section passing through the entire spore, 



X 200. 



5. A more advanced prothallium, in which the first archegoniuin is developed; 



in longitudinal section, X 200. 

 6 — 7. Longitudinal section of unimpregnated archegonia, in which germinal 

 vesicles are visible, x 300. 



8. Transverse section of an unimpregnated archegonium with two terminal 



vesicles. The observer is looking into it from below, and in the middle 

 of the figure the interior of the mouth of the canal is seen. 



9. Fragment of a prothallium cut through longitudinally, in which archegonia 



are visible, one unimpregnated and the other just impregnated, X 300. 



10. Longitudinal section of the angle of a prothallium, exhibiting an archego- 



nium with an unusually long canal, X 300. 



11. Three-celled embryo, entirely filling the central cell of its archegonium. 



The position of the mouth of the canal is shown by two lines, x 200. 



12. An impregnated archegonium, with an abnormally large central cell, a small 



part of the latter only being filled by the few-celled embryo, X 200. 



13. An impregnated archegonium enclosing an eight-celled embryo, x 300. 



14 A 16-celled embryo detached, a from the outside, laterally, b in longitudinal 

 section, c seen from behind. 



15. A somewhat more developed embryo detached, a from above, b from the 



side, c also from the side turned round 90°. X 20. 



16, 17, 18. More advanced embryos ; figs. 16 and 18 entirelv, and fig. 17 half 



detached. Eigs. 16 and 18 are X 200, and fig. 17 is X 300. 



19. Embryo enclosed by the prothallium, x 300. 



20. Detached embryo seen from the front surface, x 200. 



21. Embryo enclosed by the prothallium. The first leaf is beginning to develope 



itself above. X 300. 



22. Spores with prothallium and embryo somewhat more advanced, in longitu- 



dinal section, x 50. 



23. A similar embryo, X 200. 



24. A similar embryo detached and seen from the front, surface of the first leaf, 



at the lower margin of which the end of the principal axis is clearly 

 visible, x 200. 

 25°. An embryo more advanced, seen half in front. Near the first leaf a the 

 principal axis b — which is already once forked and is in the act of 

 forking again — is visible ; behind it is the hinder end of the germ-plant c. 



