EXPLANATION OF THE FIGURES. 481 



TIG. 



2. Similar section of the apex of a root in which forking has commenced a 



short time previously, X 400. 



3. Half of a forked root in longitudinal section, x 100. 



4. Transverse section of a root-tip, x 400. 



5. Transverse section of the forking tip of a root, X 400. 



6. Fragment of the lower part of the woody mass of an older plant cut through 



longitudinally in the direction of its larger transverse axis (part of the 

 preparation figured in PI. XLIX, fig. 1), x 200. 



7. Young frond of an older plant seen from the front, x 300. 



8. Similar view of a somewhat more developed frond, x 300. 



PLATE LIU. 

 Pigs. 1 — 19. Isoetes lacustris. 



1. Longitudinal section of a young fertile frond, x 300. 



2, 3. The lower portion of the front surface of a somewhat more developed 



fertile frond in longitudinal section, x 300. 



4. The lower end of a young fruit cut through longitudinally (according to its 



position it is destined to produce microspores), x 300. 

 ' 5. Longitudinal section of the base of a frond, which bears a half- ripe micro- 

 sporangium, x 20. 



6, 7. Sets of four adherent spore-mother-cells, x 400. 



8 — 17. Stages of development of the mother-cell of microspores. 



8. The mother-cell filled with homogeneous granular protoplasm, in which the 



central nucleus floats freely, X 400. 



9. Accumulations of more firm mucilage — the rudiments of the second nu- 



clei are forming at the two poles of the globular nucleus, x 500. 



10. After the absorption of the primary nucleus and the formation of the 



secondary ones the primordial utricle of the mother-cell becomes con- 

 tracted, preparatory to its division, x 400. 



11. The secondary nuclei are in the act of dissolution, before the division of 



the cavity of the mother-cell, x 400. 



12. A mother-cell divided into two halves by a septum through its equator, 



X 400. 



13. 14. Mother-cells, each with four free tertiary nuclei ; those in fig. 13 are in 



one plane, those in fig. 14 at the angles of a tetrahedron, x 400. 



15. Pour special-mother-cells united by the mother-cell; produced by the re- 



peated division of the two halves of the mother-cell (the special mother- 

 cells of the first degree). The septa dividing the latter have different 

 inclinations, X 400. 



16. A similar preparation. The four special-mother-cells are lying in the same 



plane, X 400. 



17. Set of special-mother-cells arranged decussately in the last stage of for- 



mation, immediately before their individualisation. The innermost 

 layer of the much thickened cell-membrane of the special mother-cells is 

 far more highly refractive than the already swollen outer layers. Treat- 

 ment with tincture of iodine has contracted the primordial utricles of 

 the cells. X 400. 



31 



