The histological details of the origin of the sporangium, and the earliest differentiation of its cells, we have not been able to make out in the very limited material in our possession. A tangential section of one of the earliest outgrowths shows a central cell which may be the archesporium separated by five surrounding cells from the limiting dermatogen. At a later stage, the young sporangium consists of a single layered investing membrane, a central mass of archesporial cells, and three or four layers of more elongated, and less granular, cells, filling up the space between the two. As the sporangium grows the limiting layer becomes thicker and yellow. The cells lining it become still more elongated tangentially, and the archesporial cells appear large, square, and granular, with extremely large spherical nuclei. Subsequently, the layer surrounding the archesporial cells (tapetum) breaks up, its cells undergo a mucilaginous change, and finally disappear. The archesporial cells divide crucially, each into four mother-cells, and the spore mother-cells divide by formation of transverse walls into two cells, each of which divides again. The division in the second case also takes place in a cruciate manner, and the arrangement of the developing spores is not tetrahedral as in Lycopodium. At a later stage the groups of developing spores have the form of spheres divided into quadrants: these when examined in spirit are seen to possess a wall which on slight pressure or drying comes away in flakes. On the addition of water this wall swells up, separating the segments of the sphere from one another, and surrounding the whole group with a mucilaginous envelope. The developing spores elongate and become narrower in proportion, while each develops its own wall; but up to a late stage the groups of four remain intact. The ripe spore is triangular in section, with two straight sides and an outer convex one, like a segment of an orange; the nucleus lies in the middle of the outer face, and after preservation in spirit the protoplasm is mainly aggregated round it, but remains connected by strings with the sides and extremities. In conclusion we wish to express our thanks to friends in New Zealand for valuable help in collection of material, especially to Mr. G. M. Thompson, F.L.S., and Mr. D. Petrie, M.A., of Dunedin. EXPLANATION OF PLATES I. TO V. Fig. 1. The plant in position. PLATE I. 2. Part of the "rhizome" isolated. The specimen is more than usually branched. 3. Part of the "stem," showing the similar decurrent ridges below the leaf and the sporangiferous shoot. 4. The sporangiferous shoot (a) From the front. (b) From the side. (c) In longitudinal section. (d) With ripe sporangium burst. EXPLANATION OF PLATES-continued. PLATE II. Fig. 5. Transverse section of the "rhizome." 6. Transverse section of its fibro-vascular bundle. x 25. × 120. 7. Cells of the ground tissue, with part of the brown sheath, in transverse section. One of them shows the pitted end wall. × 50. 8. Longitudinal section of the "rhizome." × 50. 9. Cell of the ground tissue in longitudinal section, showing the coil of hyphæ round the protoplasm, and their passage through the wall. × 120. 11. Transverse section of its fibro-vascular bundle. x 50. x 175. × 300. 14. Epidermal cell of the "stem," showing the peculiar thicken ing of the wall. × 200. 15. Stoma from the epidermis of the "stem." × 100. 16. The same in cross section. (a) Xylem. × 100. (a') "Primitive fibres" (Bertrand) in the axis of the bundle. (b) Phloem. (c) Phloem sheath. (d) Brown sheath. (e) Ground tissue. (ƒ) Epidermis. EXPLANATION OF PLATES-continued. PLATE III. Fig. 17. Longitudinal section through a growing point, showing scalariform tracheides developing from procambium. x 25. 18. Surface view of growing point, showing the apical cell. × 300. 19. Transverse section of the leaf. x 25. 20. Epidermis of the leaf with stoma. × 300. 21. Fibro-vascular bundle of the leaf in transverse section. × 300. 22. The same in longitudinal section. × 300. (m) Mesophyll. 23. Part of transverse section of the leaf, showing the ingrowth of the thickening ridges of the outer wall. x 175. 24, 25. Stoma. × 300. 26. Epidermal cell, seen with oblique light, and showing the thickening of the outer wall. × 300. PLATE IV. Fig. 27. Appearance of a tip of a stem which is developing sporangiferous shoots; natural size. 28. Apex of a stem just before development of sporangiferous shoots; natural size. 29. The same as 28, after removal of the surrounding leaves. × 30. 30. The same growing point, seen from above. × 30. 31, 32. Stages in development of the sporangiferous shoots. x 10. 33. Longitudinal section of one of the developing shoots. x 10. 34. Tangential section of developing sporangium at an early stage, showing a central cell which is perhaps the archespore. × 60. 35. Longitudinal section at a later stage, where the archesporial cells are distinctly marked off from the surrounding tapetal layers. × 60. 36. Longitudinal section at a later stage, after division of the sporangium into two chambers. x 25. R.I.A. PROC., SER. III., VOL. II. с EXPLANATION OF PLATES-continued. PLATE V. Fig. 37. Transverse section of a sporangium just before the disintegration of the tapetum. x 50. 38. Transverse section of a sporangium at a later stage, during the disintegration of the tapetum. 39. Disintegration of the tapetal cells. × 30. > 200. 40. Archesporial cells dividing into spore mother-cells. (a) Nucleus breaking up. (b) Nuclear division. x 200. (c) Division complete. (d) Pair of cells redividing into spore mother-cells. 41. Spore mother-cells divided into groups of four spores. × 200. (a) Groups of developing spores as seen when in spirit. (b) The same, showing flaking off the wall. (c) The same after addition of water, showing the relative position of the spores, separated by and enclosed in mucilage. (d) A similar group in optical section. (e) Groups of spores whose mucilaginous investment has more or less fused with that of surrounding groups. 42. Stages in development of the spore. |