illumination, though no exact investigations were undertaken to demonstrate the influence of light in this matter. The prevailing form, however, while still unistratose, is spatulate, though flabelliform and linear outlines are not uncommon and grotesquely lobed and branched stages are often met with. Linear or ribbon-shaped forms, like that shown in f. 36, are especially common when the young plants are from the first well covered with water. Just below the chief growing point c, in the stage illustrated by f. 34, there are cell divisions in the plane of the paper, which possibly foreshadow the axis or stem, though they may have arisen in this case through changed relations to the light. We have not yet been able to follow the development much beyond this point, but we have grounds for believing that the subsequent history is essentially as described by Goebel (l. c.) for young plants of A. Battandieri with the exception that the wing is probably more lobed than in that species. In most cases, the development is confined to practically a single plane, though occasionally, as in the case represented by f. 35, the wing-lobes show an irregularly spiral arrangement, due perhaps to changed positions in regard to the light.
Explanation of Plates
[The drawings have been prepared by M. A. Howe, chiefly with the aid of a camera lucida.]
Plate 11. Riella Americana Howe & Underw.
1 and 4. ♂ gametophytes, natural size.
2 and 3. ♀ gametophytes, natural size.
5. Terminal portion of a branch of the ♂ gametophyte, with a young branch at the apex, × 11.
6. Terminal portion of a branch of the ♀ gametophyte, × 11.
7. An involucre, with mature sporogonium, × 16.
8. A scale, × 55.
9. Portion of axis near the apex, showing gemmae and scales, × 40. The gemmae are shown at a and below; the scales are, in this case, nearer the wing.
10–18. Stages in the development of the gemmae. See text, pp. 219 and 220.
10. A trichome destined to become a gemma, in distal (outer) aspect, × 193.
11. A similar stage in lateral view, × 193.
12–15. Later stages; 12, 13, and 15 in distal aspect, 14 in proximal, × 193; b, point of attachment.
16. A more advanced stage, inverted as regards its original relations to the axis and as regards the preceding figures, × 193.
17. Outline of a gemma in a later stage, × 55.
18. A young gametophyte derived from a gemma, × 28.
