EXPLANATION OF FIGURES. 



Plate I. 



Fig. I. Stem-apex of Naias graminea Del., showing the arrangement of 

 the appendicular oY%3in?,—ff ',//",//'", fertile leaves; yir, sterile leaf; //', 

 fl", floral rudiments;/',/", lateral branches (after Schumann). 



Fig. 2. This and all the succeeding figures are drawn from the author's 

 own preparations of A^. flexilis Willd. Fig. 2 is a cross-section of the 

 terminal bud, X about 70. The older leaves are numbered; x, x\ the apices 

 of the main axis and oldest branch, sq.^ squamulae intra vaginales; $, $ ', ?", 

 young female flowers. 



Fig. 3. Longitudinal section of the stem-apex, X about 200; y, stem-apex; 

 k, young primordium, subtended by the leaf, /; ? , young female flower. 



Fig. 4. Longitudinal section of an abnormal stem-apex, where the prim- 

 ordium had developed directly into a lateral sterile branch, v'; X about 

 400. 



Fig. 5. Longitudinal section of a normal stem-apex, showing the youngest 

 pair of leaves, /', /-, and the primordium, k, subtended by the latter; X 400. 



Fig. 6. Cross-section of a young internode of the stem, showing the 

 arrangement of the tissues and the intercellular spaces, i; X about 300. 



Fig. 7. Longitudinal section of a young internode, showing the central 

 annular vessel of the vascular bundle; X 400. 



Figs. 8-10. Early stages in the development of the male flower seen in 

 longitudinal section, X about 400; in, inner envelope (integument), p, outer 

 envelope. 



Fig. II. Section of a young primordium divided into the floral rudiment, 

 /, and the lateral branch, v. The subtending leaf is seen below. 



Fig. 12. Longitudinal section of male flower after the differentiation of 

 the archesporium; X 200. The archesporial cells are the nucleated ones. 



Fig. 13. Cross-section of an older anther. 



Fig. 14. Longitudinal section of a male flower shortly before the separa- 

 tion of the sporogenous cells; X 70. 



Fig. 15. Upper part of the same more highly magnified. The enlarged 

 upper part of the integument, /«, is conspicuous. 



Fig. 16. Longitudinal section through the base of a slightly older flower, 

 showing a rudimentary partition in the lower part of the theca; X 200. 



Figs. 17-21. Successive stages in the first division of the spore-mother- 

 cell. Leitz, immersion jV. ocular 3; reduced by one-third. 



Fig. 22. Two pollen-tetrads; X about 400. 



Fig. 23. Young pollen-spore, with the primary antheridial cell; X about 800. 



Fig. 24. A slightly older stage. 



Fig. 25. A nearly mature spore, with the large vegetative nucleus, and the 

 two small generative cells. The spore is nearly filled with starch granules; 

 X about 400. 



Fig. 26. A ripe pollen-spore; X 300. 



