A GUIDE AND KEY TO AQUATIC PLANTS 



31 



13. Submerged leaves long and ribbon-like, at 14. 



least 3 mm. wide. 

 13. Submerged leaves not ribbon-like; often 17. 



thread-like but if wider than 3 mm., less 



than 2.5 cm. long. 



14. Leaves all borne from one point 16. 



14. Leaves scattered along the stem 15. 



15. Leaves without midribs evident when exam- Heteranthera, p. 106. 



ined against transmitted light; flowers 



yellow (fig. 12). 

 15. Leaves with midribs evident when examined Potamogeton, p. 69. 



against light; flowers inconspicuous, not 



yellow (fig. 13). 



Figure 12. 



Figure 13. 



16. Leaf, when examined with hand lends, showing a cen- Vallisneria, p. 75. 



tral dense and a peripheral less dense zone (fig. 14). 

 16. Leaf, when examined with hand lends, not showing AHsmaceae, p. 72. 



zones (fig. 15). 



Figure 14. 



Figure 15. 



I7. Leaves all from a hardened base or from 'a Isoetes, p. 68. 

 single point; base of leaves with spore-sacs 

 (fig. 16) 



17. Leaves borne along stem "l8. 



18. Leaves opposite or whorled 19. 



18. Leaves alternate or singly borne or crowded 24. 



19. Leaves opposite, but sometimes appearing 20. 

 whorled because of fascicles of leaves in the 

 axils of the opposite leaves. 

 19. Leaves whorled 23. 



