(Bull. U. S. F. C, 1 901 . (To face page 72.) 
Plate 13. 
3. POTAMOGETON ZOSTER/EFO LI US, CROSS SECTION OF 
CENTRAL CYLINDER. 
5. CERATOPHYLLUM DEMERSUM, CROSS SECTION 
OF THE CENTRAL CYLINOER 
6. CERATOPHYLLUM DEMERSUM, CROSS SECTION OF A STEM. 
