68 IOWA ACADEMY OP SCIENCE 



ing the pistillate flowers at the lower end of the spike. Also the pro- 

 duction of seeds at the lower, rather than at the upper end of the spike, 

 would favor both their nutrition and protection. There are, thus, ap- 

 parent advantages in the separation of the flowers of Myriophyllum 

 spicaimn, this monoecism doubtless favoring both cross pollination and 

 seed production. Finally the occasional separation of the flowers on 

 different plants would suggest a probable tendency toward dioeeism in 

 this form, in which case all the advantages of monoecism would be con- 

 served, w4th the added advantage that self-pollination could never occur. 



Explanation of Plates I-IV. 



All figures were made with Spencer camera lucida with the exception 

 of the floral diagram in Plate II. Original drawings were reduced one- 

 half in reproduction. The figures of Plates 1, 3, and 4 were made with • 

 Spencer 16 mm. objective and 8 ocular. Figures 2 and 3 were made 

 with Bausch and Lomb li/o inch objective and 4 ocular. Figure 4 was 

 made with Bausch and Lomb 4 inch objective and 4 ocular. The orig- 

 inal magnifications in diameters were approximately as follows: Fig. 1, 

 160; Figs. 3 and 4, 90; Fig. 5, 50; Figs. 6-22. 160. 



The abbreviations employed in describing figures are as follows: a, 

 air chamber; b, large bract; bb, lateral bract; f, flower; is, stamen of 

 inner whorl ; o s, stamen of outer whorl ; o, ovule ; pe, petal ; pi. pistil ; 

 s, stamen; se, sepal; st, stigma. 



