66 IOWA ACADEMY OF SCIENCE 



filament is very short while the anther is long and introrse (Fig. 4). 

 The four microsporagia which develop within the anther are almost 

 radially symmetrical with respect to its axis. During the development 

 of the pollen the two sporangia of each half of the anther unite by the 

 breaking down of the sterile tissue between, forming two pollen-sacs. 

 At maturity the spores are shed by the longtitudinal dehiscence of the 

 fused sporangia. The petals are larger in the staminate than in the 

 pistillate flowers, necessitated by the greater length of the flower. They 

 also persist longer than the petals of the pistillate flowers, and remain 

 closed tightly over the stamens until the pollen matures (Fig. 4). 



Each staminate flower bears four rudimentary carpels. These vary 

 in size and development similarly to the staminodia of the pistillate 

 flower, but in the opposite direction, the best developed carpels being 

 found near the center of the spike next to the pistillate flowers and be- 

 coming more and more rudimentary toward the tip, where they may be 

 entirely suppressed. Two ovules arise in each (Fig 14) rudimentary 

 carpel. Competition sets in early, checking one, while the other 

 developes to about the spore mother cell stage. At this time the integu- 

 ments are partly formed and the ovule has turned at right angles in 

 the progress toward the anatropous condition. 



Summary and Discussion. 



We have seen that the spike of Myriophylhmi spicatum L. has stamin- 

 ate flowers aboA^e and pistillate below, with occasionally a few perfect 

 flowers in the center. Apparently, there is an abrupt shift from one 

 kind of flower to the other. This study, however, reveals a- gradual 

 transition in structure from one end of the spike to the other, rudiments 

 of the non-functional parts being present in every degree of reduction. 

 "We have thus a complete gradation from monosporangiate flowers of 

 both kinds, without a trace of the aborted parts, to perfect flowers. 

 The uppermost are stariiinate with only a suggestion of a pistil. Be- 

 low this, the pistils of the pollen bearing flowers become increasingly 

 prominent toward the center of the spike, and of course function in the 

 perfect and pistillate flowers below. Similarly, among the pistillate 

 flowers, there is a corresponding series of staminodia. The uppermost 

 of these are but little smaller than the true stamens; lower flowers may 

 have but four, and these further reduced ; while at the base of the spike 

 no trace of stamens may be found. 



Evidently this species is in a process of floral transition. Either, it 

 is changing from perfect to monosporangiate, or else from monospor- 

 angiate to perfect flowers. We may discard the latter supposition with- 



