62 IOWA ACADEMY OF SCIENCE 



tion of these was the species in question. The plants seem to thrive 

 best in water from 4 to 6 feet deep, yet in a little cut-off bay where the 

 water was from 6 to 18 inches in depth and usually much warmer than 

 that of the general body of the lake, they were growing, flowering, and 

 fruiting luxuriantly. Here, too, they blossomed and fruited about two 

 weeks earlier than those in the larger body of water. 



In the summer of 1909, the first flowers were found July 8th, and 

 the height of the season was reached near the end of July. The next 

 year they were fully three weeks earlier in their time of blossoming. 

 This Avas probably due to the fact that the early spring of 1910 was very 

 warm which started these plants growing vigorously, and the cold 

 weather which came during the latter part of April, checking for a long 

 time the growth of land plants, did not proportionately affect these sub- 

 mersed forms. 



The plants live rooted in mud at tlie bottom of a shallow lake or 

 pond, and send their long, slender stems upward. The plants are much 

 too weak to support their own weight in air, but are buoyed up readily 

 by the water, as the stems have large air-chambers extending from node 

 to node. In cross section, the stem resembles a wagon-wheel ; a central 

 area of tissue suggests the hub, a peripheral zone, the rim, with radiating 

 spoke-like structures between. The spaces between the spokes are air- 

 chambers, wliich together make up one-half the volume of the stem. The 

 vascular bundles, as is common with submersed plants, are very small 

 and inconspicuous. The cortex is delicate without any apparent thick- 

 enings and without cutinized walls. During the months of June and 

 July, the tips of the plants become emergent from the water, and these 

 bear the spikes of flowers. The young flowers are formed beneath the 

 water in a very close head or spike, and as they emerge, the spike elong- 

 ates by the lengthening of the internodes. 



Floral Development. 



The infloresence of this species is a terminal, naked, interrupted spike, 

 bearing from 40 to 60 flowers in whorls of four, or rarely five. Each 

 whorl is rotated through 45° from the adjoining one, thus producing the 

 spiral arrangement so clearly seen in the young spike before elongation. 



Each flower is borne nearly sessile in the axil of a strong bract, while 

 two smaller but heavy bracts bound the flower on the sides (Figs. 2, 15). 

 These three bracts entirely enclose and protect the young flower, opening 

 gradually as the spike elongates and the flowers develop (Fig. 1). 



Generally the flowers are functionally monosporangiate, though rudi- 

 ments of the suppressed parts are present. The upper flowers of the 



