Minnesota Plant Life. 



281 



Riverweeds. Of the riverweed family there is a single Min- 

 nesota species which grows attached to stones under water in 

 strong rapids or cataracts. The riverweed belongs to a family 

 of herbs best developed in the tropics and very remarkable for 

 marvelously perfect adaptation to the submerged life. The 

 plant-body of many of the riverw^eeds resembles that of an 

 alga, the leaves being poorly distinguished from the stem on 

 which they are borne. The flowers and fruits are produced 

 entirely under water and are surrounded by involucres resem- 

 bling the spathes of the arum family. In the Minnesota va- 

 riety — the only one common in North America — the flowers 



are sessile, there is no perianth 

 and there are two stamens united 

 together at the base. The fruit- 

 rudiment is ovoid, with two short 

 stigmas. The general appearance 

 of the plant is that of a dense tuft 

 of finely divided leaves attached 

 to the stones at the bottom of 

 the water. The flowers are small 

 and easily recognized by the two 

 partly fused stamens, standing- 

 like a little fork beside the ovary. 

 In the fruit arise a number of 

 small seeds with straight embryos 

 and without albumen. The 

 ,„^, ,.. . r • , riverweed has been collected in 



Fig. 136. River-weed. After Britton and 



Brown. Minucsota on the International 



boundary, in the Granite lake rapids, at Minnehaha falls and at 

 Lake Pepin. The most interesting thing about the riverweed 

 is its entire abandonment of terrestrial methods of flower-pro- 

 duction and pollination. While pondweeds are compelled to 

 lift their spikes of flowers above the surface of the water to 

 accommodate themselves to the persistence of ancient methods 

 of wind-distribution in vogue during the days when their an- 

 cestors were dwellers on the land, the riverweeds have freed 

 themselves from this necessity and have the ability to main- 

 tain themselves quite submerged in deep water, as if they were 

 algae. Some other varieties of flowering plants flower under the 



