444 Minnesota Plant Life. 



upper side exposed to the air, as do the duckweeds, certain 

 special structures are necessary. First of all, proper counter- 

 poises must be carried to prevent the plant from being turned 

 upside down by the ripples. In the duckweeds these counter- 

 poise areas are readily seen to be roots, or groups of roots, but 

 in the smallest of the duckweeds no counterpoise exists as a 

 special organ, since from the very small size of the plants they 

 are able to ride the waves without their upper surfaces being wet. 

 In one of the water ferns — the little Salvinia plant, common in 

 greenhouses, where it grows in tanks — two rows of leaves on 

 the floating stem are thrust down into the water and act as 

 counterpoises; but in the wild water fern — the tiny Azolla — 

 just as in the duckweeds it is the slender roots that perform 

 this function. Sometimes skillful combinations of counter- 

 poises and floating apparatus are effected, as in the handsome 

 water-hyacinth of conservatories — a species which has become 

 a great pest in the rivers of Florida. In these plants the bases 

 of the leaves, which arise in rosettes, are swollen into spongy, 

 spherical floats, an inch or more in diameter. Below such a 

 circle of floats a tuft of roots hangs down into the water, and it 

 is quite impossible for any ordinary gust of wind to turn the 

 plant upside down. 



A second group, also, of adaptations are interestingly de- 

 veloped in the free-floating higher plants by means of which 

 the upper surfaces of natant organs are protected against wet- 

 ting. Sometimes the surface is very smooth and glistening 

 and the water rolls from it in the spheroidal form just as it 

 does from a buttered plate. This is the condition to be ob- 

 served in the larger duckweeds. The upper surface of the small 

 disc-like stem of these plants is somewhat convex and very 

 smooth. If water is poured upon a group of duckweeds as they 

 float upon the surface of a pool it will roll ofl' as from the pro- 

 verbial duck's back, and will leave the little plants as dry and 

 glistening as before. One of the duckweeds, the three-cor- 

 nered variety, is a partially submerged i)lant and therefore has 

 not this power of shedding water. Another arrangement is 

 seen in the water fern, Salvhiia. The upper surfaces of the 

 floating leaves are proxided with curious tufted hairs, the lips 

 of which spread otU in three or four branches If one of the 



