Minnesota Plant Life. 433 



floating leaves as those of the water-shield, in which the upper 

 side of the leaf is green and the lower is purple. The giant lily 

 of the Amazon, sometimes cultivated in aquatic gardens of parks 

 in Minnesota, has great shield-shaped leaves, two feet or more 

 in diameter, and fitted to float upon the surface of the water. 

 The edge of the leaf is turned up to prevent ripples from break- 

 ing over the surface. While the upper side is green the under 

 side is violet or purplish. The surplus light, not used in starch- 

 making, is, in such a leaf converted, by the purple coloring mat- 

 ter, into heat, and is not lost. 



When leaves form rosettes an adaptation for saving the sur- 

 plus light is often noticeable. Thus, the under sides of dande- 



FIG. 217. The Virginia creeper on the walls of the old round tower, Fort Snelling. This 

 plant does not turn towards the sun, but clings to the shaded wall. After photograph 

 by Williams. 



lion leaves are commonly purple; and in a great many other 

 rosette-forming plants the leaves of the rosette will have the 

 color scheme which has been described. Hanging or swing- 

 ing leaves that are swayed by the wind, for reasons that 

 are sufficiently obvious, do not so often have the two sides 

 colored in this manner. 



Protection of the leaves against an illumination strong 

 enough to injure the starch-making machinery within, is of 

 various types. Sometimes the leaves are covered with scales, 

 or hairs, thus tempering the light. Sometimes they are 

 capable of changing their positions, so that when exposed to 



direct sunlight they shift from the transverse to a more ver- 

 29 



