426 Minnesota Plant Life. 



So, too, if a thermometer bulb is imbedded in a bunch of violets 

 and laid in the sun, while another is imbedded in a bunch of 

 primroses, the one in the violets will register a higher temper- 

 ature after both have been lying together in the sunlight. The 

 recognition of the warming-up color of plants as a heat-pro- 

 ducing substance gives a basis for the explanation of a great 

 many facts that would otherwise be difficult to understand. 

 Autumn foliage can be understood to be a definite response of 

 the plant to the falling average temperature, by the development 

 of a heat-producing area of its own. The purple bud scales, so 

 common in trees that grow outside the tropics, purple tints 

 in bark and in leaves, and the purplish or reddish colors of 

 flowers, are all recognized as having the same general signifi- 

 cance. 



At this point it is possible to understand how, in a great many 

 species, the color of flowers may have originated. As has already 

 been explained, the flower is essentially the end of a branch, and 

 the coloring substances which it contains are possibly to be 

 attributed to protective heat-producing substances present in 

 the race-history long before flowers were developed as such. 

 When flowers came to be formed it was important that their 

 pollen should be protected against cold, and the heat-producing 

 colors were not abandoned but accentuated. In many blue, 

 violet and red flowers it is possible to read a story of defence 

 against cold. Hence is made clear the reason for so many 

 very early flowers, like the pasque flower, maple flowers, violets 

 and anemones, being purplish in color, and for late flowers, 

 such as the gentians, having the same blue or purple hue. It 

 is also easy to understand how flowers on the mountain tops 

 should be so often blue and that flowers of the polar regions 

 should show the blue, violet or reddish tints. With such facts 

 in mind it is possible to recognize violets and anemones as, for 

 the most part, northern plants, while goldenrods and evening- 

 primroses indicate a southern origin. 



Another useful habit of plants in polar regions is their tend- 

 ency to form stores of reserved food-material, such as under- 

 ground fleshy roots, stems or tubers, or subterranean bulbs. A 

 great variety of plants with such habits are common in Minne- 

 sota. On account of its provident behaviour the plant is able 



