Minnesota Plant Life. 425 



dormant condition. Not all of them hibernate in the same way. 

 A few polar species are annuals, shooting up from the seed dur- 

 ing the short polar summer, rapidly maturing their flowers and 

 fruits, and passing the long cold winter as little plantlets, 

 wrapped up in their protective seed coats. There are, however, 

 a greater number of biennial and perennial polar plants. Some 

 of these are shrubs, like the poplars and willows, which grow a 

 very little from year to year, quickly mature their flowers and 

 fruits, and pass the winter in a leafless, dormant state. Such a 

 rhythm in the plant, established to meet the rhythm in the outer 

 world, results, as has been previously noted, in that general 

 habit of shedding the leaves which characterizes so many plants 

 of the polar and temperate regions. 



Besides their dwarfed size and their various habits of hiberna- 

 tion, plants of cold regions exhibit numerous protective devices 

 against the cold. The seed coats are thick, and sometimes the 

 seeds are inclosed in warm fruits ; the leaves and stem are often 

 clothed with hairs ; the young twigs are covered over with pro- 

 tective scales, forming the well-known object that is called a 

 bud; and "warming-up colors" are developed, especially where 

 there is need for them to protect the delicate portions of the 

 plant. The ends of the shoots are particularly rich in the warm- 

 ing substances. Peat-mosses are good examples of this, for in 

 a great many of them the ends of the branches are violet, red 

 or purple, while lower down they remain green. Bud scales 

 are commonly purple, and alpine and polar flow r ers are more 

 likely to be blue or violet than yellow or red. By this means 

 the temperature of a flower in which the delicate pollen spores 

 are formed, or the temperature of a young twig, surrounded by 

 its purple bud scales, is raised somewhat, and the unfavorable 

 influence of the cold is, to some extent, counteracted. The 

 efficiency of the "warming-up color" may be tested experiment- 

 ally by any one who cares to take the trouble. If two similar 

 thermometer tubes are selected and around the bulb of one of 

 them a green leaf is tied, while around the bulb of the other a 

 purple leaf, such as that of the beet or of certain begonias, is 

 wrapped, and the two thermometers are then laid in the sun, 

 after a short time it will be found that the one with the purple 

 leaf is registering from six to ten degrees higher than the other. 



