464 Minnesota Plant Life. 



their leaves after the short rainy season is over and pass into 

 a winter-like condition, during which they evaporate much less 

 moisture than would be necessary if their leaves were expanded 

 to the ardent rays of the sun. For this reason, too, a great 

 many desert plants are annual, and in the compact form of 

 seeds withstand the dry season of the year. When the rain has 

 come again the seeds germinate rapidly and in a short time the 

 flowering and fruiting processes are completed. It is remark- 

 able, in a desert, when the rainy days are at hand, to observe 

 the extraordinary rapidity with which plant organs of various 

 sorts appear upon what was before an arid waste. 



Many desert plants develop underground bulbs and root- 

 stocks which persist without sign of life during the dry time 

 of the year and, when there has been a short season of rain, 

 suddenly put forth flowering stems and foliage. Another way 

 of meeting the aridity and dryness of the air is by the rolling 

 up of the leaves. This is well seen in the buffalo-grasses, in 

 some of the wormwoods or sage-brushes, and in a variety of 

 sedges, grasses and mosses. By means of this reduction of 

 the evaporating surface at the critical time, the desert plants 

 which employ these adaptations are able to prevent too great 

 evaporation. Another method adopted by plants in arid re- 

 gions is the modification of their leaf positions, and a number 

 of leaf-movements are regarded as protective against undue 

 evaporation of moisture. Many plants of the pea family in 

 desert regions have such special leaf-movements. Still another 

 method is the setting of the leaves on edge, as in compass- 

 plants; and the rosinweed compass-plant and the lettuce com- 

 pass-plant of the Minnesota flora, by twisting their leaves so 

 that only their edges are presented to the direct light, illustrate 

 such adaptations. Since the strongest illumination in the 

 northern hemisphere is from the south, these compass-plants 

 turn most of their leaves so that the flat sides face east or west, 

 and thus they are exposed to the least direct sunlight possible. 



In the desert regions of the world, leaves sometimes hang 

 from the stems so that the light strikes them as little as pos- 

 sible during the day. Still another device for limiting tran- 

 spiration, common among desert plants, is the formation of leaf 

 structures that evaporate moisture but slowly. The prismatic 



