210 



Minnesota Plant Life. 



Distribution of grass grains. A few grasses in the state 

 have interesting special methods of distributing their fruits. 

 The sand-bur, for example, encloses its fruits in bur-like scales. 

 If carefully examined, the points on the burs will be found to 

 have barbs directed backwards along their sides, so that a bur 

 sticks very closely to the fur of an animal or to clothing and 



thus brings about the dissemina- 

 tion of the fruits within. An- 

 other grass known as spear-grass, 

 or to children as "fairy's spears," 

 is remarkable for the "self- 

 planting attachment" of the 

 grain. In this variety the grain 

 is enclosed in a chaffy scale, the 

 end of which is prolonged into a 

 slender awn, while the base is 

 sharply pointed, hard, and pos- 

 sessed of hairs pointing back- 

 wards. When such grains fall 

 upon the soil the tips penetrate 

 a little, owing to the heaviness 

 iof the seed. The 

 slender bristle 

 then begins to 

 coil and uncoil 

 under the stim- 

 ulus given to it 

 by changes in 

 the moisture of 

 the air. Since 

 the grain holds 

 all the ground 

 that it gains on 



FIG. 91. A cluster of sedge-flowers (Carex-type), a single 

 pistillate flower with one fruit rudiment, and a stami- 

 nate flower with three stamens. After Atkinson. 



account of its backward-pointing hairs it is slowly driven into 

 the soil and thus enjovs a certain advantage over varieties 

 which have not such self-planting mechanism. Grasses are not 

 alone in apparatus of this sort, for the fruits of the clematis and 

 of some geraniums are similar to a degree. Other species of 

 grass are provided, upon their fruits, with expansions or tufts 

 of cottony hairs, by which the wind assists them in their dis- 



