436 Minnesota Plant Life. 



numerable delicate hairs and these are thrust between the crev- 

 ices of the soil to collect whatever moisture there may be 

 present. If, however, the roots are immersed in the water and 

 hang down like the little balancing roots of the duckweeds, root 

 hairs are not then so abundantly produced because moisture is 

 plentiful everywhere around the root and no special arrange- 

 ments for its collection are necessary. Moisture, after having 

 been collected by the plant through the activities of the absorb- 

 ing surfaces, is evaporated, and the residue is left in the plant, 

 either to be combined with other substances in the plant chem- 

 istry or to remain as a useless by-product. Evaporation and 

 transpiration of water vapor are the ordinary methods by which 

 plants rid themselves of the superfluous water they have ab- 

 sorbed ; but some varieties exude it in drops from special water- 

 excreting glands. Thus, fuchsia leaves, if well supplied with 

 water at the root, will excrete it in little drops from each tooth 

 of the leaf margin. 



There are at least three conditions under which plants 

 find it undesirable to excrete or transpire water rapidly. One 

 condition is that of the desert, where there is very little 

 water to be obtained, and its rapid transpiration by large 

 evaporative surfaces would result in the wilting of the whole 

 plant. Another condition under which rapid evaporation is 

 undesirable is that of bogs and marshes, and the reason is just 

 the opposite. Here there is such an abundance of moisture 

 that the rapid evaporation of it might maintain an unneces- 

 sarily strong stream through the plant tissues. A third con- 

 dition is where the soil-water is impregnated with salts, and 

 if rapidly evaporated it would be as rapidly absorbed, and the 

 salts would accumulate in the plant tissues to such an extent 

 that they might interfere with vital processes. Consequently 

 there are three groups of plants so situated that, while they 

 permit water to evaporate from their leaves and stem, their 

 adaptations of structure are for slow transpiration. The cacti, 

 with their solid stems and reduced leaf surfaces, are examples 

 of one class. The tamaracks and spruces, with their small 

 needle-shaped leaves, are examples of another, and the sea- 

 blites and glassworts of the salt marshes furnish examples of 

 the third. 



