6lO RELATION TO ENVIRONMENT. 



the pond is filled, the ground formation in the center becomes 

 more elevated than toward the margin. Transeau (1903) has 

 shown in an interesting way how these plant societies of peat- 

 moors are related to the arctic tundra. The evidence seems- to 

 show, as earlier students have also pointed out, that during the 

 glacial period these arctic plants moved southward in advance 

 of the great ice-sheet. When the glacier retreated to the north 

 many of these plants were left behind. In situations like the 

 present moors, where the climatic type of vegetation could not 

 take possession of the ground because of soil peculiarities, these 

 arctic plants have retained possession; the very soil conditions 

 here which prevent the growth of the forest favor the growth 

 of these plants. It has long been known that a number of 

 the heaths and other shrubs growing in these moors were very 

 similar to and in a few cases identical with certain species grow- 

 ing in the cold wastes of arctic regions. For example, the moss 

 plant (Cassiope hypnoides), the four-angled cassiope (Cas- 

 siope tetragona), and others extend from the Northern States to 

 arctic America. 



1101. Insectivorous plants on moors. Most plants growing 

 in moors have difficulty in obtaining food because of the small 

 amount of mineral substances, the unavailable form of much 

 of the nitrogen, and the interference with absorption due to 

 lack of oxygen and the presence of humus acid. Some moor 

 plants, however, are assisted in obtaining food because of their 

 carnivorous habit. These are the insectivorous plants like the 

 sundews (Drosera), the pitcher plant (Sarracenia purpurea), 

 the butterwort, or bog violet (Pinguicula vulgaris), the horned 

 bladderwort (Utricularia cornuta), etc. In the sundews there 

 are viscid glandular hairs (fig. 127) on which flies get stuck. 

 These hairs are sensitive, they bend inward, bearing the fly 

 against the leaf so that it folds over it, and certain juices from 

 the leaf digest the insect. In the butterwort the flies are stuck 

 to the surface of the sticky leaf. In the bladderwort, many 

 species of which grow -in water, there are bladder-like leaves 

 with a trap-door at the opening, so that insects can easily swim 



