996 MINNESOTA BOTANICAL STUDIES. 



example the Polygonum and Utricularia types. When any one 

 of the types has become established the constant addition of 

 decaying organic substance builds up what has been termed wet 

 humus and gradually a spongy coherent substratum is elabor- 

 ated and upon this numbers of plants of hydrophytic tendencies 

 will find a home. Numerous small Carices and Ranunculi, 

 Caryophyllaceae and La.biatae establish themselves, together 

 with grasses, and the bog shows a transition to the dry type. 

 It should be noted that the rhizomes and roots of the plants are 

 the active agents in binding together the humus in some cases, 

 while in others the floating areas of the plants, as in the 

 Panicularia and Utricularia varieties, play an important part. 



Dry morass. This name may be applied to those morassic 

 shores on which the accumulation of humus has reached a point 

 where every year it remains for the most of the time fairly 

 above the average lake level. In the piling up of the humus 

 in this fashion a number of causes co-operate. Among the 

 interesting influences under which wet morassic areas may 

 become dry should be mentioned the following: 



1. The influence of ice-floes. These, crowding against a 

 wet morassic shore in the spring sometimes pile up the humus 

 in such fashion that it is condensed and elevated into a perma- 

 nent ridge. In this condition its plant population changes. 



2. The influence of outgrowing roots of shoreward estab- 

 lished plants. By the pushing in of heavy roots from trees, 

 usually willows, birches or tamaracks, and in some cases 

 spruces, the wet morassic soil is lifted and gradually brought 

 up where it is for the most part out of reach of inundation. 



3. The lifting force of gases of decomposition developed in 

 the humus itself. By this means a general elevation of the 

 humus mass is brought about in some instances and the whole 

 substratum is, as it were, leavened and by the growth of root- 

 lets and further deposition of humus the spaces thus formed 

 become filled first with water, then with soil components. 



These are in addition to, and an accentuation of the steadily 

 progressing increase in bulk of the wet morass due to the 

 formation of generation after generation of leaves, roots and 

 stems by its plant population. Thus, beginning in a variety of 

 ways, — as a bed of Utricularia, a growth of reeds, a mat of 

 sedges or a tangle of willows — the wet humus may be gradu- 

 ally changed to dry and in this process its plant-population 

 slowly but definitely changes. 



