BOG XEROPHYTES AND ACID SOILS. 119 



been much modified if the author had determined the transpiration 

 at the different levels, since it is almost certain that this would have 

 shown Phragmites and Cladium to be hydrophytic in both transpira- 

 tion and stomatal behavior, as Scirpus and Typha are known to be. 



Coville (1910) has demonstrated that the swamp blueberry {Vac- 

 cinium corymbosum) requires a well-aerated soil for active growth 

 and can not grow readily in soil saturated with water, Sandy soil 

 and drained fibrous peat offer satisfactory conditions, as well as live 

 moist sphagnum hummocks, which furnish permanent moisture and 

 thorough aeration. The rootlets of the blueberry contain an endo- 

 trophic mycorrhiza, probably belonging to the genus Phoma. It is 

 assumed as possible that this is able to assimilate atmospheric nitro- 

 gen much more actively than Clostridium, and also that it makes the 

 non-available nitrogen of the peaty soil available to the plant, thus 

 making up for the great lack of available nitrates, due to the inability 

 of nitrifying bacteria to thrive in the acid soil. The swamp blueberry 

 grows in peaty soils that contain acids or other toxic substances and 

 suppresses the root-hairs as a protection against these. As a conse- 

 quence, both absorption and transpiration are low, and many bog 

 shrubs show devices for retarding water-loss similar to those of desert 

 plants. Low absorption leads to insufficient nutrition, and the 

 danger of nitrogen starvation is especially great, owing to the lack of 

 nitrates. In the swamp blueberry the necessary nitrogen is secured 

 by means of the mycorrhizal fungus, and conveyed into the plant 

 without a large amount of the poisonous water. 



Burns (1911 : 124) concluded that the bogs around Ann Arbor 

 contain xerophytic, hydrophytic, and even mesophytic areas. The 

 presence of definite communities in each zone is due chiefly to soil 

 conditions, especially temperature, and also to the position of the 

 water-table and to aeration. Of the 7 zones described, but 3, the 

 floating sedge, bog shrub, and tamarack, have dominants that are 

 xerophytic in nature. In the sedge community only the plants that 

 root deep in the floating mat are regarded as box xerophytes, while 

 those rooting in the surface are hydrophytes. The chief mat-form- 

 ing plants are Carex filiformis and C. oligosperma. Associated with 

 these, some playing an important part in mat formation, are Meny- 

 anthes trifoliata, Dulichium arundinaceum, Eriophorum viridi-cari- 

 natum, Drosera rotundifolia, Aspidium thelypteris, Onoclea sensibilis; 

 Equisetum limosum, Eupatorium purpureum, E. perfoliatum, Mentha 

 arvensis glabrata, Scutellaria galericulata, Utricularia, Calopogofi 

 pulchellus, Campanula aparinoides, Arethusa bulbosa, Galium trifidum. 

 Aster junceus, Potentilla palustris, Solidago serotina, Lysimachia ier^ 

 restris, etc. The characteristic plants of the bog-shrub zone in- 

 clude Chamcedaphne calyculata, Andromeda polifolia, Betula pumila, 

 Nemopanthes mucronata, Sarracenia purpurea, Vaccinium oxy coccus, 

 V. macrocarpum. The principal plants of the tamarack zone are 



