126 AERATION AND AIR-CONTENT. 



areas. While the temperatures in southern Michigan are regarded 

 as inadequate to render cold a factor in xerophily, it is probably a 

 powerful factor further north, an assumption borne out by the bogs 

 of northern Minnesota in which ice persists at certain depths until 

 midsummer or later. The high water capacity of peat is a factor of 

 some importance, in that it serves to prevent proper aeration of the 

 substratum. The activity of nitrifying bacteria is almost completely 

 inhibited in natural bog-soils, owing to the acidity, deficiency in 

 oxygen, and low temperatures, and denitrification often further 

 reduces the supply of available nitrogen. 



Davis (1907) ascribed the xerophytic nature of bog-plants to the 

 tenacity with which peat retains water, as a consequence of which 

 plants wilt in it at 49.7 parts of water per 100 of dry soil, and crops 

 require more than 60 per cent of water in peat to be productive. The 

 drying-out of the peat in years of minimum rainfall was also regarded 

 as a factor of importance. It was noted that Betula and Salix en- 

 dured a foot rise in the water-level for 3 years, while more sensitive 

 plants perished. 



Dachnowski (1908 : 134) has grown gemmae of Marchantia in bog- 

 soil extract, bog-water untreated, bog-water aerated, bog-water 

 neutral, obtained by shaking with dry calcium carbonate and filter- 

 ing, bog-water filtered, shaken with lamp-black and filtered, distilled 

 water in which bog-plants had been grown in culture, and spring- 

 water. In the case of water from the central zone, growth in the 

 aerated, neutral, and filtered cultures was greater than in spring- 

 water, while that in the extract and untreated water was much poorer. 

 Growth in water from the maple-alder zone was best in the aerated 

 solution, practically as good in the neutral and filtered, and but little 

 less in the untreated and bog-plant water. Similar cultures of a num- 

 ber of cultivated plants gave much the same results, though the 

 aerated solution was generally less favorable than the neutral and 

 filtered ones. This does not seem strange, as shaking and filtering 

 and the diffusion of the dry particles should have brought about 

 much more effective aeration of these two cultures. The conclusion 

 is reached that the inhibiting factors of bogs are in part injurious 

 water-soluble substances, the toxicity of which can be corrected by 

 aeration and by the use of calcium carbonate and lampblack. The 

 author further says : 



"It may readily be questioned whether part of the response arises from a 

 deficiency of oxygen in the soil. The evidence obtained by Bennett is against 

 aerotropism in roots. It follows, therefore, that results reported as due to 

 lack of aeration in the bog substratum are really due to toxicity. Under 

 natural conditions, the inhibiting effect is eliminated by aeration, a slow 

 process of oxidation preventing the accumulation of injurious plant excreta 

 in the soil. However, on account of the great demand for oxygen, this process 

 can be carried on efficiently only near the surface." 



