160 DIRECTION OP DEVELOPMENT. 



the point where plants are still able to obtain the necessary water supply. 

 Wherever the swamp is built above the level of the ground water, trees, espe- 

 cially alders and oaks, enter and form forests. Often, however, the swamps 

 remain treeless. The swamp peat has the peculiarity that it conducts water 

 very poorly, in contrast to heath moor peat, which has a marked conductive 

 power. As a result, the swamp plants disappear as soon as their roots are no 

 longer able to penetrate into the subsoil, and at the outset the flat-rooted 

 plants disappear. There remains a community of tall perennials, mostly 

 grasses. 



"This is the point at which the change to heath moor begins. Sphagnum 

 colonizes the lower moister places, and in similar fashion as upon the moist 

 sandy soils, the cushions run together and first fill the hollows and ditches in 

 the swamp. As soon as the Sphagnum has reached a certain extent, and has 

 filled the bottom of a ditch or hollow, other conditions of moisture begin to 

 appear. While previously a single dry sunny day sufficed to dry out and heat 

 up the black surface of the moor, the Sphagnum cushions now hold the water 

 with great tenacity. Even after a long dry period, the moss turf is still 

 moderately moist within, while elsewhere it is dried out. In early stages, the 

 Sphagnum occm-s only in ditches and hollows, which soon become completely 

 filled. When the moss layer has reached a certain thickness, it forms a great 

 reservoir of water, and the upward growth of the moss constantly increases. 

 It then spreads laterally over the level surface of the swamp, always carrying 

 larger quantities of water, which is unable to sink away because of the marked 

 imperviousness of the swamp peat which underlies the heath peat. After a 

 time, the various Sphagnum masses grow together and close over the swamp. 



"The primary requisite for such a moor, in so far as an actual inflow of 

 water is concerned, is that the annual precipitation should be greater than the 

 loss of water by evaporation and percolation. Here must be noted the fact 

 that the marked affinity of Sphagnum and heath peat for water, as well as 

 the very impervious nature of peat when it is satiu-ated, produces very dif- 

 ferent water relations than those which prevail in the swamp. The depend- 

 ence of such moors upon the rainfall of a region also explains the great fre- 

 quency of heath moors in the great heath regions, and their infrequence or 

 absence in dry climates (98-100). 



"In cases where a forest has developed upon a meadow moor before the 

 beginning of a moss moor, the development of a heath moor takes place more 

 rapidly. This is obviously due to the protection which the trees afford the 

 Sphagnum against sudden drouth. In such forests one almost never finds 

 small shattered cushions, but nearly always great masses or connected mats. 

 In an open swamp in which heath moor is beginning its development, one finds 

 on the contrary that the small dense moss cushions, located in small depressions 

 under the scanty shade of grass tufts, have their stems much compacted and 

 often show a red color. This indicates that the mosses live there only on 

 sufferance, and that they scarcely secure enough water to last through a dry 

 period. 



"The second method of origin of heath moor upon bare soil is that found 

 in some meadow moors. One very often finds in moors of great depth that 

 there is at bottom a more or less thick layer of black swamp peat, which 

 passes through a definite zone, often with tree trunks, into the heath peat 

 above. Not rarely, especially in northwestern Germany, the heath peat 

 shows an upper and lower layer. The development of heath moor in swamp 

 in such cases must have been due to a change of water relations, as a conse- 

 quence of which the swamp was flooded with enutrient water. Such instances 

 must, however, occur but rarely. In the majority of cases, heath moor arises 



