I9 6 OXYLOPHYTES SECT, vi 



always acid; humous acids depress tte root's activity and render it 

 more difficult for the plant to replace the water lost by transpiration. J 



In fact, various factors enter into the question, and possibly all of 

 them play a part in evoking xeromorphy. 



Finally it may be noted that there are not only moors inclining to 

 xerophily, but also others leaning rather to hydrophily, and that, in 

 addition to the structural types 1 and forms of leaves mentioned, there 

 are others which apparently show no signs of xerophily and cannot be 

 shown to be in harmony with this habitat : for example, broad, hastate, 

 sagittate, or cordate leaves occur in many Araceae concerned, while 

 broad, orbicular or reniform leaves are shown by Rubus Chamaemorus, 

 Caltha palustris, and Viola palustris. 



The formations growing on acid soil may be arranged according to 

 the following scheme : 



1. Low-moor formation : Often represents the first stage and the 

 one most nearly related to hydrophytic formations. 



2. Grass-heath. Tussock-formation (Peculiar to the Southern 

 Hemisphere). 



3. High-moor formation : Likewise very wet. 



4. Moss-tundra formation. 



5. Lichen- tundra formation. 



The others are drier formations in which woody plants are the domi- 

 nant constituents : 



6. Dwarf-shrub heath formation. 



7. Bushland and forest formation : The formations 4, 5, 6 may 

 occur as undergrowth here. 2 



CHAPTER XLVII. LOW-MOOR FORMATION 



THE vegetation of low-moor requires less water than does that of 

 reed-swamp, with which it is often continuous on the landward side 

 and at the expense of which it often develops. It shows less open water, 

 which, moreover, is visible to a less extent than in reed-swamp and 

 frequently is only to be seen at certain times. The water-table is always 

 at a high level. The vegetation is closer, and the vegetative shoots 

 project nearly entirely into the air. The water is still or flows 

 but slowly ; the land flat and horizontal, though in arctic countries 

 it may slope slightly. Humous acids arise in the soil, which becomes 

 moor-like because of the accumulation of vegetable fragments 3 ; thick 

 layers of peat may be produced, especially from certain species, including 

 at times some, such as Phragmites, also present in reed-swamp. The 

 peat contains much nitrogen, which, however, is not always in a form 

 easily available to plants. 



1 In regard to the great variation in the anatomical characters of monocotylous 

 marsh-plants, see Grabner, 1895. 



1 In reference to these formations, special reference should be made to the 

 great work by Fruh and Schroter, 1904. See also Pound and Clements, 1900; 

 Clements, 1904; Weber, 1902, 1903; MacMillan, 1893, 1896, 1897; Livingston, 

 1904; Ramann, 1895 and 1906; Yapp, 1908. * See Chap. XVI. 



