CHAP. XI AIR IN THE SOIL 43 



during growth is small ^ ; in this way social growth is promoted, and the 

 landscape may even acquire a special uniform physiognomy, for example, 

 through Ammophila and Elymus on dunes, or Phragmites and Scirpus in 

 swamps. Firm, very tenacious clay, on the other hand, on drying becomes 

 hard and cracked, and therefore is not well suited to such plants ; in it 

 are found plants with a vertical, short, thick root-stock (tuber or bulb), 

 or with a multicipital rhizome and caespitose habit, for instance, on the 

 compos of Brazil.- Rigid plastic clay is no favourable soil for plants, 

 indeed (when it occurs beneath other layers) it may form an impenetrable 

 obstacle to plants. Sohd rock (without any deposit of loose soil) does 

 not in the least suit plants of the former habit, but may permit the enter- 

 tainment of plants of the second kind in its splits and clefts (chasmo- 

 phytes), and beyond this only such plants can settle on its surface 

 (lithophytes) as have special organs of attachment.^ 



It remains to be said that the root-structure of the various species 

 is very little known, and distinctions in this may perhaps often afford an 

 explanation of the distribution of species. 



The capillary action of soil plays a very important part in the physical 

 constitution. It depends especially upon the size and arrangement of 

 the particles. The smaller are the particles and the more closely are 

 they packed, the greater is the capillary action ; soil with compound 

 particles has less capillary action than if consisting of simple particles ; 

 stones and coarse grit in the earth likewise depress capillary action. 



CHAPTER XI. AIR IN THE SOIL 



Air in the soil is of most fundamental significance to plant life ; all 

 living subterranean parts, like all other living parts, require air (oxygen) 

 for respiration. In very wet soil, normal plants, adapted to soil rich in 

 air, are suffocated ; alcoholic fermentation, the evolution of carbon dioxide, 

 and consequently death and putrefaction * ensue ; in soil poor in oxygen 

 decomposition takes place in a manner different from that in aerated soil ; 

 humous acids are formed in great quantities, so that the soil becomes 

 'sour'. The aeration of soil depends essentially upon the structure; 

 the more porous and loose the soil is, the more free is the aeration. Farmers 

 and gardeners break up the soil with plough and spade, and drain and 

 harrow it, so that, among other reasons, air may be freely admitted. In 

 order to aerate the soil, the Dutch farmer causes the water-table of his 

 meadows to sink to a depth of one metre during autumn and winter, but 

 during the remaining months only to a depth of half a metre ; this is also 

 the practice in the meadows of Soborg in Denmark.' A production of 

 acid humus in the forest leads to an exclusion of the air, and consequently 

 to an extinction of the forest. 



Air in the soil is somewhat different in composition from that in the 

 atmosphere ; it contains more carbon dioxide and less oxygen, parti- 



' Henslow, 1895. 



- Warming, 1892 ; Lindman (1900) terms certain woody subterranean tuberous 

 structures ' xylopodia '. 



' Ottli, IQ03. See Section VIII. * See Sorauer, 1886. 



^ See Feilberg, 1890. 



