32 A. G. Tansley. 
squarrosa (annual), and Schedonnardus paniculatus, Opuntia polya- 
canthyn, 0. fragilis, Ma miliaria vivipara, Malvastum coccinenm, 
Gvindelia squarrosa, Ptiloria paucijlora, Gaum coccitiea, Aragallus 
Lambertii, etc. (perennial). In 1913 Grindelia squarrosa occurred in 
enormous quantity in the Akron district, its erect bright-green 
shoots, a foot or more high, forming a striking constrast to the grey 
of the short grasses. After rain the short grasses turn rapidly 
green—only to revert to the grey colour of their dry state in the 
succeeding drought. 
The competition for water in the upper layers of the loamy soil 
is very severe. The short grasses by their shallow rooting habit 
are well adapted to utilise the rain directly it falls, and owing to 
this fact and to the intense evaporation and the slow percolation 
through the heavy fine-grained soil, very little water ever reaches 
lower levels. Soil boring clearly shows that while a certain amount 
of moisture is retained during drought in the surface-layers of soil, 
the underlying layers are extremely dry and the water they hold is 
unavailable to plants. The factor of the capillary rise of water 
does not enter into the conditions. Local irregularities in the 
compactness of the soil, however, cause considerable differences 
in the water-relation, and where the soil is looser percolation 
increases, deeper rooting plants flourish, and partly or completely 
exclude the short grasses. These water relations are no doubt 
the main explanation of the absence of trees in the Great Plains. 
When the soil is broken and trees are planted they make healthy 
growth though they do not attain any considerable size. It is 
certainly not true that the dryness of the air excludes tree-growth 
in this region. If the soil is broken by ploughing and left fallow 
the ground is first colonised by an open association of ruderals 
including Amaranthus grtzcizans, Salsola pestifer, Festuca octoflora, 
Plantago Purshii, Munroa squarrosa, Clieuopodium spp., etc. This 
gradually gives place to an association in which the yellow flowered 
composite Gutierrezia sarothree and the grey Artemisia frigida may 
share dominance. The former species is on the whole more southern 
and the latter more northern in distribution, but in the central region 
of the plains the two are generally associated, and in this association 
also occur Bouteloua oligostachya, B. curtipendula, Mentzelia nuda, 
Petalostemum purpureum, Paronychia jantesii, Eriogonum effusum, etc. 
The Artemisia-Gutierrezia association gradually passes over, by 
the increasing dominance of the short grasses, into the typical 
climax or “chief” association of the formation. The whole 
