ECADS. 99 



more pronounced output of branches when growing in a sandy sub- 

 stratum than when grown in soils of firmer texture. These branches 

 were also often more widely spreading, especially in shallower soils. 

 Artemisia filifolia and Muhlenbergia pungens, species examined only in 

 the sandhills, showed onty slight variations, but Redfieldia flexuosa, 

 Calamovilfa longifolia, and Andropogon hallii, all sandhill dominants, 

 in general showed greater depth of penetration where the precipitation 

 was actually greater or where the topography or other local conditions 

 were such as to supply an unusual amount of soil-moisture. Convinc- 

 ing evidence on the relation of root distribution to soil-moisture in these 

 semiarid regions may be found in Chapter VI on the development of 

 cereal-crop plants. 



It should be noted further that root-length, closeness of branching, 

 etc., may have different causes and effects in different species or under 

 different conditions and consequently a different meaning to the plant. 

 The remarkable constancy of the specialized root habit of such species 

 as Asclepias arenaria and Mentzelia nuda (where the tap-root is the 

 dominant feature) is worthy of note. The retention of this habit 

 under sandhill conditions, where 85 per cent of the species show the 

 apparently much more efficient generalized absorbing system, strongly 

 suggests their relation to an ancestral habitat or condition of growth. 

 As shown by Cannon (1911:94), plants with generalized roots (where 

 both laterals and taps are well developed) are aided in their distribution 

 by their root plasticity, while those of the specialized type are restricted 

 because of their rigidity. It may be noted that none of the plants with 

 highly specialized root systems are dominants, but rank only as sub- 

 dominants and, with the exception of cacti, are of no great importance. 



The relation of root development to the various factors and the 

 proper evaluation of each demand a long series of ecological-physio- 

 logical investigations of a very intensive nature. A knowledge of the 

 exact position of root systems in various soils and descriptions of root 

 habits of native vegetation and crop plants under field conditions is an 

 essential prerequisite to such studies. It was with this purpose in 

 mind that the writer has earned out the investigations here recorded. 



