96 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



upon the effects of the environment upon their development. Until 

 very recently (Cannon, 1913a, 1915; Waterman, 1919), intensive 

 studies of root systems along both morphological and physiological 

 lines have been made mostly by European investigators (cf. Kroemer, 

 1903; Tschirch, 1905; Von Alten, 1909). The chief records of the 

 influence of the soil environment upon root extension and development 

 are mainly incidental to other investigations. The role played by soil 

 temperature has been discussed on page 34 in comparing mixed and true 

 prairie environment. While its ecological importance has been em- 

 phasized, in the absence of further quantitative data we must for the 

 present accept its influence in the development of grassland vegetation 

 as of minor importance. The extension of the grassland associations 

 throughout such wide areas of latitude with little change in their 

 floristic composition would seem sufficient evidence to warrant this 

 conclusion. 



Soil aeration, which is closely connected with water-content and soil 

 texture, is an important factor in root development. As shown by 

 Livingston and Free (1917), the exclusion of oxygen from the roots of 

 certain species interferes with the respiration of the protoplasm of the 

 root-cells, resulting in the death of this protoplasm and the consequent 

 failure of the roots to function as water-absorbers for the plant. The 

 cessation of water intake is soon followed by progressively lessened 

 turgor of the shoot and leaves and finally by wilting and death. Cannon 

 and Free (1917) have shown that roots of various plants respond quite 

 differently to variations in the composition of the atmosphere. An 

 increased air-supply to the roots of certain species favors root-branching 

 and probably accelerates root-growth. Their results indicate that 

 plants growing in well-drained soil are much more sensitive to the 

 composition of the soil-atmosphere than those from poorly drained and 

 poorly aerated habitats. 



The soil-atmosphere is important not only as affecting directly the 

 respiration of the roots of higher plants and consequently their normal 

 functioning and development, but it also plays a part in the life 

 activities of both aerobic and anaerobic micro-organisms of the soil. 

 These in turn alter its chemical composition, and this indirectly in- 

 fluences both root and shoot development. The deficiency in soil 

 aeration and its bearing upon problems of agriculture and forestry 

 have been clearly presented by Howard and Howard (1917) and Hole 

 (1918). To grasp its full ecological significance, as a factor affecting 

 root habit, and one always to be considered along with soil-moisture, it 

 should be borne in mind, as is pointed out by Cannon and Free (1917) 

 that different species may have great differences in the oxygen require- 

 ment of their roots and may show responses which may be quite as 

 specific and significant as responses to temperature and to available 

 water. It seems probable that one of the beneficial effects of heavy 



