AERATION: TEMPERATURE 45 



out very clearly in these conditions, as it is not complicated 

 by changes in moisture or salt supply. Stiles and Jorgens- 

 sen (1917) found that well-aerated cultures showed a 70 per 

 cent, increase of root growth over non-aerated. 



In nature the effects of lack of aeration may occasionally 

 be observed clearly. Thus, Emerson (1921) found that the 

 tap root of seedlings of Picea and Pinus, growing in 

 Sphagnum tussocks, ceased growth or bent sharply in a 

 horizontal direction near the water-table. 



Some indirect effects of bad aeration have already been 

 referred to. They include modification of bacterial activity, 

 inhibition of the destruction of humus compounds, and the 

 production of organic toxins. Such effects have a very 

 important influence on plant growth. For a detailed 

 discussion the monograph on " Aeration " by Clements 

 (1921) should be consulted. 



6. The influence of temperature on root growth has been 

 the object of frequent study. For the main root of Pisum, 

 Leitch (19 1 6) has shown that the growth rates at 29° C, 

 20° C, 10° C., and 0° C. are as 48, 24, 8, and i. Above 

 30° C. (at which temperature irregularities occur), a progres- 

 sive diminution takes place. As to the effect of different 

 soil temperatures on the growth of root systems, there seems 

 to be no definite information. One may be sure that in 

 desert soils, in the surface layers of which temperatures of 

 50°-6o° C. frequently occur, growth may be inhibited. 

 Cannon (19 15) found that root growth of such desert 

 plants as Prosopis and Fouquieria is appreciable only 

 above i5°-20° C. and is active at 35° C. In the 

 soils of temperate regions inhibition must often be caused 

 by low temperatures. The greater uniformity of soil 

 temperature must mean a steadier though perhaps slower 

 development of the root system. It has been generally 

 believed that the growth of tree roots ceased in summer and 

 winter in temperate climates, but McDougal (19 16) has 

 shown for certain American species that if water is abundant 

 growth continues through the summer, and that the only 

 regular cessation is that due to low winter temperatures. 



