PLANT PHYSIOLOGY 29 



rooting Opuntias to develop long tap roots. The treatment 

 consisted in providing adequate aeration and warmth through- 

 out the length of the root, and Cannon is therefore inclined to 

 regard root development as a process depending on environment 

 rather than as a distinctive characteristic of the type. 



Experiments on aeration of soil- and water-cultures have 

 been carried out by Livingston and Free and by Free {Johns 

 Hopkins Univ. Circ, 191 7, Contributions to Plant Physiology, 

 pp. 182 and 198), and have shown that the deleterious effect of 

 lack of aeration is generally not wholly due to the poisonous 

 effect of CO2, but rather to lack of oxygen. Buckwheat, on 

 the other hand, does not benefit from aeration in water culture, 

 nor does it suffer if nitrogen is bubbled through its nutrient 

 solution. Carbon dioxide, however, quickly causes wilting and 

 death. This work again indicates that the response of a plant 

 to lack of aeration may be due either to the resulting increased 

 CO2 concentration or to the lack of oxygen. Noyes and his 

 co-workers have followed up their preliminary experiments 

 {Science, n.s., 1914, 40, 792) which showed the response of 

 roots of tomatoes and maize to the influence of CO2, by a more 

 detailed investigation. Noyes, Frost, and Yoder {Bot. Gaz., 

 191 8, 66, 364-373) have described some pot experiments in 

 which the roots of plants were exposed to a stream of carbon 

 dioxide through the soil. The results showed the importance 

 of soil aeration to plant growth, whether the immediate result 

 is the maintenance of the supply of oxygen or the removal of 

 carbon dioxide. In all cases, the inhibiting effect of COg was 

 more apparent in the development of roots than in the growth 

 of the aerial parts, and in addition the extent to which growth 

 was affected was different in different plants ; Phaseolus 

 vulgaris, for example, proving indifferent to treatment. The 

 question of the real value of decaying organic matter in the soil 

 is raised, the authors expressing the opinion, as the result of 

 their experiments, that the carbon-dioxide content of garden 

 soils may at times become actually harmful to plants. 



Noyes and Weghorst {Bot. Gaz., 1920, 69, 332-336) have 

 described the results of growing plants in the soils used in the 

 experiments of Noyes, Frost, and Yoder. They report that 

 nine months after the treatment of a soil with carbon dioxide, 

 the soil still retains to some degree the capacity for inhibiting 

 root growth. No analysis of this result has been attempted, but 

 the authors point out its importance from the standpoint of 

 CO2 production from soil organic matter. 



Howard has devoted considerable attention to the problem 

 of soil aeration in India, and in an article {Agr. J. India, 191 8, 

 13, 416-429) on the general aspect of the question has reported 

 some experiments on the opening of a calcareous silt for 



