DEPARTMENT OF BOTANICAL RESEARCH. 49 



1 . The reaction of roots to a deficiency of oxygen or to an excess of 

 carbon dioxid in the soil was investigated in thirty species of plants 

 and observed to a certain extent in several more. The habitats from 

 which the species were derived or the cultural conditions under which 

 they are growing are extremely varied, ranging, in the case of the 

 former, from swamps on the one hand to the arid mesa of southern 

 Arizona on the other. 



2. All of the species studied appear able to maintain root-growth 

 in an atmosphere containing 2 per cent oxygen, provided the amount 

 of carbon dioxid present is not as great as 30 to 50 per cent. 



3. Several species, including Baccharis vimines, Citrus sinensis, 

 Juncus effusus var. hrunneus, Potentilla anserina, and others, are able 

 to continue root-growth, although at a relatively slow rate, for more 

 or less extended periods in a soil-atmosphere containing 5 per cent 

 oxygen, the remainder nitrogen, and in relatively large amounts of 

 carbon dioxid, as in 50 per cent, provided the oxygen partial pressure 

 is normal or not far below. 



4. When oxygen is entirely removed from the soil-atmosphere, no 

 root-growth in any species takes place. 



5. The species mentioned in paragraph 3 occur naturally m sub- 

 strata saturated all or most of the time. It does not follow, however, 

 that all species growing in water have similar oxygen relations. For 

 example. Nasturtium officinale appears to require as high a percentage 

 of oxygen for root-growth as many plants from well-aerated soils. 

 A good condition of aeration in the case of Nasturtium is possibly 

 provided by active water movements. 



6. The studied species of Avena, Hordeum, Oryza, Phalaris, Triti- 

 cum, and Zea can maintain root-growth, although at a slow rate, 

 in a soil-atmosphere containing as little as 0.5 per cent oxygen, re- 

 mainder nitrogen. These species, however, appear not to hold equal 

 relations to a deficiency of oxygen. Thus, Zea is dependent on a fairly 

 good oxygen-supply. 



7. Species from well-aerated soils may have unlike oxygen relations. 

 Thus, in the sandy soil at Carmel may be found Mesembryanthemum 

 and Erigonum, which are relatively insensible to oxygen deprivation, 

 and also Stachys, which, on the other hand, requires a good oxygen- 

 supply. The soil of this habitat is never puddled. Opuntia, Fou- 

 quieria, and Covillea, which are relatively intolerant of poor conditions 

 of soil-aeration, occur on the bajada slopes near Tucson. The bajada 

 soil is coarse and is well aerated most of the year, but it is puddled 

 during the short rainy seasons, at which time gaseous exchange is 

 slow. On the flood-plain of the Santa Cruz River at Tucson, Prosopis 

 velutina occurs under somewhat similar conditions and Medicago 

 sativa is cultivated. The soil of the flood-plain is of fine texture and 

 puddles easily. Indeed, in former times the flood-waters spread as a 



