LABORATORY FOR PLANT PHYSIOLOGY. 6J 



ments are greater at higher than at lower temperatures for other organs than 

 roots and the same should hold with these organs. 



Although the data at hand are not sufficient to permit satisfactory compari- 



T 



son of the -^ for the different species studied, yet they suggest the lines on 



which such comparison might be instituted. For example, it is instructive 

 to examine what species were found to have normal growth, or a ratio of 1, 

 at parallel temperatures, and under what oxygen partial pressure. Thus, at a 

 soil temperature of about 17°, normal growth was found in Potentilla 2 per 

 cent oxygen; in Zea 10 per cent oxygen. At a temperature of 20° to 22° 

 normal growth took place in Salix in 1.6 per cent oxygen and in the rough 

 lemon with 2.8 per cent oxygen; and, finally, at a temperature of 27° the 

 growth was at the normal rate in the Brazilian sour orange in 1.2 per cent 

 oxygen and in Allium in 3 per cent oxygen. So far as the species mentioned 

 are concerned, therefore, the results suggest the relatively great tolerance for 

 oxygen deficiency of Potentilla, Salix, and the orange as contrasted with the 

 tolerance of Zea, the lemon, and Allium. A comparison of the growth 

 ratios for higher soil temperatures should yield even more striking results. 



Diffusion of Oxygen through Leafy Bases op Herbaceous Plants and 



THROUGH Bulbs. 



During the course of the experiments on the oxygen relations of roots it 

 has occasionally been observed that there is sometimes an inward diffusion 

 of oxygen not to be accounted for by faulty seals or connections and appar- 

 ently attributable only to diffusion in some manner through the plant itself. 

 This was seen in Potentilla, Zea, and Allium especially, and was not found 

 in young plants of other species, as Prosopis, in which the leaves take their 

 origin wholly above the seals of the cultures. It was concluded, therefore, 

 that the enriching of the soil atmosphere with oxygen was in some manner 

 connected with plants with overlapping leaves or scales. 



In the earlier experiments with Allium, Potentilla, and Zea, the base of the 

 plant was sealed within the tube in such manner that it was included with 

 the subterranean portion beneath the seal. Under such conditions there was 

 no outward gas leak, but analyses showed occasionally that the experimental 

 atmosphere contained as much as 1 per cent more oxygen than the gasometer 

 from which it was taken. Blank controls indicated that the inward diffusion 

 was not associated with connections or seals. In the case of Allium, when 

 the bulb was coated with vaseline or sealing-wax, the variation in oxygen- 

 content was much less than in the cultures with uncoated bulbs, and when 

 the seal was at the crown of the main root, the bulb remaining outside the 

 culture, there was no inward diffusion. Similar results were obtained in 

 Potentilla and Zea. In the former, the cut ends of the root-stock were waxed 

 and the seal placed below the origin of the leaves, under which condition 

 inward diffusion of oxygen ceased. 



The oxygen may have been derived, in part at least, from the tissues of the 

 plants or from between the closely overlapping leaf bases or scales. Owing, 

 however, to the apparently unequal capacity of these organs to contain the 

 gas, which did not appear to be reflected in the amount of oxygen added to the 

 experimental atmospheres used, such tissues were not considered to be the 

 sole source of the supply. This conclusion was supported by the relatively 

 long period over which the inward diffusion continued. It appears probable, 



