LABORATORY FOR PLANT PHYSIOLOGY. 57 



first on roots per se, to be followed by investigations of root-shoot relations. 

 Thus step by step, as the progress of the work indicates, different phases of 

 the topic are brought into focus and subjected to scrutiny. The present report 

 deals mainly with introductory studies on the relation of roots growing in a 

 deficiency of oxygen in the atmosphere of the soil to the soil temperature. 



Experimental Methods. 



The methods employed in the investigations on various activities of roots 

 are in conformity to the present trend of experimentation on the relations of 

 plants to their physical environment, in that, so far as possible, such environ- 

 mental factors are brought under control and the influence of each studied and 

 evaluated. 



The investigations are at present confined wholly to roots and no attempt 

 is made to control the subaerial environment. The methods employed have 

 the following especially important features : The plants experimented with are 

 grown in sand and in glass tubes of a size sufficient to permit a fair amount of 

 root extension and growth in length. The culture tubes are provided with 

 inlet and outlet lateral tubes for the ingress and egress of the experimental 

 gases used. They are kept in thermostats at an angle sufficient to induce the 

 tip of the roots to follow the inner concave surface and thus to be in view. 

 The thermostats as employed are accurate to about 1° C. and are maintained 

 at temperatures between 18° and 30° C. Commercial nitrogen, carbon 

 dioxide, and oxygen are used. The gases are stored at pressure in cylinders 

 from which they are passed into gasometers, where they are mixed as desired, 

 and from which the gaseous mixture is passed under slight pressure to the 

 culture tubes. Volumetric determinations are made of the amount of oxygen 

 present in each culture before and after each experiment or at any other 

 desirable time. These are accurate to about one-fifth volume per cent. 



It can be pointed out that the method as employed is apparently appro- 

 priate for use in connection with root studies, aside and in addition to those 

 directly concerned with soil temperature and the oxygen supply, including, 

 among others, investigation on root respiration in which quantitative deter- 

 minations of the carbon-dioxid-oxygen ratio are made, studies on the immediate 

 effect on root-growth of soil solutions different in H-ion contents, as well as 

 work on root-growth in sterilized and unsterilized soils, special phases of the 

 activities of roots, and others. 



A detailed account of the methods used and of the apparatus employed 

 is in course of preparation. 



Oxygen Deficiency in the Atmosphere of the Soil as a limiting Factor in the Rate 



OF Growth of Roots. 



Temperature is known to directly affect the rate of respiration. Thus 

 at medium temperatures, 10° to 15° C, the respiratory ratio is least. The 

 value of the ratio increases at lower temperatures and also at higher temper- 

 atures up to about 40°, above which it remains constant until death of the 

 plant takes place. The value of the respiratory ratio is apparently unaf- 

 fected by the partial pressure of oxygen, at least in aerobes when the defi- 

 ciency of oxygen is not extreme. It varies with acidity among other factors, 

 and hence plants of unlike habit of growth may exhibit different values of the 

 respiratory ratio. 



Although an increase in temperature is usually accompanied by more 

 active respiration, the consumption of oxygen, as referred to above, may not 



