9£ 



of plant roots in the soil with relation especially 

 to temperature and aeration. Dr. Cannon's data have 

 not yet been published in detail and I am indebted to 

 him for fuller memoranda of his results than have ap- 

 peared in the preliminary notices which have been publish- 

 ed. His technique differs essentially from any hereto- 

 fore employed in this field. A small plant of the 

 species to be examined is grown in sand in a glass 

 tube 2 cm. in diameter and 30 cm. long. This tube 

 can be sealed at the top, around the plant, with modeling 

 clay or other plastic. and side tubes at the top and 

 bottom permit the control of the atmosphere inside 

 the culture tube. 7/ater can be supplied when necessary 

 through a perforation in the seal at the top and can 

 be allowed to drain off at the bottom. Under these con- 

 ditions some one or more of the plant roots will lie 

 close to the glass tube and can be observed through it. 

 The method consists in observing the rate of elongation 

 of such an individual root over short periods v;liQn the 

 atmosphere in the tube is of known character. Observa- 

 tions are made by a horizontal microscope or by an auto- 

 matic device w.hich photographs the position of the root 

 tip at regular time-intervals. The rate of elongation 

 is first measured in air, then the gas under examination 

 is allowed to flow through the tube and the rate of elon- 

 gation is measured again. The temperature must be kept 

 constant. 



