INTRODUCTION. I i 



The root-systems of perennials had to be studied in another way and 

 always in situ. After the earth above the most important roots was 

 removed, the root-area of the plant was considered as a square and was 

 surveyed in the following manner. Tapes divided according to the metric 

 system were stretched along the east and west sides of the imaginary square, 

 in a north-and-south direction, always at a certain distance, 1.5 meters 

 from the base of the stem. These tapes were firmly fastened to the ground. 

 A third movable tape was extended in an easterly and westerly direction 

 so that it connected the two permanent tapes. This was also ruled metri- 

 cally. By means of moving the latter tape and noting its position on the 

 stationary tapes, the north-and-south as well as the east-and-west exten- 

 sions of the roots were learned with fair accuracy. The record was made 

 on a square of metrically ruled paper, each square on the paper correspond- 

 ing with an imaginary square of the root-area as delimited by the divisions 

 of the tapes. Wherever possible a reduction to one-tenth was employed in 

 sketching the roots and preparing the field charts. 



The data on the temperature of the air and soil, on water in the soil, and 

 on its physical nature are taken from records made at the Desert Labora- 

 tory or from studies made elsewhere but now available at the Laboratory. 

 The soil temperatures are mostly from the continuous soil-thermograph 

 record which has been kept since 1904-05, and partly from thermometer 

 readings made by Dr. V. M. Spalding. The other data on the soil are 

 mainly from studies by Dr. B. E. Livingston (Distribution and Movements 

 of Desert Plants, Carnegie Institution of Washington, Publication 113, pp. 

 83-93)- Acknowledgment should also be made to Prof. J. J. Thornber, 

 University of Arizona, for the determination of several of the plants studied, 

 as well as for much information on the distribution of the plants of the 

 domain of the Desert Laboratory (Vegetation Groups of the Desert Labora- 

 tory Domain, Carnegie Institution of Washington, Pub. 113, pp. 103-112). 



