de;partme;nt of botanicai, research. 63 



illumination, the evaporating power of the air, and the daily curve of 

 stomatal movement. In connection with this work a modification of the 

 porous cup-atmometer has been found to serve admirably as a solar radio- 

 integrator, and promises to be of great usefulness in ecological research. 



Soil-moisture in Relation to Plant Grozvth (by Dr. B. E. Livingston) : 



Progress has been made in the improvement of methods for measurement 

 of moisture conditions in the soil. Data have been obtained of the actual 

 moisture-content of four types of soils in the domain of the Desert Labora- 

 tory for a complete year and a number of other determinations are being 

 made. These, together with the data of precipitation and of general plant 

 activity, will go far toward solving a number of problems connected with the 

 seasonal fluctuations in growth and other functions of desert vegetation. 



The Root-Habits of Desert Plants (by Dr. W. A. Cannon) : 



The study of the mode of branching, extent, and position in the soil of a 

 number of types of annual and perennial desert plants indigenous to the 

 Tucson region has been brought to a stage approaching completion. It has 

 been necessary to take into account the interrelations of the roots of differ- 

 ent species growing in the same habitat, the influence of the substratum, and 

 the amount and character of the rainfall with respect to its periodicity in 

 applying the results to the interpretation of distribution. 



The roots of the more pronounced types of desert species, such as those 

 inhabiting the arid bajadas and mountain slopes, do not penetrate deeply, 

 partly by reason of the presence of rock strata or impenetrable hardpan and 

 partly because the moisture from the rains does not penetrate beyond a very 

 few feet, the amount of soil-moisture being less at 8 feet than at i or 2 feet 

 below the surface. Where rock-crevices containing water are found in the 

 alluvium along streamways, a penetration similar to that in well-watered 

 regions is found. 



A general census of the plants examined shows that the spinose forms, 

 such as Ka:herlinia, send their roots more deeply than those which accumu- 

 late a large water-balance like the cacti. Although no minute examination 

 has been made, there does not appear to be any special type of structure of 

 the absorbent organs of desert plants, their fitness being wholly one of habit. 

 Many of the species observed show a relatively great development of long 

 thin roots capable of rapid absorption with the beginning of the moist mid- 

 summer, and these organs perish with the decrease of the soil-moisture 

 content. 



The thickness of the layer of soil above an underlying hardpan or imper- 

 vious layer naturally affects the character of the root-systems and also the 

 nature of the competition among species. The roots of individuals of the 

 same species may interweave to make very dense tangles, but no direct 

 competition may ensue between different species growing near together, as 



5— YB 



