8 BULLETIN 1018, U. S. DEPARTMENT OF AGRICULTURE. 



continued through a longer period during the day than on the other 

 plats. Increasing the soil moisture in plat 1 was effective in pre- 

 venting this wilting for only a short time. In six or seven days 

 subsequent to an irrigation the plants would again show a loss of 

 turgor of the leaves under the heat of the sun toward the middle of 

 the day. 



Balls (-5), in considering the functions of the cotton root system, 

 states that "since deficient water absorption is the real limiting 

 factor in the sunshine effect, it follows that the size of the root sys- 

 tem is the true limiting factor of water absorption and that the 

 humidity of the soil may vary between wide limits without affecting 

 it." In the effort to determine whether there was any difference in 

 the root systems of plants on the four plats which might account 

 in part for a deficiency in water absorption by the larger plants, 

 excavations were made on March 1, 1920, and several plants were 

 removed from the ground with all of the roots that could be traced 

 in the upper 18 inches of soil. It was found that the roots had 

 lived through the winter, and only a few of those uncovered showed 

 any signs of decay. A count was made of all lateral roots attached 

 to the 18 inches of taproot, a summary of which is given in Table III. 



Table III.— Count of lateral roots attached to top 18 inches of the tapi-oot of 



Pima cotton in 1919. 



Plat. 



No. 1. 



Mean num- 

 ber of 

 laterals. 



34. 6± 1.56 



Number 



of 

 observa- 

 tions. 



13 



No. 2 J ! 32. 1± 1. 49 15 



No. 3 ; 24.4±1.0S 17 



No. 4 1 3S. 2± 1. 09 30 



It is not a far-fetched supposition that the earlier irrigation re- 

 ceived on June 10 by the plants on plats 1 and 2 would have encour- 

 aged the production of lateral roots near the surface of the ground 

 and that the roots of plants not receiving early irrigations would 

 have been forced to make their way into the deeper subsoil in order 

 to obtain moisture, while the formation of laterals in the dry topsoil 

 would have been somewhat suppressed. It appears, from the results 

 shown above that this did not occur. While a considerably greater 

 number of laterals was found on plants from plats 1 and 2 than 

 on those from plat 3, the greatest number of laterals was found on 

 the plants removed from plat 4. It is not inconceivable that the 

 liberal application of fertilizer supplied to plat 1 might have some 

 bearing on the occurrence of this large number of surviving laterals 

 in the topsoil, but until more detailed investigation has been com- 



