DEPARTMENT OF BOTANICAL RESEARCH. 93 



than when the parent joint was absorbing distilled water. These 

 were the concentrations of acid used in the swelling experiments and 

 are much lower than the concentration within the plant, which in the 

 species used varies approximately between N/10 and N/20. In solu- 

 tions of neutral salts of strength equimolar with that of the acid used, 

 the growth-rate lay between the acid and distilled-water extremes. 



A final experiment showed that the swelling power of the same joint 

 was much greater at 5 p. m. (period of lowest acidity) than at 5 a. m. 

 (period of highest acidity) , the difference between the acidity values 

 at the two times being greater than could be accounted for by the 

 osmotic-pressure variance due to the day's transpiration. 



From these experiments it seems that there is a parallelism between 

 growth and colloid hydratation, and that acidity, instead of being a 

 favoring factor, is, at least in the plants used, inhibitive to both. 



Further experiments showed that dilute alkalies induced a some- 

 what greater hydratation than distilled water. The interesting fact 

 developed that in nutrient solution — containing K3PO4, MgS04, KNO3, 

 NaCl, and Ca(N03)2 — not only was there an increased growth-rate 

 of cacti over that in distilled water, but also the hydratation of isolated 

 pieces was greater — that is, that the greater growth-rate and hydrata- 

 tion took place where the difference in osmotic pressure was smaller. 

 The field of salt action in colloid hydratation entered here is undoubtedly 

 an enormous one. 



Relation of the Rate of the Growth of Roots to Soil Temperature, 



by W. A. Cannon. 



Studies on the response of the roots of desert perennials to different 

 soil temperatures have been carried on by means of especially devised 

 apparatus, by which the growth of the roots can be observed and the 

 cultures can be kept at any desired temperature for any desired period. 

 As a general result of these studies, it has been learned that represen- 

 tative perennials, such as Prosopis velutina, Fouquieria splendens, and 

 Opuntia versicolor, exhibit characteristic responses to parallel tempera- 

 tures, and that such responses may be unlike. As examples of such 

 reactions, the following measurements on root growth can be given : 



During 42 days of spring (April-May) the roots of the species which 

 were growing in glass tubes and at air temperatures increased in length 

 as follows: Opuntia, 341 mm.; Prosopis, 309 mm. In 13 days in April 

 the root of Fouquieria grew 117 mm., and those of Prosopis and Opuntia 

 181 and 162 mm., respectively. The temperature of the soil was 

 26° C, and over. For a period of 24 days in November-December, 

 the root of Prosopis increased 315 mm. in length, that of Fouquieria 

 136 mm., and that of Opuntia 89 mm. The soil temperature ranged 

 from 12.3° C. to 21.3° C, which is the average of readings made at 



