148 PHYSIOLOGICAL KEGULATIONS 



load being unknown, nothing can be done to test quantitatively the 

 notion that the rate of exchange is proportional to the gradient of 

 believed "osmotic" pressure; it is indeed improbable that the con- 

 centration inside the body is the same in all loads. 



§ 54. Ameba 



The belief is wide-spread that if Ameba regulates its water con- 

 tent, then any animal can. In this freshwater rhizopod, deficits of 

 water (-25% of Bq) are produced by immersion of individuals in 

 0.2 M to 0.4 M lactose-saline solution (Mast and Fowler, '35, p. 160 ; 

 '38, p. 303). Recovery of volume, after return to the weak saline 

 solution in which the Ameba proteus has been reared, proceeds dur- 

 ing the initial 0.5 hour with velocity quotients of about 0.8/hour. 

 In solutions of similar weak concentrations but of other ions, recov- 

 ery is slower. Whether any of the recovery occurs by reduction 

 in rate of output through the path of contractile vacuoles was not 

 ascertained. 



The maximal rate of turnover measured in contractile vacuoles 

 of A. proteus is about 24% of Bo/hour, with a mean at 23° C. of 

 10% of Bo/hour (Adolph, '26, p. 377). The rate of output through 

 vacuoles varies with the salt concentrations of the medium (Miiller, 

 '36, p. 360) ; it is not certain that the body volume is unchanged in 

 these concentrations. 



Excesses of water may be introduced into Ameba dubia by 

 micro-injection. According to Chambers and Reznikoff ('26) the 

 water, even in an amount of one-third the body volume, mixes with 

 the body substance. Measurements of subsequent vacuolar output 

 (Howland and Pollack, '27) indicate faster excretion, lasting 0.1 to 

 0.3 hour, at augmentations 1.16 to 2.9 times the control rate. The 

 response was observed three times in one individual after as many 

 successive injections. Following an injection of one-half the body 

 volume, an augmentation ratio of 3.9 was recorded. 



Apparently the rates of compensation of water content in 

 Ameba are, like Zoothamnium, proportioned to its high rate of 

 turnover. That freshwater protozoa are fully equipped for deal- 

 ing with both deficits and excesses of water is worthy of explicit 

 statement. 



§ 55. Note on plants 



The water relations studied are equally prevalent in plants. 

 Measurements of transpiration and of potometry now call for cor- 



