TRUE & HARVEY: ABSORPTION OF CALCIUM SALTS 505 



by the distilled water, and seems to represent a fairly well-defined 

 irreducible minimum. This point is marked by a lower ion content 

 than the similar minimum seen in the distilled water. It seems clear 

 that the substances giving to distilled water its conductivity are not 

 absorbed to as dilute a minimum concentration as is calcium nitrate. 

 It will be noted that only in the two more concentrated members 

 of the series are there any absorbable electrolytes remaining at the 

 end of the experiment, and therefore only in these cultures is the total 

 capacity of these plants to absorb this salt measured. In these cases 

 the total absorbed salt equals 565 and 713 grm. norm. X io~^ re- 

 spectively. 



An inspection of the graphs shows that in nearly all solutions of 

 the salt there occurs a time at which the plants reduce the conductivity 

 of the solution to a minimal concentration, a point that may be 

 assumed for present purposes to represent that of maximum absorption. 

 In some cases experiments have been closed before this point has been 

 reached in a number of members of the series (usually the more con- 

 centrated ones), owing to the well-marked exhaustion of the plants 

 in a number of the cultures. In such a case the absorption maximum 

 may not have been reached, although probably in most cases it has 

 been approached. 



It is interesting to compare the original concentration of the differ- 

 ent solutions with the corresponding concentration at the time of 

 maximum absorption. By this means one is able to ascertain how 

 much of the salt is absorbed or how much net loss the plants have 

 suffered calculated at the time of greatest absorptive efificiency. 



Carrying out the calculation referred to for the cultures included in 

 this experiment the results seen in the following table (i) are obtained. 



A glance shows that about 37.0 grm. norm. Ca(N03)2 are required 

 by five squash seedlings growing in 500 cc. of solution to enable them 

 to protect such ions as are mobilized from their reserves against the 

 leaching action of the distilled water. This stated in terms of the 

 quantity of Ca(N03)2 per plant would be about 0.00028 grm. absorbed 

 during a period of about two weeks. 



As the salt content is increased, absorption increases in approxi- 

 mately the same magnitude leaving an unabsorbable residue of 

 approximately 34 grm. norm. X IQ-" Ca(N03)2. This minimum is 

 here calculated as Ca(N03)2, but it should be clearly understood that 

 the substances actually composing this residue may be, and probably 

 in considerable part are, of quite other composition. Indeed, the 

 substances indicated as Ca(N03)2 at all stages of the experiment, 

 especially the later ones, doubtless consist in part of other materials, 

 largely coming by exosmosis from the interior of the test plants. The 



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