PHYSIOLOGY OF THE RED MANGROVE. 



649 



tube of standard dextrose solution of known strength is the basis 

 of these records. The standards are in five grades, each being a 

 certain definite percentages of Merck's standard tannic acid, or Kahl- 

 baum's standard dextrose. The amounts by this comparative 

 method of testing were placed in the five arbitrary units, approxi- 

 mating the same color as that for 0.5 per cent, standard dextrose 

 solution on the one hand, and a 0.125 per cent, standard tannic acid 

 solution on the other, with successive dilutions by half of these 

 standard solutions. 



ur^iTS or o£ ^ Tfcoje 



Graph No. 3. 



The tests were made in series of twelve, that is a dozen seed- 

 lings of progressive increase in size were selected from which to 

 make extracts at one time. More could not be handled conveniently 

 at one time, since the length of time required to carry the extracts 

 through the various precipitations, filtrations, etc., gave opportunity 

 for mould spores to germinate in the flasks, a difficulty very hard 

 to control in a warm, moist climate. About two hundred of these 

 tests were made and the various series of twelve seedling-extracts 

 were averaged to obviate errors in judgment regarding color inten- 

 sity, etc. Graph No. 3 illustrates the relation of the two substances 

 in question as they occurred in seedlings of progressively larger 

 growth according to the above tests. The ratio may also be ex- 

 pressed by the equation 3' = /v.r+C, where C approximates 1.05 

 and i<r = %, the ordinates, Y, express units of tannin and the 

 abscissae, X, units of dextrose. By this graph then it is seen that the 



