364 



EFFECT OF DENSITY OF THE MEDIUM [Cn. XIII 



stunt. As the concentration increases from 29%, instead of 

 the grains all being killed or failing to germinate, we have the 

 interesting result that the percentage of germinating individ- 

 uals rises rapidly from 4 to 56. VANDEVELDE hazards the 

 following explanation of this result : " Dilute solutions are 

 easily absorbed ; the more concentrated the solution, the 

 smaller the power of diffusion ; in a saturated solution the 

 seeds do not swell and the action of the surrounding solution 

 is less injurious." But we need to know more of the condi- 

 tions under which this result occurs before we can accept any 

 interpretation. 



With animals we find growth similarly affected as with 

 plants. LOEB ('92) first showed this in his experiments upon 

 the regenerative growth of decapitated tubularian hydroids. 

 The regenerating hydroids were kept in water more and less 

 dense than, and equally dense with, sea water. At the end of 

 eight days the length of the regenerated piece was measured in 

 seven to nine individuals at each concentration. The results 

 may be given in the form of a curve (Fig. 98) by laying off as 

 abscissae the percents of sodium chloride in the water and as 

 ordinates the corresponding average growth in millimetres : 



1 2 



DILUTE SEA WATER 



SEA WATER 



FIG. 98. Curve showing the relation between the density of the medium and the 

 proportional rate of growth of regenerating Tubularia. The maximum ordinate 

 indicates 10.5 mm. growth in 8 days. The numbers at the base of the curve are 

 per cents of density in excess of distilled water; thus "3" signifies a specific 

 gravity of 1.03. (From LOEB, '92.) 



This curve shows that the optimum concentration for growth 

 is not, as might have been expected, the normal concentration, 

 but one considerably below the normal, namely 2.5% instead 



