137 



about the time when the diameter of a spore becomes 

 about 18 or 19 units, there occurs a period of relative 

 rest in their growth. I find just the same thing in the 

 spores of Mucor Mucelagineus which hâve also been 

 measured by Mr. Postma. As has already been pointed 

 out (art. 1), we hâve to consider such a resuit rather as 

 an indication (as a working hypothesis) than as a well 

 established fact. In the présent case there is outside 

 évidence for the belief that our interprétation is the correct 

 one. This évidence, to which my attention was kindly 

 drawn by Prof. Hugo de V r i e s, is to be found in the 

 resuit, arrived at already in 1884 by Prof. Errera, that 

 there is a period of rest in the growth of the sporangia 

 of some of the fungi of the same family. 



Much weight ought not to be attributed to the downward 

 slope of the reaction curve at both its extremeties. As 

 already explained (art. 14, Remark II) the uncertainties 

 near the limits are usually very considérable. They are 

 so in the présent case. 



The fact hère found, that to an abnormal accumulation some- 

 where in the frequency-curve there corresponds a minimum 

 in the reaction curve, is gênerai. The converse holds too; 

 wherever there is in the frequency curve an abnormal dépres- 

 sion, there we will find high ordinates in the reaction curve. 



A good illustration is furnished by 



Example V. Length of wheat-ears, grown under unfa- 

 vorable circumstances (closely sown in poor soil), meas- 

 ured by Dr. C. de Bruyker (Handelingen 13^ Vlaamsch 

 Nat, en Geneesk. Congr. p. 172) (tab. 9, fig. 10). 



The frequency curve is double topped. It is usual in 

 such cases to conclude that we hâve to do with hybrids 

 or with a mixture of two différent species. In the présent 

 case there seems to be no reason whatever for such a 

 supposition. Turning to our solution we find that the 

 reaction curve shows a growth, which, for the smaller 



