DISCONTINUOUS VARIATION. 



43 



diameter of stem taken at half the height, and the width 

 of the largest leaf at its widest part, are reproduced in 

 Figs. 11, 12, and 13. Here we see that the stem 

 height shows no differentiation, the curve being more 

 or less symmetrical. The mid-stem diameter shows a 

 slight second hump, but this is obviously insufficient to 

 indicate the presence of two species. The leaf width, 



Dm.T 8 9 101112131415161718192021 



Stem-Height 



FIG. 11. Distribution of Stem-Heights in Typha. 



however, shows a marked differentiation, the index of 

 isolation being 75 per cent. Of the other characters 

 measured, the diameter of the stem at its base, the 

 diameter of the pistillate spike, and the interval be- 

 tween the staminate and pistillate spikes, had indices of 

 isolation of respectively 79, 89, and 83 per cent., or 

 showed even greater differentiation than the leaf width. 

 The curve for the pistillate spike length was, however, 

 symmetrical. Thus this plant shows distinct differen- 



