GRADATION 



201 



There is a distinct breach of continuity [discontinuous 

 gradation) in each of the above curves between the longest 

 ordinate and the next one. This breach is a quantitative 

 expression of the differentiation of the stem (individual of ist 

 order) into two segments (individuals of 2nd order : vegetative 

 part of the stem and panicle). The longest internode is the 

 upper internode of the basal part ; the next one is the first 

 internode of the panicle. I have measured the length of the 

 intemodes of numerous stems belonging to a number of species 

 of Gramineae : the general (ascending-descending) form of the 

 gradation curve is the same in each and all. An exact com- 

 parison between two given stems,^ in respect of the primordia 



zoo 



Fig. 26. — Poa trivialis. Gradation Curve of the length of the intemodes 

 of one fertile stem. /, Base of the stem ; I, V, . . . successive intemodes. 

 Scale in mm. See the figures, p. 200, ist stem. 



(length, etc.) of their intemodes is easily obtained by compar- 

 ing the longest internode and the first internode of the panicle 

 of both stems. The compared intemodes are socially equiva- 

 lent and they represent the maximal value of the length in each 

 of the segments of 2nd order. ^ 



SECOND EXAMPLE : THE FERTILE STEM OF HOL- 

 CUS MOLLIS (A GRASS).— This object has the same con- 

 stitution as the first example. I have measured the following 

 primordia, which are all ordered along the longitudinal axis of 

 the stem : 



(i) The length of the intemodes from the base to the summit 

 (the terminal spikelet being excluded) ; 



1 Belonging to the same or different species. 



* It may be remarked that each of the above series of figures is a bcrtillonage — 

 that is to say, a quantitative description of a specimen complete enough to enable 

 us to distinguish it from any other fertile stem of any grass whatever, because 

 the same combination of figures practically never occurs a second time. 



