362 LECTURE XV. 



the action of gravity exhibit only some form of simple or undulating (see 

 infra) nutation. 



We will now discuss the form of the orbit described 

 by the apex of a growing organ which exhibits revolving 

 nutation. The form of the orbit depends upon two factors: 

 first, upon the form of the outline (as seen in transverse 

 section) of the growing organ : secondly, upon the relative 

 rapidity and duration of the more active growth attained 

 in succession by the various segments. If the increased 

 rapidity of growth attained by each segment, and the time 

 of duration of this more rapid growth, be the same for all, 

 it is clear that the form of the orbit will be determined by 

 the form of the transverse section of the region in which 

 the unequal growth is taking place : for instance, if, as is 

 very commonly the case, the transverse section of the organ 

 is approximately circular, then the form of the orbit will 

 be approximately circular. But if one or more of the seg- 

 ments attain a greater rapidity of growth than is attained 

 by the others, or if the duration of the period of more 

 rapid growth is longer in some segments than in others, then 

 the form of the orbit will not correspond to the outline of 

 a transverse section of the organ, but will deviate more or 

 less from it. These factors cooperate to produce the different 

 forms of the orbit which vary from the circular, in which all 

 diameters are equal, to forms in which the longest diameter is 

 very much greater than the shortest, to forms, that is, which 

 approximate to a straight line. We see then that we have 

 all intermediate forms of orbit, from the straight line in simple 

 nutation, to the circular which is the most perfect expression 

 of revolving nutation. It is because of the existence of all 

 these intermediate forms that Darwin has ceased to distin- 

 guish between "simple" and "revolving" nutation, but in- 

 cludes all these phenomena under the one term Circum- 

 nutation. 



The length of the diameter of the orbit is determined by 

 the length of the portion of the growing organ which inter- 

 venes between the apex and the region of nutation, and upon 

 the curvature of the growing portion. For instance, Darwin 



