GROWTH. 363 



observed in the case of a Hop-plant, that the length of the 

 circumnutating portion of the stem was about 15 inches, and 

 the curvature was such that the diameter of the orbit was 

 19 inches: in the case of a plant of Ceropegia Gardnerii the 

 circumnutating stem, 31 inches long, was nearly horizontal, 

 so that the diameter of the orbit was about 5 feet. 



We have dealt, so far, with those phenomena of nutation 

 which would be best observed by looking down from above 

 upon a nutating apex, that is, with the horizontal motion, 

 and we have therefore spoken as if the orbit lay accurately 

 in one plane. We must remember, however, that while the 

 apex of a growing organ is describing its orbit it is also 

 elongating, that it has also a vertical motion ; its path is then 

 such that the orbit does not lie exactly in one plane. The 

 path described by the apex of a nutating organ will of 

 course depend upon the form of its nutation. The apex of 

 an organ exhibiting simple nutation will trace a zig-zag course 

 about the prolongation of the axis of the portion which has 

 ceased to grow, whereas that of an organ exhibiting revolving 

 nutation will trace a spiral of some form. In the latter case 

 the direction of the spiral is not always the same : in some 

 plants (Hop, Scyphanthits elegans^ Tamus communis, Lonicera 

 brachypoda, etc.) the direction of nutation is that of the 

 sun, or of the hands of a watch, in others (Akebia quinata, 

 Wistaria chinensis, Phaseolus vulgaris, Ceropegia Gardnerii, 

 various species of Convolvulaceae, Aristolochia gigas) the direc- 

 tion of nutation is contrary to that of the sun or of the hands 

 of a watch. 



The rate at which the orbit is described varies widely in 

 different plants, as the following observations made by Darwin 

 will shew: 



Time for one complete revolution. 

 Plant. Longest. Shortest. 



H. M. H. M. 



Tamus communis 3 10 2 30 



Hop 2 20 20 



Akebia quinata 4 i 45 



Wistaria chinensis 3.21 2 5 



Ceropegia Gardnerii 7 55 5 J 5 



Adliatoda cydon&folia 48 26 30 



