STEMS 



76 



common wax-plant of the greenhouses sometimes describes 

 a five-foot circle, the tip moving at the rate of thirty-two 

 inches per hour.^ " This circular motion results from 

 some cause not yet fully understood by botanists.^ 



The direction in which twiners coil about a supporting 

 object is almost always the same for each species of plant, 

 but not the same for all 

 species. In the hop it is as 



Fig. 41.— Coiling of Petiole of Dwarf 

 Tropseolum. 



Fio. 42. — Twining Stem of Hop. 



shown in Fig. 42. Is it the same as in the bean ? in the 

 morning-glory ? 



91. Underground Stems. — Stems which lie mainly or 

 wholly underground are of frequent occurrence and of 

 many kinds. 



In the simplest form of rootstock (Fig. 43), such as is 



1 See article on Climbing Plants, by Dr. W. J. Beal, in the American 

 Naturalist, Vol. IV, pp. 405-415. 



2 See Strasburger, Noll, Schenk, and Schimper, Text-Book, pp. 258-262 ; 

 also Vines, Students' Text-Book of Botany, London and New York, 1894, 

 pp. 769, 760. 



