SPIRAL TORSION. 



323 



but more commonly form a single continuous line; 

 when arranged in leaf-whorls it generally happens 



Fio. 173. Stem of Galium spirally twisted. From a specimen com- 

 municated by Mr. Darwin. 



that some of the leaves are turned downwards, while 

 others are erect. It has been said that this condition 

 occurs particularly frequently in plants growing in 

 damp places. It is certainly true that spiral torsion of 

 the stem is specially frequent in the species of Equi- 

 setum, most of which grow in such spots. In these 

 plants either the whole of the upper part of the stem is 

 thus twisted, or a portion only : thus Reinsch* cites 

 a case in Equisetum Telmateia, where the upper and 

 lower portions of the stem were normal, while the 

 intermediate portion was twisted spiraUy. In this 

 instance the whorl next beneath the spiral had twenty- 

 eight branchlets, and that immediately above it thirty. 

 Along the course of the spire there were two hundred 

 and three ; dividing this latter number by the mean of 



' ' Flora,' Feb. 4, 1858, p. 69, tab. ii, f. 3, and also ' Flora,' 1860, p. 737. 

 tab. vii, f. 9. 



