M. WICHURA ON THE WINDING OF LEAVES. 273 



helices which the margins of the leaf describe, at the same time 

 as the other parts, must be further removed from the axis in the 

 broad than in the narrow leaf. 



§23. 

 Consequently the greatest number of revolutions, from twenty 

 to twenty-five or even more, are found in the narrowest leaf- 

 structures, e.g. in the awns of the Grasses, the leaves of many 

 species of Gethyllis, &c., while elsewhere the most common 

 number of revolutions is from J to 2. Although accurate pre- 

 determination is mostly impossible here, since the length and 

 breadth of the leaf, which influence the number of revolutions, 

 usually vary in the same plant, still at least approximative deter- 

 minations may be given for each species, and it is to be desired 

 that descriptive botany should include this in the subjects of its 

 observation. In flower-stalks which, as regards size and shape, 

 appear always more constant than the stem-leaves, the number 

 of revolutions of the heliacal windings admits of a tolerably exact 

 definition. 



V. Rapidity of the Movement and Epoch of its Commencement, 



§24. 



The winding movement takes place either rapidly or slowly. 

 In the appendages of the carpels which separate at the epoch of 

 maturity in the Geraniaceae, it is so rapid as to be visible to the 

 eye. In the other cases with which I am acquainted, we can 

 only conclude that a movement has taken place by recognizing 

 after long observation that a change of place has occurred. Days 

 or even weeks may elapse before the leaf has completed even 

 one revolution. 



§25. 



In regard to the epoch of commencement, the movement is 

 connected with the stages of growth of the plant. It here fol- 

 lows definite laws, which however differ very much in different 

 plants. 



§26. 



The " aestivatio contorta " affords the earliest traces of the 

 commencement of a movement of curvature. The extremely 



SCIEN. MEM.— iVa/. Hist. Vol. I. Part IV. 18 



