M. WICHURA ON THE WINDING OF LEAVES. 279 



X. Change of the Direction in the Winding Leaves of the 

 same plant, 



§39. 



Plants exist in the leaves of which only one direction, to the 

 right or left, is represented, others in which the leaves of one 

 individual exhibit both directions. Such a change of direction 

 in all cases presupposes a difference in the age or situation of 

 the parts of the plant, since two heliacal lines, one turning to 

 the right, the other to the left, form an absolute contrast, mutu- 

 ally exclusive, and so can only occur either on different bodies, 

 that is, in distinct places, or at different epochs of the existence 

 of the same body. 



§40. 



The distinctions which, from the observations I have hitherto 

 made, usually accompany the change of direction, are: 1. The 

 different periods of age of the same leaf; 2. The differentiation 

 of the leaf in reference to point and base; 3. The relative alti- 

 tude of the insertions of different leaves ; 4. The dissimilar lateral 

 insertion, both of the solitary and the verticillate leaves; and 

 5. The metamorphosis of the leaf. 



§41. 



Since, however, the change of direction depends, in some 

 plants on one, in others on another of these distinctions, there 

 exists a great variety in plants with winding leaves, which is 

 increased moreover by the fact that the lateral direction of the 

 helix, formed by the arrangement of the leaves, may affect in an 

 opposite way the direction of the heliacal winding of the leaves 

 themselves, so that the leaves are wound either in the same, or 

 in the opposite direction, to that of the leaf-spiral. We have 

 now to furnish some instances of the effect of those differences 

 of the position or relative age of the plant on the direction of 

 the heliacal winding. 



§42. 



1 . Different periods of the age of the same Leaf, 



The leaves of the inner circle of the perianth oiPuya ccerulea, 

 Micrs, P, guianensisy KL, Bilbergia zebrina — Fam. of Brome- 



