62 THE PALM-STEM: 



He starts from the fact, that the leaves of the arborescent 

 Monocotyledons, e. g. of Yucca, lie in a heliacal line, 

 which in the flattened bud of these plants passes into a 

 true spiral. The heliacal line marks a constant relation ; 

 this does not exist in the spiral, for this passes, with the 

 further development of the leaves lying in it, and the 

 conversion of the bud into stem, into the heliacal line 

 running upon the side of the stem ; in this, the same 

 lines still remain, since they always gain at their inner 

 end as much as they lose at their outer end by passing 

 into the form of a helix. Each leaf originates in the 

 centre of the bud, and passes, when a new leaf arises 

 in the centre, into the place of the preceding one, till 

 at last it comes to be situated on the outer surface of 

 the stem. In this movement each leaf follows a spiral 

 course, in which the mutual relations of the leaves remain 

 the same, and all the leaves are carried back uniformly. 

 Each leaf therefore exhibits, besides the movement from 

 within towards the outside, and from above downwards, 

 also a horizontal motion in a spiral direction ; on the first 

 of these motions depends the divergence of the vascular 

 bundle from the vertical line ; the second gives rise to a 

 divergence in the horizontal direction, since the upper 

 part of the vascular bundle, running from the centre of 

 the stem to the leaf, follows the lateral motion of the 

 latter. Since the turns of the spiral are closer in the 

 middle, and the motion of the leaves becomes slower in 

 proportion as they pass further out, the greatest bending 

 of the vascular bundles takes place in the centre of the 

 stem (1. c. p. 16.) 



Against the idea that, in the development of the ter- 

 minal bud into stem, the leaf-spiral passes into a helix, 

 and that each leaf traverses the length of this spiral, no 

 objection can be urged ; this traverse of the spiral, how- 

 ever, is only a seeming movement, and by no means con- 

 nected with an actual motion to the side. If Meneghini's 

 idea were correct, it is clear that the most external leaf 

 seated on the spiral, could not advance with it in a lateral 



