398 DLSTRIISU'l'ION OF THE GREEN LEAVES OX THE STEM. 



originates below and ends above exactly like tlie one ovei* and the one under it, and 

 the only difference rests in the fact that the sections closer to the summit of the 

 branch have smaller diameters, and often also a somewhat different outline of their 

 members. The plan of construction is, however, as stated, exactly the same in the 

 successive stories. 



In those instances where each story consists of two whorls of leaves, which are 

 displaced with regard to one another through a certain angle, especially in the very 

 common case where the whorl is two-membered, i.e. where the leaves are opposite 

 one another in paii's, and where the successive pairs of leaves are alternately 

 displaced through a right angle from one another, appearing thus like a cross, the 

 leaves are said to be decussate. This arrangement is seen especially in maples and 

 ashes, in lilac and olive-trees, in elder and honeysuckle, in labiates, gentians, 

 Apocynacete, and numerous other families of plants. 



Still more common than this arrang-ement of the leaves is that which has been 

 called the spiral. Here at one and the same height only a single leaf originates 

 from the stem, and therefore all the leaves of a stem are not only shifted with 

 respect to one another in a horizontal, but also in a vertical direction. If one 

 imagines the nodes of a stem with decussate leaves to be so arranged longitudinally 

 that the leaves are inserted no longer at the same heights, but at definite intervals 

 above one another, then from the decussating, i.e. whorled, arrangement a spiral is 

 produced. In many willows {e.q. Salix purpurea), and very regularly also in some 

 bucktiiorns {e.g. Rhainvus cathartica), in the speedwells {e.g. Veronica sp'uxita and 

 longifolia), and also in many composites leaves ai'ranged jjartly in whorls and 

 partly in spirals occur on the same axis, and doubtless the one merges into the 

 other, but for the sake of clearness it is better to keep them distinct, and to draw 

 a line between them, even though it be an imaginary one. 



It niay be observed that stems with spirally-arranged leaves are constructed 

 exactly like those which bear leaf-whoi-ls, and that they consist of many stories 

 each displaying a similar plan of construction, so that the number, position, and 

 distribution of the leaves is repeated in each story, and as a matter of fact the 

 followin ; plans of construction are actually to be found very frequently. 



First case. In each story only two leaves arise from the circumference of the 

 stem. These two leaves are displaced with regard to one another in a horizontal 

 as well as vertical direction, and their horizontal divergence amounts to half the 

 circumference of the circle (180°) as shown in the plan in fig. 99 ^. If a continuous 

 line be drawn from the point of insertion of each lower older leaf to the younger 

 one next above it on the surface of the stem, this will display the form of a spiral. 

 It has been called the genetic spiral. In the first case here discussed it forms in 

 each story only a single spiral band. This arrangement is repeated in the second, 

 third, and perhaps in many other stories which follow successively on the same 

 axis. In this way the lower leaf of the second, third, or fourth story alwaj's lies 

 exactly above the lower leaf of the first story. The same applies to "the upper 

 leaves of all the stories. Thus two rectilineal lines or oi-thosticliies ai-e formed on 



