MORPHOLOGY 



41 



successive nodes of a stem upon a plane by means of a series of 

 concentric circles, on which the position of the leaves may be 

 indicated (Fig. 44). The angle made by the intersection of the 

 median planes of any two successive leaves is called 

 their DIVERGENCE, and is expressed in fractions of 

 the circumference ; for example, when the angular 

 divergence between two successive leaves is 120, 

 their divergence is expressed by the fraction |. 

 In the adjoining diagram (Fig. 44) a f divergence 

 is shown. Where the lateral distance between 

 two successive leaves is -?- of the circumference of 

 the stem, the sixth leaf is above the first, the 

 seventh above the second, and so on. The leaves 

 form on the axis five vertical rows, which are 

 spoken of as ORTHOSTICHIES. Where the leaves 

 are very much crowded, as in dwarf-shoots, a 

 set of spiral rows called PARASTICHIES, due to 

 the contact of the nearest laterally adjacent mem- 

 bers, becomes much more noticeable than the orthostichies. If the 

 surface of such an axis be regarded as spread outl horizontally, 

 the parastichies become at once distinguishable (Fig. 45), and it will 

 be evident that the sum of the parastichies cut by every cross-section 



FIG. 43. Theoretical 

 diagram of the flower 

 of the Iris. The ab- 

 sent whorl of stamens 

 is indicated by 

 crosses. 



FIG. 44. I )iagram showing j position of 

 leaves. The leaves numbered according 

 to their genetic sequence. 



Fio. 45. The \ position on the outspread 

 surface of the axis, o, Orthostichies ; p, 

 parastichies. The leaves are numbered 

 according to their genetic sequence. 



through such an axis must equal the number of the orthostichies. 

 On objects like pine cones, in which the parastichies are easily 

 recognised, they may be used to determine the leaf arrangement. 

 If a line be drawn on the surface of a stem, so as to pass in the 



