SECT. I 



MORPHOLOGY 



41 



stamens were present. From this similarity of arrangement, 

 despite the absence of the one cycle of stamens, the conclusion 

 has been drawn that, at one time, the inner row of stamens 

 was actually present, but has now disappeared. In constructing 

 a THEORETICAL DIAGRAM of the Iridaceae the missing cycle of 

 stamens is indicated by some special sign (by crosses in Fig. 43); 

 a diagram in which theoretical suppositions are not taken into 

 consideration is called an EMPIRICAL DIAGRAM. Diagrams showing 

 the alternate arrangement of leaves, in cases where only a single 

 leaf arises from each node, may be constructed by projecting the 

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



Fig. 44. — Diagram showing § position of 

 leaves. Tlie leave.s numbered according 

 to their genetic sequence. 



Fig. 45. — Tlie f position on the outspread 

 surface of tlio axis, o, Oithosticliies : p, 

 parastichies. Tlie leaves are numbered 

 according to their genetic sequence. 



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 -| divergence is shown. Where the lateral distance 

 between two successive leaves is f 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, a set of spiral rows called parastichies, due to the 

 contact of the nearest laterally adjacent members, becomes much 

 more noticeable than the orthostichies. If the surface of such an 

 axis be regarded as spread out horizontally, the parastichies become 

 at once distinguishable (Fig. 45), and it will be evident that the 



