MORPHOLOGY 



the arrangement is exactly similar, except that one whorl, that of 

 the inner cycle of stamens, is lacking, but the three carpels are 

 situated exactly as if the missing cycle of stamens 

 were present. From this similarity of arrangement, 9 



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. 41) ; a diagram in which theoretical sup- 

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

 indicated (Fig. 42). The angle made by the intersection of the 

 median planes of any two successive leaves is called their divergence, 



Fig. 41. — Theoretical 

 diagram of the flower 

 of the Iris. The ab- 

 sent cycle of stamens 

 is indicated by 

 crosses. 



Fig. 42.— Diagram showing £ position of 

 leaves. The leaves numbered according 

 to their genetic sequence. 



Fig. '43. — The % position on the outspread 

 surface of the axis, o, Orthostichies ; p, 

 parasticliies. The leaves are numbered 

 according to their genetic sequence. 



and is expressed in fractions of the circumference ; for example, in 

 case the angular divergence between two successive leaves is 120°, 

 their divergence is expressed by the fraction J. In the adjoining 

 diagram (Fig. 42) a f- divergence is indicated. 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, 



