40 



BOTANY 



torsion of their parent stein. Thus the leaves of Pandanus first 

 appear in three straight rows on the vegetative cone, and their 

 subsequent spiral arrangement results from the torsion of the stem. 

 Irregular arrangement of the leaves, such as occurs, for example, on 

 the flower-stalk of the Crown Imperial (Fritillaria imperialis), may 

 result from the unequal size of the leaves at the time of their 

 inception on the vegetative cone. Further observation has revealed 

 an increasing number of cases in which definite leaf arrangements 

 cannot be explained by the spatial relations, and as the effect of 

 contact and pressure. It can only for the present be assumed that 

 the cause of the leaf-arrangement in these cases lies in the inherited 

 organisation of the plant (). 



A frequent mode of arrangement of foliage leaves is the decussate, 

 in which two-leaved whorls alternate with each other (Figs. 31, 33). 

 A whorled arrangement is characteristic of floral 

 leaves. When the number of leaves in each 

 whorl is the same the whorls usually alternate. 

 On the other hand, the number of members in 

 the different whorls of floral leaves will often 

 be found to change, especially on passing from 

 the petals to the stamens, or from the latter to 

 the carpels. In many cases a whorl, the existence 

 of which would be expected from the position of 

 other whorls and from a comparison with allied 

 Fl : .,; - Dia 8 ram r a plants, may be altogether wanting. In this con- 



Liliaceous (lower. The r . ' * * i T 



main axis is indicated nection a comparison of the flowers of the Liliaceae 

 by a black dot, oppo- an( j Iridaceae will be instructive. The flowers of 



site to which is the .1 T . /& . \ j n 



bract the Liliaceae (.tig. 42) are composed of five 



regularly alternating, three-leaved whorls or cycles, 

 viz. a calyx and a corolla (each consisting of three leaves, and, on 

 account of their similar appearance, usually referred to conjointly as 

 the PERIANTH), an outer and an inner whorl of stamens, and finally, 

 in the centre of the flower, an ovary of three carpels. In the flowers 

 of the Iridaceae (Fig. 43) 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 whorl of 

 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 



