40 



BOTANY 



PART I 



affected when the size of the rudimentary leaves diminishes while that 

 of the vegetative cone remains the same, or when the leaf-rudiments re- 

 main of the same size but the circumference of the apical cone decreases. 

 In developing flowers- sudden changes in the relative position of 

 the parts often occur in which the apical cone undergoes rapid 

 increase in size while the leaf rudiments become smaller. Changes 

 in the usual position of the leaves may also be occasioned by the 

 torsion of their parent stem. 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 increasine; number of cases in 

 ^ which definite leaf arrangements 



cannot be explained by the spatial 



relations, and as the eff"ect 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 arrange- 



FiG. 43. — Theoretical 

 diagram of the flower 

 of the Iris. The absent 

 whorl of stamens is 

 indicated by crosses. 



Fig. 42.— Diagram of a ^ j- r i • i -^.11 



liiiiaceous flower. The ii^eut 01 toliage Icaves IS the decus- 

 main axis is indicated sate, in whicli two-leaved whorls 

 byabiack dot, opposite .,|teniate with cach other (Figs. 



to which IS the bract. ^ ~ 



3 1 , 3 3 E). A whorled arrangement 

 is characteristic of floral leaves. AVhen the number of leaves in each 

 whorl is the same the whorls usually alternate. On the other 

 hand, the number of members in the diff'erent 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 Avould be expected 

 from the position of other whorls and from a comparison Avith. 

 allied plants, may be altogether wanting. In this connection a 

 comparison of the flowers of the Liliaceae and Iridaceae will be 

 instructive. The flowers of the Liliaceae (Fig. 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 conjointl}'' as the 

 rERiANTii), 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 whoi'l, that of the inner cycle of stamens, is lacking, but 

 the three carpels are situated exactly as if the missing Avhorl of 



