42 THE STRUCTURE OF FLOWERS. 



On tTie other hand, whorls of threes, and fives, and others 

 in flowers are compressed cycles of spiral arrangements. They 

 are, therefore, attempts at simulating ancestral or the verti- 

 cillate conditions, but cannot possibly be primitive whorls 

 themselves. That the petals can thus become decussating 

 with the sepals is a result of the fact that their cords are not 

 strictly superposed to and confluent with those of the latter. 

 The total number of cords in the pedicel being usually limited 

 to the same number as there are parts in the perianth, i.e. 

 the calyx and corolla together, there is ample room for them 

 to arrange themselves at equal angular distances around the 

 central medulla of the pedicel. Then from the vascular 

 cylinder thus formed, they pass off into the sepals and petals 

 respectively.* 



The sepals and petals or the two whorls of a perianth 

 being thus provided for as to their fibro- vascular cords, the 

 stamens and carpels, as already stated, generally depend upon 

 these latter for their positions, and various arrangements 

 arise according as the cords of the perianth-leaves give off new 

 members or not. Theoretically there should be at least one 

 whorl of stamens superposed to the sepals, another superposed 

 to the petals, and two whorls of carpels as w^ell ; but while 

 many flowers have both staminal whorls {Caryophyllece, 

 Leguviinosce, Ericacece, etc.), many others, as the Gamopetalce 

 retain only one, and more generally the first formed or 

 sepaline, but sometimes it is the petaline, as in Primulacece; 

 the probable cause in each case being certain exigencies in 



* That foliar organs possess this power of rearranging themselves 

 according to requirements is evident from other considerations ; thns, 

 many plants having freely growing erect shoots — as, for example, 

 the common Laurel — have their leaf -arrangements represented by the 

 fractions | or |, but when extending horizontally, as in the usual con- 

 dition, they are distichous. Similar features are seen in the Jerusalem 

 Artichoke, which often changes its phyllotaxis on the same stem. 



