506 



PART III. THE CLASSIFICATION OF PLANTS. 



composition of the calyx : thus, if the calyx be trimerous or penta- 

 merous, the odd sepal is, as a rule, median, generally posterior or, 

 less commonly, anterior (e.g. Leguminosse) : the tetramerous calyx 

 has usually two sepals in the median plane and two in the lateral 

 (e.g. Philadelphus; Isnardia among Onagraceae; Rhamnus cathartica, 



A 



FIG. 320. Floral diagrams of Jasminum nudiflorum (after Eichler) : A with normal lateral 

 position of the prophylla : B with antero- posterior position of the prophylla : the calyx is 

 hexamerous and dicyclic, the two sepals of the outer whorl alternating with the prophylla : 

 the symmetry of the flower is isobilateral. 



Ilex). In some few cases, when there are two lateral prophylla, 

 the four sepals of an apparently tetramerous calyx are arranged 

 diagonally (e.g. Veronica and other Scrophulariacese, Plantago, 

 probably also Mimosa, Fig. 321); this exceptional position is due 

 to the fact that in these plants the calyx is typically pentamerous, 

 though, in consequence of the suppression of the posterior median 

 sepal, it appears to be tetramerous. 



As a general rule, the genetic relation between the sepals and 

 the prophylla is that the first sepal of a 

 successively-developed calyx arises on 

 the opposite side of the axis (though 

 higher) to the single prophyllum or to the 

 upper (/?) when there are two. Hence 

 the position of the prophylla affects that 

 of the sepals and, consequently, that of 

 all the floral organs. Thus, it is not un- 

 common in the Monocotyledons for the 

 single prophyllum to be placed either 

 obliquely or laterally (some Liliacese, 

 Canna, etc.), and then all the whorls of the flower present a 

 corresponding displacement (Fig. 319 B) ; the same thing occurs 



FIG. 321. Floral diagram of 

 Mimosa, showing the diagonal 

 position of the sepals (after 

 Eichler). 



