206 



EVOLUTION OF PLANTS 



of the flower as well ; but in a larger number the carpels 

 are more or less completely coherent, and the sepals are 

 also frequently united into a cup-shaped or tubular calyx. 

 The lowest of the Choripetalse are the Arnentacese, so 

 named from usually having the simple flowers in elon- 

 gated catkins or aments. The willows (Fig. 49, A-D), 

 poplars, and various nut-trees are familiar examples of 

 this group. A second order allied to these includes the 



FIG. 49 (Lower Dicotyledons Amentaceae, Centrospermfe). A, male 

 inflorescence of a willow; B, an individual staminate floWer consist- 

 ing of two stamens surrounded by inconspicuous bracts ; C, a female 

 inflorescence of a willow, each flower (D) consisting of a single pistil 

 made up of two coherent carpels; E, flower of a knot-grass (Polygo- 

 num) : the perianth consists of five colored sepals ; F, the pistil of E, 

 with the side of the ovary cut away to show the single ovule borne at 

 the apex of the floral axis : G, section of the flower of a scarlet catchrly 

 (Silene) ; the sepals are united into a tube enclosing the free petals and 

 stamens : the petals are showy, and the flower is pollinated by insects ; 

 H, diagram of the flower of Silene ; the pistil is composed of three 

 carpels ; the central axial placenta bears numerous ovules. 



pepper family, a tropical group which superficially, at 

 least, shows a curious similarity to the aroids, and may 

 prove to comprise connecting forms between Mono- 

 cotyledons and Dicotyledons. In these low types the 

 flowers are often diclinous, i.e. stamens and carpels are 

 in separate flowers and no perianth is present, or the 

 perianth is reduced to inconspicuous scales. It is gen- 

 erally supposed that these amentaceous Dicotyledons 



