598 PART III. THE CLASSIFICATION OF PLANTS. 



and may be regarded from the two points of view in both orders (see 

 Lauraceae). 



The typical structure of the flower is to be found in the genus Pterostegia. 

 As regards the perianth, it is dimerous in Oxyria, it is 5-leaved in most species 

 of Polygonum, in Coccoloba, Muehlenbeckia, etc., and is apparently quincuncial 

 () ; however, the development of the flower shows that this is not really the 

 case, but that it is the result of reduction by the absence of the anterior seg- 

 ment of the inner perianth-whorl : it may be mentioned further that when the 

 perianth is trimerous, there is (if any) but a single prophyllum, whereas when it 

 is pentamerous there are two prophylla. As regards the andrcecium, in a trimer- 

 ous flower, only a single whorl of stamens may be developed ; either the outer 

 (e.g. Eumex, Kcenigia, as also in the dimerous flower of Polygonum diospyri- 

 folium), or the inner (Leptogonum). Duplication obtains in the single whorl of 

 stamens of Kumex and Polygonum diospyrifolium, and in the outer whorl of 

 many genera, in which either all the typical number of stamens are duplicated 

 (e.g. Kheum, Oxyria), or some only (e.g. species of Polygonum, in which also 

 suppression is frequent). It is important to note that the number (usually 5-8) 

 of stamens in a flower with a pentamerous perianth bears no direct relation to 

 the number of the perianth-leaves ; whereas in dimerous flowers the number of 

 the stamens is a multiple of 2. In some rare cases the number of the stamens 

 is large (12-50). When two whorls of stamens are present, the outer being dupli- 

 cated, it is frequently the case (e.g. species of Polygonum) tbat the anthers of 

 the external stamens are introrse and those of the inner stamens extrorse. 

 The gynseceum is sometimes dimerous when the perianth is pentamerous (e.g. 

 Polygonum orientale, amphibium and other species) ; always when the perianth 

 is dimerous. The endosperm is sometimes ruminated (Coccoloba). The British 

 genera are the following: Polygonum, the Bistort or Knot-Grass, of which 

 there are many species (e.g. Bistorta, Hydropiper, Persicaria, amphibium, avi- 

 culare, Convolvulus) ; Kumex, the Dock or Sorrel, the leaves of which contain 

 a large quantity of oxalic acid ; Oxyria, the Mountain Sorrel. 



The most important economic plants are : Kheum, the Khubarb ; the 

 Khubarb of the pharmacopoeia is obtained from H. palmatum tanguticum and 

 R. officinale, (China) ; the garden Rhubarbs are jR. rhaponticum and undulatum: 

 Fagopyrum esculent-urn, the Buckwheat. 



Order 6. CERATOPHYLLACE.S. Submerged water-weeds of doubtful 

 affinity, with whorled sessile leaves dichotomously branched and 

 subdivided ; in the axils of some of these the diclinous flowers 

 occur. The <$ flowers consist of from 6-12 perianth-leaves and 

 about as many stamens ; the ? flowers have a similar perianth and 

 a monomerous ovary with a single suspended orthotropous ovule : 

 seed with endosperm. 



Ceratophyllum demersum and submersum occur submerged in ponds and 

 ditches. Pollination takes place under water, and in correlation with this the 

 wall of the pollen-grain is not cuticularised (see p. 434). 



Cohort III. Caryophyllales. Flowers cyclic, generally actino- 



