506 



BOTANY 



for that purpose. Other frequently cultivated plants belonging to this family 

 are the Buckwheat, Fagopyrum esculentum, and the different species of garden 

 Rhubarbs. 



Fig. 456. — JUieum oflwiTude. A, Flower ; B, the same cut through longitudinally ; C, gynoecium with 

 disc ; Rheum compactum, D, fruit. (After Luhssen, magnified.) 



Officinal. — The rhizome of Rheum officinale (Figs. 455, 456) and R. palma- 

 turn var. tanguticum is the officinal Radix Rhei. 



Order 4. Centrospermae 



Flowers hermaphrodite, usually hypogynous, pentamerous with 

 calycoid perigone, or with calyx and corolla, rarely naked ; 

 androecium haplostemonous or diplostemonous ; ovary commonly 

 unilocular, with a single, basal ovule, or with a free-central 

 placenta and numerous campylotropous ovules ; seeds with peri- 

 sperm and a curved embryo. 



The Centrospermae are for the most part herbaceous, rarely woody 

 plants with simple, exstipulate leaves. The flowers are either incon- 

 spicuous, white or highly coloured, according to the method of, pollina- 

 tion. As regards their structure, the flowers of the different members 

 of this order may be arranged in an ascending series, beginning with 

 the simplest forms, resembling those of the Urticaceae and gradually 

 advancing to the more highly developed, constructed after the penta- 

 cyclic, pentamerous type, characteristic of the Dicotyledons, and having 

 a perianth differentiated into calyx and corolla. The Centrospermae 



THUS LINK TOGETHER THE APETALOUS AND COROLLATE DICOTYLEDONS. 



The unilocular character of the ovaries in most members of this 

 order is due, no doubt, to the disappearance of the dissepiments, as 

 in some cases they are partly retained (Fig. 458). 



In the simplest cases the flowers consist typically of three whorls (e.g. Cheno- 

 podiaceae) ; the number of the whorls is in other instances increased to five (e.g. 

 most Caryophyllaceae), but in other cases it is reduced again, by suppression, to three 

 (e.g. the Caryophyllaceous Paronychioideae). At the end of the series, accordingly, 

 flowers occur with a structure apparently similar to those at the beginning ; but 



