SUPPLEMENT. 



THE FAMILIES OF FLOWERING PLANTS. 

 By Chaeles Louis Pollaed. 



CH.\PTER XVn. 



Orders Polygonales and Centrospermae. 



The order Polygonales comprises the single family Polygonaceae, 

 and its characters are those of the family. The Centrospermae, on the 

 other hand, form a group of considerable size, embracing no less than 

 ten families, of more or less economic or ornamental interest. This 

 order is distinguished from the Polygonales by the variously curved or 

 coiled embryo * in the seed, and by the fruit, which is not an achene. 

 The flowers are mostly perfect, rarely monoecious or dioecious, and the 

 ovary is entirely superior or free from the calyx in both groups. 



Family Polygonaceae. Buckwheat family. Contains about 30 gen- 

 era and 800 species, of very wide geographic distribution. They are 

 herbs, frequently of twining habit, shrubs or even trees, their most con- 

 spicuous feature being the sheathing united stipules around the bases 

 of the leaves. This sheath is so distinct in appearance from the ordi- 

 nary type of stipule, that it is known by a special name, ocrea. The 

 leaves are simple, mostly entire; the flowers are small and regular, 

 variously clustered, with a perianth consisting of calyx alone, which, 

 however, is frequently colored like a corolla. The stamens are 2 to 9 

 in number, the ovary 1-celled, becoming in fruit a shining angled or 

 lenticular (prune-shaped) achene. 



The Polygonaceae are Avell represented in our country, not only by 

 numerous species of smartweed (Polygonum) and of dock {Buniex), but 

 in the western States by many species of the genus Eriogonum, which 

 may be known by the umbelled or capitate flower-clusters, each group 

 springing from a bell-shaped or cup-like involucre. Polygonum is the 

 largest genus of the family, however, and has also the widest distribu- 

 tion, being found from the far north all the way to the tropics, the 

 species usually of rank growth and weedy appearance. 



Buckwheat, an important article in the daily dietary of many per- 



* This can be seen very plainly by cross-sectioning the seeds of some one of the 

 common goosefoots {Chenopodium). 



