SUPPLEMENT. 
THE FAMILIES OF FLOWERING PLANTS. 
By Caarzes Lovrs Porwarp. 
CHAPTER XVII. 
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 well represented in our country, not only by 
numerous species of smartweed (Polygonum) and of dock (Rumex), but 
in the western States by many species of the genus Hriogonum, 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). . 
