286 



BOTANY OF CROP PLANTS 



pericarp is hard or leathery. The single seed in each fruit 

 assumes the shape of the pericarp; the seed coat (testa) is 

 membranaceous, the endosperm is abundant and mealy, and 

 the embryo is straight or curved. 



Rey to Principal Genera 



Flowers subtended by involucres; ocreae wanting, Eriogonum. 

 Flowers not subtended by involucres; ocreas present. 

 Calyx six-parted (rarely four). 



Stamens nine (very rarely six), Rheum (rhubarb). 

 Stamens six, Rumex (dock). 

 Calyx five-parted (rarely four). 



Achene much surpassing the calyx, Pagopyrum (buckwheat). 

 Achene enclosed by the calyx. Polygonum (bistort, persicaria, knot weed, 

 bindweed, etc.). 



RHEUM RHAPONTICUM (Rhubarb, Pie Plant) 



Roots, Stems, Leaves, Flowers.— This plant is a perennial 

 from large, quite woody rhizomes which have a fibrous and 



Fig. III. — Rhubarb (Rheum) flower, external view, median lengthwise 

 section, and with perianth and stamens removed. (After Lurssen.) 



well-developed root system. The rhizome is used in the 

 propagation of the plant. In the spring, a number of large 

 leaves are sent up from the underground stem, and, later in 



