394 



GENERAL BOTANY 



the tree are of various sorts, some being perfect, with both pistils 

 and stamens in the same flower, while others are imperfect, with 

 either stamens or pistils in a single flower. In the latter instances 

 both sets of organs are usually present, but one set (either pis- 

 tils or stamens) is abortive and hence useless in reproduction. 

 These abortive organs furnish an interesting example of the per- 

 sistence of structures which were once useful but have ceased to 

 function. Such a floral condition, with a mixture of perfect and 

 imperfect flowers on the same tree, is designated as polygamous. 



FIG. 246. Natural habitat of elms on a creek bottom 

 Photograph furnished by the United States Forest Service 



Pollination is necessarily either self-pollination or cross- 

 pollination and is effected by wind or by insects. The fruit is a 

 winged samara, or key fruit, the wings being an outgrowth of 

 the ovary wall which takes place as a secondary result of fertili- 

 zation along with the formation of the embryo and seed. This 

 winged fruit aids materially in the dissemination of the elms. 



Commercial importance. In addition to its value as a shade 

 and ornamental tree the elm is of considerable importance in the 

 production of lumber for certain purposes. The wood is of the 

 ring-porous type, with a single circle of large spring ducts in 



