148 



THE NETTLE FAMILY— URTIOACEAE. 



This family contains a great number of representatives, the ma- 

 jority of which are tropical. It contains trees, shrubs, and many 

 other small plant forms. The trees and shrubs alone comprise over 

 3,000 species and are found in the temperate and tropical regions of 

 both hemispheres. They grow usually at relatively low altitudes fre- 

 quenting wet and swampy as well as dry and arid habitats. 



Several representatives of this family are important timber trees 

 while others are of less commercial importance. Occasionally they 

 may form pure stands but usually are mixed with other species. 

 This family also contains representatives which are attractive orna- 

 mentally and used for hedges. 



The leaves are simple, alternate, and usually deciduous. The fruit 

 matures in one season, in some species in spring shortly after the 

 blossoms while in others in fall. The seeds may germinate the same 

 season or lie dormant over winter and germinate the following 

 spring. The fruit of some genera is edible. It is very variable in 

 form and structure. The subjoined key based primarily on fruit 

 will aid in distinguishing the genera of this family native to Penn- 

 sylvania : 



KEY TO THE GENERA. 



Page. 



1. Fruit a berry; pith chambered, Celtis 152 



1. Fruit not a berry, pith not chambered, 2 



2. Fruit dry, a samara, winged all around; flowers mostly polygamous, sap not 



milky, Ulmus 149 



2. Fruit not dry, an acheue, not winged; flowers unisexual; sap milky, 3 



3. Fruit elongated, edible; leaves dentate 3-nerved; branches unarmed; both staminate 



and pistillate flowers in separate spikes, Morus 154 



3. Fruit round, not edible; leaves entire; branches armed; staminate flowers in racemes, 



pistillate in heads Madura 153 



