EXOGENOUS OR DICOTYLEDONOUS PLANTS. 



477 



toi'ia. Galls are swellings on the leafstalks, &c., when wounded by 

 certain insects ; those of commerce are derived from Q. infectoria of 

 Asia Minor. Cork is the exterior corky layer of the bark of the 

 Spanish Quercus Suber. 



919. Orel. MyricaccCB {Sweet-Gale Family). Shrubs, with. altei«- 

 nate and simple aromatic resinous-dotted leaves, monoecious or dioj- 

 cious. Differs from the next principally by the one-celled ovary, 

 with a single erect orthotropous ovule, and a drupe-like nut. — Ex.. 

 Myrica, Comptonia, the Sweet Fern. The drupes of M. cerifera 

 (our Candleberry or Bayberry) yield a natural wax. 



920. Ord, Betnlaceae {Birch Family). Trees or shrubs, with al- 

 ternate and simple straight-veined leaves, and deciduous stipules. 

 Flowers monoecious ; those of both kinds in aments (Fig; 312), and 

 commonly achlamydeous, placed three together in the axil of each 

 three-lobed bract. Stamens definite. Ovary two-celled, each cell 

 with one suspended ovule : styles or stigmas distinct. Fruit mem- 

 branaceous or samara-like, one-celled and one-seeded, forming with 

 the three-lobed bracts a kind of strobile. Albumen none. — Ex. 

 Betula (the Birch), Alnus (Alder). The bark is sometimes astrin- 



FIGr. 1164. Quercus Chinquapin in fruit: a, cluster of sterile aments. 1165. A magnified 

 staminate flower. 1166. Transverse section of an ovary, showing the three cells with twoi 

 ovules in each. 1167. The immature seed, with the accompanying abortive ovule. 1168. Thte 

 nut (acorn), in its scaly involucre, or cupule. 1169. Vertical section of the same, and of the- 

 included seed and embryo, showing the thick cotyledons. 



