DEVELOPMENT OF PLANTS 433 



known as the nut. Certain bracts of the flower increase greatly 

 in size as the ovary matures and form a conspicuous part of the 

 fruit. Thus in the hornbeam (Fig. 298, D), one of the bracts 

 develops into a large, three-lobed green leaf, in the hop horn- 

 beam the bract forms a papery sac about the nut, in the hazel 

 a leafy husk (Fig. 298), in the birch and alders a woody peg- 

 like structure. In the chestnut and beech the pistils are com- 

 pletely enveloped by prickly bracts or outgrowths that form the 

 bur (Fig. 300, A-C), while in the oak these structures only cover 

 the lower portion of the ovary, forming the cup (Fig. 299, C-E), 

 The flowers of this order are typical of those that are wind polli- 

 nated. Note the small and simple flowers, absence of showy 

 perianth, nectar, and odor glands, dry and light microspores 

 lavishly produced to ensure seed formation and the delicate 

 bushy stigmas for catching the spores. The stigmas appear a 

 day or so before the adjoining microspores are being shed so 

 that a crossing from an earlier flowering plant is necessitated. 

 These plants form the larger part of our deciduous forests and 

 their association in colonies is doubtless connected with the dis- 

 tribution of pollen by the wind, as is also the appearance of the 

 flowers before the leaves are fully developed. The positions as- 

 sumed by the staminate aments is of service in protecting the 

 microspores against wetting and also to assist in their distribu- 

 tion. During the winter, these aments are quite erect but as the 

 flowering stage approaches they become pendulous, the larger 

 bracts protecting the sporophylls, like the shingles on a roof 

 (Fig. 298, A). These bracts, being hygroscopic, remain closed 

 during wet weather but on dry days they curve back, each bract 

 forming a shelf which serves to catch the spores when no winds 

 are stirring and thus prevent their falling to the ground. 



140. Other Tree Orders Suggestive of the Fagales. — Closely 

 allied to the Fagales is the order of the walnuts, Juglandales, 

 including the black walnut and butternut {Juglans) and the hick- 

 ories {Hicoria, Fig. 301). These trees are of very common 

 occurrence in the northern United States and are characterized 

 by aromatic oils and large compound leaves. The flowers are 

 very similar in structure and arrangement to those of the beech 

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