SPERMATOPHYTA 399 



pushes upward. Where the development of the primary root is accom- 

 panied by elongation of the hypocotyl, the cotyledon or cotyledons are 

 pulled out of the testa and are carried above the ground, where they often 

 expand and function as foliage leaves. In such seedlings the develop- 

 ment of the plumule into the shoot is usually considerably delayed. 



Chief Orders of Angiosperms 



It would be beyond the scope of the present work to include an exten- 

 sive account of the classification of angiosperms, a subject of concern 

 mainly to the taxonomist. A general survey of the chief orders, however, 

 will demonstrate the complexity of the group and illustrate its principal 

 evolutionary trends, many of which have already been mentioned. The 

 orders in each of the three series do not represent a phylogenetic sequence 

 but merely different levels of progress. The interrelationships of many 

 groups is obscure, so that the tracing of lines of descent is difficult and will 

 not be attempted here. 



1. Archichlamydeae 



The Dicotyledoneae include the Archichlamydeae and Metachlamyd- 

 eae. The Archichlamydeae are the primitive stock of angiosperms from 

 which both the Metachlamydeae and Monocotyledoneae have been 

 derived. Their flowers are naked, apetalous, or choripetalous, the parts 

 being usually cyclic but frec^uently more or less spiral. 



Piperales. The Piperales comprise 4 families and about 1 ,200 species of 

 mostly tropical herbs and shrubs, nearly all belonging to the Piperaceae. 

 The peppers {Piper} and peperomias are the best-known examples. The 

 flowers, borne in spikes, are perfect or imperfect, mostly naked, typically 

 trimerous but usually reduced, hypogynous, and mostly apocarpous. 



Salicales, Juglandales, and Fagales. These orders, with several others 

 of minor importance, were once grouped together as the Amentiferae, 

 because their flowers are borne in aments or catkins. The Salicales 

 include a single family, the Salicaceae, to which belong the willows (Salix) 

 and poplars (Populus). The Juglandales also comprise a single family, 

 the Juglandaceae, including the walnuts (Juglans) and hickories {Carija). 

 The Fagales contain two families, the Corylaceae and Fagaceae. To the 

 Corylaceae belong the birches (Behda), alders (Alnus), etc., and to the 

 Fagaceae the beeches {Fagus), chestnuts (Castanea), and oaks (Quercus). 

 The Amentiferae are woody plants with imperfect flowers. In the Salica- 

 ceae and Corylaceae both kinds of flowers are in aments, but in the Juglan- 

 daceae and Fagaceae only the staminate flowers are. In all families 

 except the Salicaceae, the flowers of some members have a simple bract- 

 like perianth. The pistillate flowers are hypogynous in the Salicaceae 

 and epigynous in the other families; they are syncarpous in all. 



