CHAPTER XXVIII 

 THE DICOTYLEDONES: ARCHICHLAMYDEAE 



DICOTYLEDONES 



The Dicotyledones are Angiospermae in which the embryo on germination 

 possesses two cotyledons. These may be retained within the seed coat at 

 germination (hypogeal), or they may grow out and become green (epigeal). 

 The primary root of the seedHng generally persists and becomes the main 

 root of a branched root system in the mature plant. The leaves are normally 

 petiolate and net-veined. The flowers may be either pentamerous, tetra- 

 merous or dimerous, but are rarely trimerous. They may be polypetalous, 

 gamopetalous or apetalous. The carpels may be free or united. The vas- 

 cular bundles of the stem are generally arranged in a ring, around a pith. 

 A cambium is normally present, which frequently builds up a woody stem. 



The Dicotyledones are a larger group than the Monocotyledones, with 

 about 90,000 species. They may be trees, shrubs or herbs and they include 

 the more primitive groups of the Angiosperms. Many are of great economic 

 importance, for they supply, among many other materials, all the hardwoods 

 of commerce, most of the fruits and vegetables, as well as many of the spices 

 and Materia Medica. We shall consider this aspect more fully in Volume IV. 



The Dicotyledones are divided into two classes: the Archichlamydeae 

 and the Metachlamydeae. 



ARCHICHLAMYDEAE 



The Archichlamydeae are Dicotyledones in which the perianth may be 

 formed of sepals only, but if petals are present they are seldom united. A 

 perianth may be sometimes completely absent. In the primitive families, 

 the floral parts may be spirally arranged, wholly or in part, and the carpels 

 may be free. 



The Archichlamydeae as here defined include both the Benthamian 

 groups of the Polypetalae with free petals and the Monochlamydeae with a 

 simple perianth, but it must be borne in mind that in thus separating oiT 

 from the group those families in which gamopetaly is characteristic it is not 

 suggested that gamopetaly is entirely absent in the Archichlamydeae. It is 

 an attempt rather to indicate a general tendency. In fact we must recognize 

 that in the evolution of the Dicotyledons both divergence and convergence 

 have played a part, and it would be unwise to assume that all the genera at 

 present included in a family are necessarily monophyletic, that is to say in 

 one line of descent. Two evolutionary series may have converged along 

 similar lines till they became associated in the same family, as is possibly 

 the case in the Rosaceae. 



1606 



